typesetting/quad/qtest/mds/guide.md

# The Racket Guide

Matthew Flatt, Robert Bruce Findler, and PLT

This guide is intended for programmers who are new to Racket or new to
some part of Racket. It assumes programming experience, so if you are
new to programming, consider instead reading _[How to Design
Programs](http://www.htdp.org)_. If you want an especially quick
introduction to Racket, start with \[missing\].

Chapter 2 provides a brief introduction to Racket. From Chapter 3 on,
this guide dives into details—covering much of the Racket toolbox, but
leaving precise details to \[missing\] and other reference manuals.

> The source of this manual is available on
> [GitHub](https://github.com/racket/racket/tree/master/pkgs/racket-doc/scribblings/guide).

    1 Welcome to Racket                                      
      1.1 Interacting with Racket                            
      1.2 Definitions and Interactions                       
      1.3 Creating Executables                               
      1.4 A Note to Readers with Lisp/Scheme Experience      
                                                             
    2 Racket Essentials                                      
      2.1 Simple Values                                      
      2.2 Simple Definitions and Expressions                 
        2.2.1 Definitions                                    
        2.2.2 An Aside on Indenting Code                     
        2.2.3 Identifiers                                    
        2.2.4 Function Calls \(Procedure Applications\)      
        2.2.5 Conditionals with `if`, `and`, `or`, and `cond`
        2.2.6 Function Calls, Again                          
        2.2.7 Anonymous Functions with `lambda`              
        2.2.8 Local Binding with `define`, `let`, and `let*` 
      2.3 Lists, Iteration, and Recursion                    
        2.3.1 Predefined List Loops                          
        2.3.2 List Iteration from Scratch                    
        2.3.3 Tail Recursion                                 
        2.3.4 Recursion versus Iteration                     
      2.4 Pairs, Lists, and Racket Syntax                    
        2.4.1 Quoting Pairs and Symbols with `quote`         
        2.4.2 Abbreviating `quote` with `'`                  
        2.4.3 Lists and Racket Syntax                        
                                                             
    3 Built-In Datatypes                                     
      3.1 Booleans                                           
      3.2 Numbers                                            
      3.3 Characters                                         
      3.4 Strings \(Unicode\)                                
      3.5 Bytes and Byte Strings                             
      3.6 Symbols                                            
      3.7 Keywords                                           
      3.8 Pairs and Lists                                    
      3.9 Vectors                                            
      3.10 Hash Tables                                       
      3.11 Boxes                                             
      3.12 Void and Undefined                                
                                                             
    4 Expressions and Definitions                            
      4.1 Notation                                           
      4.2 Identifiers and Binding                            
      4.3 Function Calls \(Procedure Applications\)          
        4.3.1 Evaluation Order and Arity                     
        4.3.2 Keyword Arguments                              
        4.3.3 The `apply` Function                           
      4.4 Functions \(Procedures\): `lambda`                 
        4.4.1 Declaring a Rest Argument                      
        4.4.2 Declaring Optional Arguments                   
        4.4.3 Declaring Keyword Arguments                    
        4.4.4 Arity-Sensitive Functions: `case-lambda`       
      4.5 Definitions: `define`                              
        4.5.1 Function Shorthand                             
        4.5.2 Curried Function Shorthand                     
        4.5.3 Multiple Values and `define-values`            
        4.5.4 Internal Definitions                           
      4.6 Local Binding                                      
        4.6.1 Parallel Binding: `let`                        
        4.6.2 Sequential Binding: `let*`                     
        4.6.3 Recursive Binding: `letrec`                    
        4.6.4 Named `let`                                    
        4.6.5 Multiple Values: `let-values`, `let*-values`,  
`letrec-values`                                              
      4.7 Conditionals                                       
        4.7.1 Simple Branching: `if`                         
        4.7.2 Combining Tests: `and` and `or`                
        4.7.3 Chaining Tests: `cond`                         
      4.8 Sequencing                                         
        4.8.1 Effects Before: `begin`                        
        4.8.2 Effects After: `begin0`                        
        4.8.3 Effects If...: `when` and `unless`             
      4.9 Assignment: `set!`                                 
        4.9.1 Guidelines for Using Assignment                
        4.9.2 Multiple Values: `set!-values`                 
      4.10 Quoting: `quote` and `'`                          
      4.11 Quasiquoting: `quasiquote` and `‘`                
      4.12 Simple Dispatch: `case`                           
      4.13 Dynamic Binding: `parameterize`                   
                                                             
    5 Programmer-Defined Datatypes                           
      5.1 Simple Structure Types: `struct`                   
      5.2 Copying and Update                                 
      5.3 Structure Subtypes                                 
      5.4 Opaque versus Transparent Structure Types          
      5.5 Structure Comparisons                              
      5.6 Structure Type Generativity                        
      5.7 Prefab Structure Types                             
      5.8 More Structure Type Options                        
                                                             
    6 Modules                                                
      6.1 Module Basics                                      
        6.1.1 Organizing Modules                             
        6.1.2 Library Collections                            
        6.1.3 Packages and Collections                       
        6.1.4 Adding Collections                             
      6.2 Module Syntax                                      
        6.2.1 The `module` Form                              
        6.2.2 The `#lang` Shorthand                          
        6.2.3 Submodules                                     
        6.2.4 Main and Test Submodules                       
      6.3 Module Paths                                       
      6.4 Imports: `require`                                 
      6.5 Exports: `provide`                                 
      6.6 Assignment and Redefinition                        
      6.7 Modules and Macros                                 
                                                             
    7 Contracts                                              
      7.1 Contracts and Boundaries                           
        7.1.1 Contract Violations                            
        7.1.2 Experimenting with Contracts and Modules       
        7.1.3 Experimenting with Nested Contract Boundaries  
      7.2 Simple Contracts on Functions                      
        7.2.1 Styles of `->`                                 
        7.2.2 Using `define/contract` and `->`               
        7.2.3 `any` and `any/c`                              
        7.2.4 Rolling Your Own Contracts                     
        7.2.5 Contracts on Higher-order Functions            
        7.2.6 Contract Messages with “???”                   
        7.2.7 Dissecting a contract error message            
      7.3 Contracts on Functions in General                  
        7.3.1 Optional Arguments                             
        7.3.2 Rest Arguments                                 
        7.3.3 Keyword Arguments                              
        7.3.4 Optional Keyword Arguments                     
        7.3.5 Contracts for `case-lambda`                    
        7.3.6 Argument and Result Dependencies               
        7.3.7 Checking State Changes                         
        7.3.8 Multiple Result Values                         
        7.3.9 Fixed but Statically Unknown Arities           
      7.4 Contracts: A Thorough Example                      
      7.5 Contracts on Structures                            
        7.5.1 Guarantees for a Specific Value                
        7.5.2 Guarantees for All Values                      
        7.5.3 Checking Properties of Data Structures         
      7.6 Abstract Contracts using `#:exists` and `#:∃`      
      7.7 Additional Examples                                
        7.7.1 A Customer-Manager Component                   
        7.7.2 A Parameteric \(Simple\) Stack                 
        7.7.3 A Dictionary                                   
        7.7.4 A Queue                                        
      7.8 Building New Contracts                             
        7.8.1 Contract Struct Properties                     
        7.8.2 With all the Bells and Whistles                
      7.9 Gotchas                                            
        7.9.1 Contracts and `eq?`                            
        7.9.2 Contract boundaries and `define/contract`      
        7.9.3 Exists Contracts and Predicates                
        7.9.4 Defining Recursive Contracts                   
        7.9.5 Mixing `set!` and `contract-out`               
                                                             
    8 Input and Output                                       
      8.1 Varieties of Ports                                 
      8.2 Default Ports                                      
      8.3 Reading and Writing Racket Data                    
      8.4 Datatypes and Serialization                        
      8.5 Bytes, Characters, and Encodings                   
      8.6 I/O Patterns                                       
                                                             
    9 Regular Expressions                                    
      9.1 Writing Regexp Patterns                            
      9.2 Matching Regexp Patterns                           
      9.3 Basic Assertions                                   
      9.4 Characters and Character Classes                   
        9.4.1 Some Frequently Used Character Classes         
        9.4.2 POSIX character classes                        
      9.5 Quantifiers                                        
      9.6 Clusters                                           
        9.6.1 Backreferences                                 
        9.6.2 Non-capturing Clusters                         
        9.6.3 Cloisters                                      
      9.7 Alternation                                        
      9.8 Backtracking                                       
      9.9 Looking Ahead and Behind                           
        9.9.1 Lookahead                                      
        9.9.2 Lookbehind                                     
      9.10 An Extended Example                               
                                                             
    10 Exceptions and Control                                
      10.1 Exceptions                                        
      10.2 Prompts and Aborts                                
      10.3 Continuations                                     
                                                             
    11 Iterations and Comprehensions                         
      11.1 Sequence Constructors                             
      11.2 `for` and `for*`                                  
      11.3 `for/list` and `for*/list`                        
      11.4 `for/vector` and `for*/vector`                    
      11.5 `for/and` and `for/or`                            
      11.6 `for/first` and `for/last`                        
      11.7 `for/fold` and `for*/fold`                        
      11.8 Multiple-Valued Sequences                         
      11.9 Breaking an Iteration                             
      11.10 Iteration Performance                            
                                                             
    12 Pattern Matching                                      
                                                             
    13 Classes and Objects                                   
      13.1 Methods                                           
      13.2 Initialization Arguments                          
      13.3 Internal and External Names                       
      13.4 Interfaces                                        
      13.5 Final, Augment, and Inner                         
      13.6 Controlling the Scope of External Names           
      13.7 Mixins                                            
        13.7.1 Mixins and Interfaces                         
        13.7.2 The `mixin` Form                              
        13.7.3 Parameterized Mixins                          
      13.8 Traits                                            
        13.8.1 Traits as Sets of Mixins                      
        13.8.2 Inherit and Super in Traits                   
        13.8.3 The `trait` Form                              
      13.9 Class Contracts                                   
        13.9.1 External Class Contracts                      
        13.9.2 Internal Class Contracts                      
                                                             
    14 Units \(Components\)                                  
      14.1 Signatures and Units                              
      14.2 Invoking Units                                    
      14.3 Linking Units                                     
      14.4 First-Class Units                                 
      14.5 Whole-`module` Signatures and Units               
      14.6 Contracts for Units                               
        14.6.1 Adding Contracts to Signatures                
        14.6.2 Adding Contracts to Units                     
      14.7 `unit` versus `module`                            
                                                             
    15 Reflection and Dynamic Evaluation                     
      15.1 `eval`                                            
        15.1.1 Local Scopes                                  
        15.1.2 Namespaces                                    
        15.1.3 Namespaces and Modules                        
      15.2 Manipulating Namespaces                           
        15.2.1 Creating and Installing Namespaces            
        15.2.2 Sharing Data and Code Across Namespaces       
      15.3 Scripting Evaluation and Using `load`             
                                                             
    16 Macros                                                
      16.1 Pattern-Based Macros                              
        16.1.1 `define-syntax-rule`                          
        16.1.2 Lexical Scope                                 
        16.1.3 `define-syntax` and `syntax-rules`            
        16.1.4 Matching Sequences                            
        16.1.5 Identifier Macros                             
        16.1.6 `set!` Transformers                           
        16.1.7 Macro-Generating Macros                       
        16.1.8 Extended Example: Call-by-Reference Functions 
      16.2 General Macro Transformers                        
        16.2.1 Syntax Objects                                
        16.2.2 Macro Transformer Procedures                  
        16.2.3 Mixing Patterns and Expressions: `syntax-case`
        16.2.4 `with-syntax` and `generate-temporaries`      
        16.2.5 Compile and Run-Time Phases                   
        16.2.6 General Phase Levels                          
          16.2.6.1 Phases and Bindings                       
          16.2.6.2 Phases and Modules                        
        16.2.7 Syntax Taints                                 
      16.3 Module Instantiations and Visits                  
        16.3.1 Declaration versus Instantiation              
        16.3.2 Compile-Time Instantiation                    
        16.3.3 Visiting Modules                              
        16.3.4 Lazy Visits via Available Modules             
                                                             
    17 Creating Languages                                    
      17.1 Module Languages                                  
        17.1.1 Implicit Form Bindings                        
        17.1.2 Using `#lang s-exp`                           
      17.2 Reader Extensions                                 
        17.2.1 Source Locations                              
        17.2.2 Readtables                                    
      17.3 Defining new `#lang` Languages                    
        17.3.1 Designating a `#lang` Language                
        17.3.2 Using `#lang reader`                          
        17.3.3 Using `#lang s-exp syntax/module-reader`      
        17.3.4 Installing a Language                         
        17.3.5 Source-Handling Configuration                 
        17.3.6 Module-Handling Configuration                 
                                                             
    18 Concurrency and Synchronization                       
      18.1 Threads                                           
      18.2 Thread Mailboxes                                  
      18.3 Semaphores                                        
      18.4 Channels                                          
      18.5 Buffered Asynchronous Channels                    
      18.6 Synchronizable Events and `sync`                  
      18.7 Building Your Own Synchronization Patterns        
                                                             
    19 Performance                                           
      19.1 Performance in DrRacket                           
      19.2 The Bytecode and Just-in-Time \(JIT\) Compilers   
      19.3 Modules and Performance                           
      19.4 Function-Call Optimizations                       
      19.5 Mutation and Performance                          
      19.6 `letrec` Performance                              
      19.7 Fixnum and Flonum Optimizations                   
      19.8 Unchecked, Unsafe Operations                      
      19.9 Foreign Pointers                                  
      19.10 Regular Expression Performance                   
      19.11 Memory Management                                
      19.12 Reachability and Garbage Collection              
      19.13 Weak Boxes and Testing                           
      19.14 Reducing Garbage Collection Pauses               
                                                             
    20 Parallelism                                           
      20.1 Parallelism with Futures                          
      20.2 Parallelism with Places                           
      20.3 Distributed Places                                
                                                             
    21 Running and Creating Executables                      
      21.1 Running `racket` and `gracket`                    
        21.1.1 Interactive Mode                              
        21.1.2 Module Mode                                   
        21.1.3 Load Mode                                     
      21.2 Scripts                                           
        21.2.1 Unix Scripts                                  
        21.2.2 Windows Batch Files                           
      21.3 Creating Stand-Alone Executables                  
                                                             
    22 More Libraries                                        
      22.1 Graphics and GUIs                                 
      22.2 The Web Server                                    
      22.3 Using Foreign Libraries                           
      22.4 And More                                          
                                                             
    23 Dialects of Racket and Scheme                         
      23.1 More Rackets                                      
      23.2 Standards                                         
        23.2.1 R5RS                                          
        23.2.2 R6RS                                          
      23.3 Teaching                                          
                                                             
    24 Command-Line Tools and Your Editor of Choice          
      24.1 Command-Line Tools                                
        24.1.1 Compilation and Configuration: `raco`         
        24.1.2 Interactive evaluation                        
        24.1.3 Shell completion                              
      24.2 Emacs                                             
        24.2.1 Major Modes                                   
        24.2.2 Minor Modes                                   
        24.2.3 Packages specific to Evil Mode                
      24.3 Vim                                               
      24.4 Sublime Text                                      
                                                             
    Bibliography                                             
                                                             
    Index                                                    

## 1. Welcome to Racket

Depending on how you look at it, **Racket** is

* a _programming language_—a dialect of Lisp and a descendant of Scheme;

  > See Dialects of Racket and Scheme for more information on other
  > dialects of Lisp and how they relate to Racket.

* a _family_ of programming languages—variants of Racket, and more; or

* a set of _tools_—for using a family of programming languages.

Where there is no room for confusion, we use simply _Racket_.

Racket’s main tools are

* **`racket`**, the core compiler, interpreter, and run-time system;

* **DrRacket**, the programming environment; and

* **`raco`**, a command-line tool for executing **Ra**cket **co**mmands
  that install packages, build libraries, and more.

Most likely, you’ll want to explore the Racket language using DrRacket,
especially at the beginning. If you prefer, you can also work with the
command-line `racket` interpreter and your favorite text editor; see
also Command-Line Tools and Your Editor of Choice. The rest of this
guide presents the language mostly independent of your choice of editor.

If you’re using DrRacket, you’ll need to choose the proper language,
because DrRacket accommodates many different variants of Racket, as well
as other languages. Assuming that you’ve never used DrRacket before,
start it up, type the line

`#lang` `racket`

in DrRacket’s top text area, and then click the Run button that’s above
the text area. DrRacket then understands that you mean to work in the
normal variant of Racket \(as opposed to the smaller `racket/base` or
many other possibilities\).

> More Rackets describes some of the other possibilities.

If you’ve used DrRacket before with something other than a program that
starts `#lang`, DrRacket will remember the last language that you used,
instead of inferring the language from the `#lang` line. In that case,
use the Language|Choose Language... menu item.  In the dialog that
appears, select the first item, which tells DrRacket to use the language
that is declared in a source program via `#lang`. Put the `#lang` line
above in the top text area, still.

### 1.1. Interacting with Racket

DrRacket’s bottom text area and the `racket` command-line program \(when
started with no options\) both act as a kind of calculator. You type a
Racket expression, hit the Return key, and the answer is printed. In the
terminology of Racket, this kind of calculator is called a
_read-eval-print loop_ or _REPL_.

A number by itself is an expression, and the answer is just the number:

```racket
> 5
5  
```

A string is also an expression that evaluates to itself. A string is
written with double quotes at the start and end of the string:

```racket
> "Hello, world!"
"Hello, world!"  
```

Racket uses parentheses to wrap larger expressions—almost any kind of
expression, other than simple constants. For example, a function call is
written: open parenthesis, function name, argument expression, and
closing parenthesis. The following expression calls the built-in
function `substring` with the arguments `"the boy out of the country"`,
`4`, and `7`:

```racket
> (substring "the boy out of the country" 4 7)
"boy"                                         
```

### 1.2. Definitions and Interactions

You can define your own functions that work like `substring` by using
the `define` form, like this:

```racket                               
(define (extract str)                   
  (substring str 4 7))                  
```                                     
                                        
```racket                               
> (extract "the boy out of the country")
"boy"                                   
> (extract "the country out of the boy")
"cou"                                   
```                                     

Although you can evaluate the `define` form in the REPL, definitions are
normally a part of a program that you want to keep and use later. So, in
DrRacket, you’d normally put the definition in the top text area—called
the _definitions area_—along with the `#lang` prefix:

```racket
#lang racket          
                      
(define (extract str) 
  (substring str 4 7))
```

If calling `(extract "the boy")` is part of the main action of your
program, that would go in the definitions area, too. But if it was just
an example expression that you were using to explore `extract`, then
you’d more likely leave the definitions area as above, click Run, and
then evaluate `(extract "the boy")` in the REPL.

When using command-line `racket` instead of DrRacket, you’d save the
above text in a file using your favorite editor. If you save it as
`"extract.rkt"`, then after starting `racket` in the same directory,
you’d evaluate the following sequence:

> If you use `xrepl`, you can use `,enter extract.rkt`.

```racket
> (enter! "extract.rkt")             
> (extract "the gal out of the city")
"gal"                                
```

The `enter!` form both loads the code and switches the evaluation
context to the inside of the module, just like DrRacket’s Run button.

### 1.3. Creating Executables

If your file \(or definitions area in DrRacket\) contains

```racket
#lang racket                      
                                  
(define (extract str)             
  (substring str 4 7))            
                                  
(extract "the cat out of the bag")
```

then it is a complete program that prints “cat” when run. You can run
the program within DrRacket or using `enter!` in `racket`, but if the
program is saved in >_src-filename_<, you can also run it from a command
line with

  `racket `>_src-filename_<

To package the program as an executable, you have a few options:

* In DrRacket, you can select the Racket|Create        Executable...
  menu item.

* From a command-line prompt, run `raco exe `>_src-filename_<, where
  >_src-filename_< contains the program. See \[missing\] for more
  information.

* With Unix or Mac OS, you can turn the program file into an executable
  script by inserting the line

  > See Scripts for more information on script files.

    `#! /usr/bin/env racket`

  at the very beginning of the file. Also, change the file permissions
  to executable using `chmod +x `>_filename_< on the command line.

  The script works as long as `racket` is in the user’s executable
  search path.  Alternately, use a full path to `racket` after `#!`
  \(with a space between `#!` and the path\), in which case the user’s
  executable search path does not matter.

### 1.4. A Note to Readers with Lisp/Scheme Experience

If you already know something about Scheme or Lisp, you might be tempted
to put just

```racket
(define (extract str) 
  (substring str 4 7))
```

into `"extract.rktl"` and run `racket` with

```racket
> (load "extract.rktl")  
> (extract "the dog out")
"dog"                    
```

That will work, because `racket` is willing to imitate a traditional
Lisp environment, but we strongly recommend against using `load` or
writing programs outside of a module.

Writing definitions outside of a module leads to bad error messages, bad
performance, and awkward scripting to combine and run programs. The
problems are not specific to `racket`; they’re fundamental limitations
of the traditional top-level environment, which Scheme and Lisp
implementations have historically fought with ad hoc command-line flags,
compiler directives, and build tools. The module system is designed to
avoid these problems, so start with `#lang`, and you’ll be happier with
Racket in the long run.

## 2. Racket Essentials

This chapter provides a quick introduction to Racket as background for
the rest of the guide. Readers with some Racket experience can safely
skip to Built-In Datatypes.

    2.1 Simple Values                                      
    2.2 Simple Definitions and Expressions                 
      2.2.1 Definitions                                    
      2.2.2 An Aside on Indenting Code                     
      2.2.3 Identifiers                                    
      2.2.4 Function Calls \(Procedure Applications\)      
      2.2.5 Conditionals with `if`, `and`, `or`, and `cond`
      2.2.6 Function Calls, Again                          
      2.2.7 Anonymous Functions with `lambda`              
      2.2.8 Local Binding with `define`, `let`, and `let*` 
    2.3 Lists, Iteration, and Recursion                    
      2.3.1 Predefined List Loops                          
      2.3.2 List Iteration from Scratch                    
      2.3.3 Tail Recursion                                 
      2.3.4 Recursion versus Iteration                     
    2.4 Pairs, Lists, and Racket Syntax                    
      2.4.1 Quoting Pairs and Symbols with `quote`         
      2.4.2 Abbreviating `quote` with `'`                  
      2.4.3 Lists and Racket Syntax                        

### 2.1. Simple Values

Racket values include numbers, booleans, strings, and byte strings. In
DrRacket and documentation examples \(when you read the documentation in
color\), value expressions are shown in green.

_Numbers_ are written in the usual way, including fractions and
imaginary numbers:

> +Numbers \(later in this guide\) explains more about numbers.

```racket
1       3.14                  
1/2     6.02e+23              
1+2i    9999999999999999999999
```

_Booleans_ are `#t` for true and `#f` for false. In conditionals,
however, all non-`#f` values are treated as true.

> +Booleans \(later in this guide\) explains more about booleans.

_Strings_ are written between doublequotes. Within a string, backslash
is an escaping character; for example, a backslash followed by a
doublequote includes a literal doublequote in the string. Except for an
unescaped doublequote or backslash, any Unicode character can appear in
a string constant.

> +Strings \(Unicode\) \(later in this guide\) explains more about
> strings.

```racket
"Hello, world!"            
"Benjamin \"Bugsy\" Siegel"
"λx:(μα.α→α).xx"           
```

When a constant is evaluated in the REPL, it typically prints the same
as its input syntax. In some cases, the printed form is a normalized
version of the input syntax. In documentation and in DrRacket’s REPL,
results are printed in blue instead of green to highlight the difference
between an input expression and a printed result.

Examples:

```racket
> 1.0000                         
1.0                              
> "Bugs \u0022Figaro\u0022 Bunny"
"Bugs \"Figaro\" Bunny"          
```

### 2.2. Simple Definitions and Expressions

A program module is written as

`#lang` >_langname_< >_topform_<\*

where a >_topform_< is either a >_definition_< or an >_expr_<. The REPL
also evaluates >_topform_<s.

In syntax specifications, text with a gray background, such as `#lang`,
represents literal text. Whitespace must appear between such literals
and nonterminals like >_id_<, except that whitespace is not required
before or after `(`, `)`, `[`, or `]`.  A comment, which starts with `;`
and runs until the end of the line, is treated the same as whitespace.

> +\[missing\] in \[missing\] provides more on different forms of
> comments.

Following the usual conventions, \* in a grammar means zero or more
repetitions of the preceding element, + means one or more repetitions of
the preceding element, and {} groups a sequence as an element for
repetition.

#### 2.2.1. Definitions

A definition of the form

> +Definitions: `define` \(later in this guide\) explains more about
> definitions.

`(` `define` >_id_< >_expr_< `)`

binds >_id_< to the result of >_expr_<, while

`(` `define` `(` >_id_< >_id_<\* `)` >_expr_<+ `)`

binds the first >_id_< to a function \(also called a _procedure_\) that
takes arguments as named by the remaining >_id_<s. In the function case,
the >_expr_<s are the body of the function. When the function is called,
it returns the result of the last >_expr_<.

Examples:

```racket
(define pie 3)             ; defines pie to be 3        
                                                        
(define (piece str)        ; defines piece as a function
  (substring str 0 pie))   ;  of one argument           
                                                        
> pie                                                   
3                                                       
> (piece "key lime")                                    
"key"                                                   
```

Under the hood, a function definition is really the same as a
non-function definition, and a function name does not have to be used in
a function call. A function is just another kind of value, though the
printed form is necessarily less complete than the printed form of a
number or string.

Examples:

```racket
> piece               
#<procedure:piece>    
> substring           
#<procedure:substring>
```

A function definition can include multiple expressions for the
function’s body. In that case, only the value of the last expression is
returned when the function is called. The other expressions are
evaluated only for some side-effect, such as printing.

Examples:

```racket
(define (bake flavor)            
  (printf "preheating oven...\n")
  (string-append flavor " pie")) 
                                 
> (bake "apple")                 
preheating oven...               
"apple pie"                      
```

Racket programmers prefer to avoid side-effects, so a definition usually
has just one expression in its body. It’s important, though, to
understand that multiple expressions are allowed in a definition body,
because it explains why the following `nobake` function fails to include
its argument in its result:

```racket                      
(define (nobake flavor)        
  string-append flavor "jello")
```                            
                               
```racket                      
> (nobake "green")             
"jello"                        
```                            

Within `nobake`, there are no parentheses around `string-append flavor
"jello"`, so they are three separate expressions instead of one
function-call expression. The expressions `string-append` and `flavor`
are evaluated, but the results are never used. Instead, the result of
the function is just the result of the final expression, `"jello"`.

#### 2.2.2. An Aside on Indenting Code

Line breaks and indentation are not significant for parsing Racket
programs, but most Racket programmers use a standard set of conventions
to make code more readable. For example, the body of a definition is
typically indented under the first line of the definition. Identifiers
are written immediately after an open parenthesis with no extra space,
and closing parentheses never go on their own line.

DrRacket automatically indents according to the standard style when you
type Enter in a program or REPL expression. For example, if you hit
Enter after typing `(define (greet name)`, then DrRacket automatically
inserts two spaces for the next line.  If you change a region of code,
you can select it in DrRacket and hit Tab, and DrRacket will re-indent
the code \(without inserting any line breaks\). Editors like Emacs offer
a Racket or Scheme mode with similar indentation support.

Re-indenting not only makes the code easier to read, it gives you extra
feedback that your parentheses match in the way that you intended. For
example, if you leave out a closing parenthesis after the last argument
to a function, automatic indentation starts the next line under the
first argument, instead of under the `define` keyword:

```racket
(define (halfbake flavor                                  
                  (string-append flavor " creme brulee")))
```

In this case, indentation helps highlight the mistake. In other cases,
where the indentation may be normal while an open parenthesis has no
matching close parenthesis, both `racket` and DrRacket use the source’s
indentation to suggest where a parenthesis might be missing.

#### 2.2.3. Identifiers

Racket’s syntax for identifiers is especially liberal. Excluding the
special characters

> +Identifiers and Binding \(later in this guide\) explains more about
> identifiers.

   `(` `)` `[` `]` `{` `}` `"` `,` `'` ` `;` `#` `|` `\`

and except for the sequences of characters that make number constants,
almost any sequence of non-whitespace characters forms an >_id_<. For
example `substring` is an identifier. Also, `string-append` and `a+b`
are identifiers, as opposed to arithmetic expressions. Here are several
more examples:

```racket
+                              
Hfuhruhurr                     
integer?                       
pass/fail                      
john-jacob-jingleheimer-schmidt
a-b-c+1-2-3                    
```

#### 2.2.4. Function Calls \(Procedure Applications\)

We have already seen many function calls, which are called _procedure
applications_ in more traditional terminology. The syntax of a function
call is

> +Function Calls \(later in this guide\) explains more about function
> calls.

`(` >_id_< >_expr_<\* `)`

where the number of >_expr_<s determines the number of arguments
supplied to the function named by >_id_<.

The `racket` language pre-defines many function identifiers, such as
`substring` and `string-append`. More examples are below.

In example Racket code throughout the documentation, uses of pre-defined
names are hyperlinked to the reference manual. So, you can click on an
identifier to get full details about its use.

```racket
> (string-append "rope" "twine" "yarn")  ; append strings       
"ropetwineyarn"                                                 
> (substring "corduroys" 0 4)            ; extract a substring  
"cord"                                                          
> (string-length "shoelace")             ; get a string's length
8                                                               
> (string? "Ceci n'est pas une string.") ; recognize strings    
#t                                                              
> (string? 1)                                                   
#f                                                              
> (sqrt 16)                              ; find a square root   
4                                                               
> (sqrt -16)                                                    
0+4i                                                            
> (+ 1 2)                                ; add numbers          
3                                                               
> (- 2 1)                                ; subtract numbers     
1                                                               
> (< 2 1)                                ; compare numbers      
#f                                                              
> (>= 2 1)                                                      
#t                                                              
> (number? "c'est une number")           ; recognize numbers    
#f                                                              
> (number? 1)                                                   
#t                                                              
> (equal? 6 "half dozen")                ; compare anything     
#f                                                              
> (equal? 6 6)                                                  
#t                                                              
> (equal? "half dozen" "half dozen")                            
#t                                                              
```

#### 2.2.5. Conditionals with `if`, `and`, `or`, and `cond`

The next simplest kind of expression is an `if` conditional:

`(` `if` >_expr_< >_expr_< >_expr_< `)`

> +Conditionals \(later in this guide\) explains more about conditionals.

The first >_expr_< is always evaluated. If it produces a non-`#f` value,
then the second >_expr_< is evaluated for the result of the whole `if`
expression, otherwise the third >_expr_< is evaluated for the result.

Example:

```racket
> (if (> 2 3)   
      "bigger"  
      "smaller")
"smaller"       
```

```racket                               
(define (reply s)                       
  (if (equal? "hello" (substring s 0 5))
      "hi!"                             
      "huh?"))                          
```                                     
                                        
```racket                               
> (reply "hello racket")                
"hi!"                                   
> (reply "λx:(μα.α→α).xx")              
"huh?"                                  
```                                     

Complex conditionals can be formed by nesting `if` expressions. For
example, you could make the `reply` function work when given
non-strings:

```racket
(define (reply s)                           
  (if (string? s)                           
      (if (equal? "hello" (substring s 0 5))
          "hi!"                             
          "huh?")                           
      "huh?"))                              
```

Instead of duplicating the `"huh?"` case, this function is better
written as

```racket
(define (reply s)                           
  (if (if (string? s)                       
          (equal? "hello" (substring s 0 5))
          #f)                               
      "hi!"                                 
      "huh?"))                              
```

but these kinds of nested `if`s are difficult to read.  Racket provides
more readable shortcuts through the `and` and `or` forms, which work
with any number of expressions:

> +Combining Tests: `and` and `or` \(later in this guide\) explains more
> about `and` and `or`.

```racket
( and >_expr_<* )
( or >_expr_<* ) 
```

The `and` form short-circuits: it stops and returns `#f` when an
expression produces `#f`, otherwise it keeps going. The `or` form
similarly short-circuits when it encounters a true result.

Examples:

```racket
(define (reply s)                             
  (if (and (string? s)                        
           (>= (string-length s) 5)           
           (equal? "hello" (substring s 0 5)))
      "hi!"                                   
      "huh?"))                                
                                              
> (reply "hello racket")                      
"hi!"                                         
> (reply 17)                                  
"huh?"                                        
```

Another common pattern of nested `if`s involves a sequence of tests,
each with its own result:

```racket
(define (reply-more s)                                          
  (if (equal? "hello" (substring s 0 5))                        
      "hi!"                                                     
      (if (equal? "goodbye" (substring s 0 7))                  
          "bye!"                                                
          (if (equal? "?" (substring s (- (string-length s) 1)))
              "I don't know"                                    
              "huh?"))))                                        
```

The shorthand for a sequence of tests is the `cond` form:

> +Chaining Tests: `cond` \(later in this guide\) explains more about
> `cond`.

`(` `cond` {`[` >_expr_< >_expr_<\* `]`}\* `)`

A `cond` form contains a sequence of clauses between square brackets. In
each clause, the first >_expr_< is a test expression. If it produces
true, then the clause’s remaining >_expr_<s are evaluated, and the last
one in the clause provides the answer for the entire `cond` expression;
the rest of the clauses are ignored. If the test >_expr_< produces `#f`,
then the clause’s remaining >_expr_<s are ignored, and evaluation
continues with the next clause. The last clause can use `else` as a
synonym for a `#t` test expression.

Using `cond`, the `reply-more` function can be more clearly written as
follows:

```racket                                             
(define (reply-more s)                                
  (cond                                               
   [(equal? "hello" (substring s 0 5))                
    "hi!"]                                            
   [(equal? "goodbye" (substring s 0 7))              
    "bye!"]                                           
   [(equal? "?" (substring s (- (string-length s) 1)))
    "I don't know"]                                   
   [else "huh?"]))                                    
```                                                   
                                                      
```racket                                             
> (reply-more "hello racket")                         
"hi!"                                                 
> (reply-more "goodbye cruel world")                  
"bye!"                                                
> (reply-more "what is your favorite color?")         
"I don't know"                                        
> (reply-more "mine is lime green")                   
"huh?"                                                
```                                                   

The use of square brackets for `cond` clauses is a convention. In
Racket, parentheses and square brackets are actually interchangeable, as
long as `(` is matched with `)` and `[` is matched with `]`. Using
square brackets in a few key places makes Racket code even more
readable.

#### 2.2.6. Function Calls, Again

In our earlier grammar of function calls, we oversimplified.  The actual
syntax of a function call allows an arbitrary expression for the
function, instead of just an >_id_<:

> +Function Calls \(later in this guide\) explains more about function
> calls.

`(` >_expr_< >_expr_<\* `)`

The first >_expr_< is often an >_id_<, such as `string-append` or `+`,
but it can be anything that evaluates to a function. For example, it can
be a conditional expression:

```racket                                
(define (double v)                       
  ((if (string? v) string-append +) v v))
```                                      
                                         
```racket                                
> (double "mnah")                        
"mnahmnah"                               
> (double 5)                             
10                                       
```                                      

Syntactically, the first expression in a function call could even be a
number—but that leads to an error, since a number is not a function.

```racket
> (1 2 3 4)                                           
application: not a procedure;                         
 expected a procedure that can be applied to arguments
  given: 1                                            
  arguments...:                                       
   2                                                  
   3                                                  
   4                                                  
```

When you accidentally omit a function name or when you use extra
parentheses around an expression, you’ll most often get an “expected a
procedure” error like this one.

#### 2.2.7. Anonymous Functions with `lambda`

Programming in Racket would be tedious if you had to name all of your
numbers. Instead of writing `(+ 1 2)`, you’d have to write

> +Functions: `lambda` \(later in this guide\) explains more about
> `lambda`.

```racket
> (define a 1)
> (define b 2)
> (+ a b)     
3             
```

It turns out that having to name all your functions can be tedious, too.
For example, you might have a function `twice` that takes a function and
an argument. Using `twice` is convenient if you already have a name for
the function, such as `sqrt`:

```racket          
(define (twice f v)
  (f (f v)))       
```                
                   
```racket          
> (twice sqrt 16)  
2                  
```                

If you want to call a function that is not yet defined, you could define
it, and then pass it to `twice`:

```racket               
(define (louder s)      
  (string-append s "!"))
```                     
                        
```racket               
> (twice louder "hello")
"hello!!"               
```                     

But if the call to `twice` is the only place where `louder` is used,
it’s a shame to have to write a whole definition. In Racket, you can use
a `lambda` expression to produce a function directly. The `lambda` form
is followed by identifiers for the function’s arguments, and then the
function’s body expressions:

`(` `lambda` `(` >_id_<\* `)` >_expr_<+ `)`

Evaluating a `lambda` form by itself produces a function:

```racket
> (lambda (s) (string-append s "!"))
#<procedure>                        
```

Using `lambda`, the above call to `twice` can be re-written as

```racket
> (twice (lambda (s) (string-append s "!")) 
         "hello")                           
"hello!!"                                   
> (twice (lambda (s) (string-append s "?!"))
         "hello")                           
"hello?!?!"                                 
```

Another use of `lambda` is as a result for a function that generates
functions:

```racket                                
(define (make-add-suffix s2)             
  (lambda (s) (string-append s s2)))     
```                                      
                                         
```racket                                
> (twice (make-add-suffix "!") "hello")  
"hello!!"                                
> (twice (make-add-suffix "?!") "hello") 
"hello?!?!"                              
> (twice (make-add-suffix "...") "hello")
"hello......"                            
```                                      

Racket is a _lexically scoped_ language, which means that `s2` in the
function returned by `make-add-suffix` always refers to the argument for
the call that created the function. In other words, the
`lambda`-generated function “remembers” the right `s2`:

```racket
> (define louder (make-add-suffix "!"))   
> (define less-sure (make-add-suffix "?"))
> (twice less-sure "really")              
"really??"                                
> (twice louder "really")                 
"really!!"                                
```

We have so far referred to definitions of the form `(define `>_id_<`
`>_expr_<`)` as “non-function definitions.” This characterization is
misleading, because the >_expr_< could be a `lambda` form, in which case
the definition is equivalent to using the “function” definition form.
For example, the following two definitions of `louder` are equivalent:

```racket                  
(define (louder s)         
  (string-append s "!"))   
                           
(define louder             
  (lambda (s)              
    (string-append s "!")))
```                        
                           
```racket                  
> louder                   
#<procedure:louder>        
```                        

Note that the expression for `louder` in the second case is an
“anonymous” function written with `lambda`, but, if possible, the
compiler infers a name, anyway, to make printing and error reporting as
informative as possible.

#### 2.2.8. Local Binding with
`define`, `let`, and `let*`

It’s time to retract another simplification in our grammar of Racket. In
the body of a function, definitions can appear before the body
expressions:

> +Internal Definitions \(later in this guide\) explains more about local
> \(internal\) definitions.

```racket
( define ( >_id_< >_id_<* ) >_definition_<* >_expr_<+ )
( lambda ( >_id_<* ) >_definition_<* >_expr_<+ )       
```

Definitions at the start of a function body are local to the function
body.

Examples:

```racket
(define (converse s)                                    
  (define (starts? s2) ; local to converse              
    (define len2 (string-length s2))  ; local to starts?
    (and (>= (string-length s) len2)                    
         (equal? s2 (substring s 0 len2))))             
  (cond                                                 
   [(starts? "hello") "hi!"]                            
   [(starts? "goodbye") "bye!"]                         
   [else "huh?"]))                                      
                                                        
> (converse "hello!")                                   
"hi!"                                                   
> (converse "urp")                                      
"huh?"                                                  
> starts? ; outside of converse, so...                  
starts?: undefined;                                     
 cannot reference an identifier before its definition   
  in module: top-level                                  
```

Another way to create local bindings is the `let` form. An advantage of
`let` is that it can be used in any expression position. Also, `let`
binds many identifiers at once, instead of requiring a separate `define`
for each identifier.

> +Internal Definitions \(later in this guide\) explains more about `let`
> and `let*`.

`(` `let` `(` {`[` >_id_< >_expr_< `]`}\* `)` >_expr_<+ `)`

Each binding clause is an >_id_< and an >_expr_< surrounded by square
brackets, and the expressions after the clauses are the body of the
`let`. In each clause, the >_id_< is bound to the result of the >_expr_<
for use in the body.

```racket
> (let ([x (random 4)]    
        [o (random 4)])   
    (cond                 
     [(> x o) "X wins"]   
     [(> o x) "O wins"]   
     [else "cat's game"]))
"cat's game"              
```

The bindings of a `let` form are available only in the body of the
`let`, so the binding clauses cannot refer to each other. The `let*`
form, in contrast, allows later clauses to use earlier bindings:

```racket
> (let* ([x (random 4)]                         
         [o (random 4)]                         
         [diff (number->string (abs (- x o)))]) 
    (cond                                       
     [(> x o) (string-append "X wins by " diff)]
     [(> o x) (string-append "O wins by " diff)]
     [else "cat's game"]))                      
"X wins by 1"                                   
```

### 2.3. Lists, Iteration, and Recursion

Racket is a dialect of the language Lisp, whose name originally stood
for “LISt Processor.” The built-in list datatype remains a prominent
feature of the language.

The `list` function takes any number of values and returns a list
containing the values:

```racket
> (list "red" "green" "blue")
'("red" "green" "blue")      
> (list 1 2 3 4 5)           
'(1 2 3 4 5)                 
```

> A list usually prints with `'`, but the printed form of a list depends
> on its content. See Pairs and Lists for more information.

As you can see, a list result prints in the REPL as a quote `'` and then
a pair of parentheses wrapped around the printed form of the list
elements. There’s an opportunity for confusion here, because parentheses
are used for both expressions, such as `(list "red" "green" "blue")`,
and printed results, such as `'("red" "green" "blue")`. In addition to
the quote, parentheses for results are printed in blue in the
documentation and in DrRacket, whereas parentheses for expressions are
brown.

Many predefined functions operate on lists. Here are a few examples:

```racket
> (length (list "hop" "skip" "jump"))        ; count the elements  
3                                                                  
> (list-ref (list "hop" "skip" "jump") 0)    ; extract by position 
"hop"                                                              
> (list-ref (list "hop" "skip" "jump") 1)                          
"skip"                                                             
> (append (list "hop" "skip") (list "jump")) ; combine lists       
'("hop" "skip" "jump")                                             
> (reverse (list "hop" "skip" "jump"))       ; reverse order       
'("jump" "skip" "hop")                                             
> (member "fall" (list "hop" "skip" "jump")) ; check for an element
#f                                                                 
```

#### 2.3.1. Predefined List Loops

In addition to simple operations like `append`, Racket includes
functions that iterate over the elements of a list. These iteration
functions play a role similar to `for` in Java, Racket, and other
languages. The body of a Racket iteration is packaged into a function to
be applied to each element, so the `lambda` form becomes particularly
handy in combination with iteration functions.

Different list-iteration functions combine iteration results in
different ways. The `map` function uses the per-element results to
create a new list:

```racket
> (map sqrt (list 1 4 9 16))                    
'(1 2 3 4)                                      
> (map (lambda (i)                              
         (string-append i "!"))                 
       (list "peanuts" "popcorn" "crackerjack"))
'("peanuts!" "popcorn!" "crackerjack!")         
```

The `andmap` and `ormap` functions combine the results by `and`ing or
`or`ing:

```racket
> (andmap string? (list "a" "b" "c"))
#t                                   
> (andmap string? (list "a" "b" 6))  
#f                                   
> (ormap number? (list "a" "b" 6))   
#t                                   
```

The `map`, `andmap`, and `ormap` functions can all handle multiple
lists, instead of just a single list. The lists must all have the same
length, and the given function must accept one argument for each list:

```racket
> (map (lambda (s n) (substring s 0 n))        
       (list "peanuts" "popcorn" "crackerjack")
       (list 6 3 7))                           
'("peanut" "pop" "cracker")                    
```

The `filter` function keeps elements for which the body result is true,
and discards elements for which it is `#f`:

```racket
> (filter string? (list "a" "b" 6))   
'("a" "b")                            
> (filter positive? (list 1 -2 6 7 0))
'(1 6 7)                              
```

The `foldl` function generalizes some iteration functions. It uses the
per-element function to both process an element and combine it with the
“current” value, so the per-element function takes an extra first
argument. Also, a starting “current” value must be provided before the
lists:

```racket
> (foldl (lambda (elem v)      
           (+ v (* elem elem)))
         0                     
         '(1 2 3))             
14                             
```

Despite its generality, `foldl` is not as popular as the other
functions. One reason is that `map`, `ormap`, `andmap`, and `filter`
cover the most common kinds of list loops.

Racket provides a general _list comprehension_ form `for/list`, which
builds a list by iterating through _sequences_. List comprehensions and
related iteration forms are described in Iterations and Comprehensions.

#### 2.3.2. List Iteration from Scratch

Although `map` and other iteration functions are predefined, they are
not primitive in any interesting sense. You can write equivalent
iterations using a handful of list primitives.

Since a Racket list is a linked list, the two core operations on a
non-empty list are

* `first`: get the first thing in the list; and

* `rest`: get the rest of the list.

Examples:

```racket
> (first (list 1 2 3))
1                     
> (rest (list 1 2 3)) 
'(2 3)                
```

To create a new node for a linked list—that is, to add to the front of
the list—use the `cons` function, which is short for “construct.” To get
an empty list to start with, use the `empty` constant:

```racket
> empty                            
'()                                
> (cons "head" empty)              
'("head")                          
> (cons "dead" (cons "head" empty))
'("dead" "head")                   
```

To process a list, you need to be able to distinguish empty lists from
non-empty lists, because `first` and `rest` work only on non-empty
lists. The `empty?` function detects empty lists, and `cons?` detects
non-empty lists:

```racket
> (empty? empty)              
#t                            
> (empty? (cons "head" empty))
#f                            
> (cons? empty)               
#f                            
> (cons? (cons "head" empty)) 
#t                            
```

With these pieces, you can write your own versions of the `length`
function, `map` function, and more.

Examples:

```racket
(define (my-length lst)                 
  (cond                                 
   [(empty? lst) 0]                     
   [else (+ 1 (my-length (rest lst)))]))
                                        
> (my-length empty)                     
0                                       
> (my-length (list "a" "b" "c"))        
3                                       
```

```racket                                         
(define (my-map f lst)                            
  (cond                                           
   [(empty? lst) empty]                           
   [else (cons (f (first lst))                    
               (my-map f (rest lst)))]))          
```                                               
                                                  
```racket                                         
> (my-map string-upcase (list "ready" "set" "go"))
'("READY" "SET" "GO")                             
```                                               

If the derivation of the above definitions is mysterious to you,
consider reading _[How to Design Programs](http://www.htdp.org)_. If you
are merely suspicious of the use of recursive calls instead of a looping
construct, then read on.

#### 2.3.3. Tail Recursion

Both the `my-length` and `my-map` functions run in _O_\(_n_\)__ space
for a list of length _n_. This is easy to see by imagining how
`(my-length (list "a" "b" "c"))` must evaluate:

```racket
(my-length (list "a" "b" "c"))        
= (+ 1 (my-length (list "b" "c")))    
= (+ 1 (+ 1 (my-length (list "c"))))  
= (+ 1 (+ 1 (+ 1 (my-length (list)))))
= (+ 1 (+ 1 (+ 1 0)))                 
= (+ 1 (+ 1 1))                       
= (+ 1 2)                             
= 3                                   
```

For a list with _n_ elements, evaluation will stack up _n_ `(+ 1 ...)`
additions, and then finally add them up when the list is exhausted.

You can avoid piling up additions by adding along the way. To accumulate
a length this way, we need a function that takes both a list and the
length of the list seen so far; the code below uses a local function
`iter` that accumulates the length in an argument `len`:

```racket
(define (my-length lst)                  
  ; local function iter:                 
  (define (iter lst len)                 
    (cond                                
     [(empty? lst) len]                  
     [else (iter (rest lst) (+ len 1))]))
  ; body of my-length calls iter:        
  (iter lst 0))                          
```

Now evaluation looks like this:

```racket
(my-length (list "a" "b" "c"))
= (iter (list "a" "b" "c") 0) 
= (iter (list "b" "c") 1)     
= (iter (list "c") 2)         
= (iter (list) 3)             
3                             
```

The revised `my-length` runs in constant space, just as the evaluation
steps above suggest. That is, when the result of a function call, like
`(iter (list "b" "c") 1)`, is exactly the result of some other function
call, like `(iter (list "c") 2)`, then the first one doesn’t have to
wait around for the second one, because that takes up space for no good
reason.

This evaluation behavior is sometimes called _tail-call optimization_,
but it’s not merely an “optimization” in Racket; it’s a guarantee about
the way the code will run. More precisely, an expression in _tail
position_ with respect to another expression does not take extra
computation space over the other expression.

In the case of `my-map`, _O_\(_n_\)__ space complexity is reasonable,
since it has to generate a result of size _O_\(_n_\)__. Nevertheless,
you can reduce the constant factor by accumulating the result list. The
only catch is that the accumulated list will be backwards, so you’ll
have to reverse it at the very end:

> Attempting to reduce a constant factor like this is usually not
> worthwhile, as discussed below.

```racket
(define (my-map f lst)                       
  (define (iter lst backward-result)         
    (cond                                    
     [(empty? lst) (reverse backward-result)]
     [else (iter (rest lst)                  
                 (cons (f (first lst))       
                       backward-result))]))  
  (iter lst empty))                          
```

It turns out that if you write

```racket
(define (my-map f lst)
  (for/list ([i lst]) 
    (f i)))           
```

then the `for/list` form in the function is expanded to essentially the
same code as the `iter` local definition and use. The difference is
merely syntactic convenience.

#### 2.3.4. Recursion versus Iteration

The `my-length` and `my-map` examples demonstrate that iteration is just
a special case of recursion. In many languages, it’s important to try to
fit as many computations as possible into iteration form. Otherwise,
performance will be bad, and moderately large inputs can lead to stack
overflow.  Similarly, in Racket, it is sometimes important to make sure
that tail recursion is used to avoid _O_\(_n_\)__ space consumption when
the computation is easily performed in constant space.

At the same time, recursion does not lead to particularly bad
performance in Racket, and there is no such thing as stack overflow; you
can run out of memory if a computation involves too much context, but
exhausting memory typically requires orders of magnitude deeper
recursion than would trigger a stack overflow in other languages. These
considerations, combined with the fact that tail-recursive programs
automatically run the same as a loop, lead Racket programmers to embrace
recursive forms rather than avoid them.

Suppose, for example, that you want to remove consecutive duplicates
from a list. While such a function can be written as a loop that
remembers the previous element for each iteration, a Racket programmer
would more likely just write the following:

```racket                                     
(define (remove-dups l)                       
  (cond                                       
   [(empty? l) empty]                         
   [(empty? (rest l)) l]                      
   [else                                      
    (let ([i (first l)])                      
      (if (equal? i (first (rest l)))         
          (remove-dups (rest l))              
          (cons i (remove-dups (rest l)))))]))
```                                           
                                              
```racket                                     
> (remove-dups (list "a" "b" "b" "b" "c" "c"))
'("a" "b" "c")                                
```                                           

In general, this function consumes _O_\(_n_\)__ space for an input list
of length _n_, but that’s fine, since it produces an _O_\(_n_\)__
result. If the input list happens to be mostly consecutive duplicates,
then the resulting list can be much smaller than _O_\(_n_\)__—and
`remove-dups` will also use much less than _O_\(_n_\)__ space! The
reason is that when the function discards duplicates, it returns the
result of a `remove-dups` call directly, so the tail-call “optimization”
kicks in:

```racket
(remove-dups (list "a" "b" "b" "b" "b" "b"))         
= (cons "a" (remove-dups (list "b" "b" "b" "b" "b")))
= (cons "a" (remove-dups (list "b" "b" "b" "b")))    
= (cons "a" (remove-dups (list "b" "b" "b")))        
= (cons "a" (remove-dups (list "b" "b")))            
= (cons "a" (remove-dups (list "b")))                
= (cons "a" (list "b"))                              
= (list "a" "b")                                     
```

### 2.4. Pairs, Lists, and Racket Syntax

The `cons` function actually accepts any two values, not just a list for
the second argument. When the second argument is not `empty` and not
itself produced by `cons`, the result prints in a special way. The two
values joined with `cons` are printed between parentheses, but with a
dot \(i.e., a period surrounded by whitespace\) in between:

```racket
> (cons 1 2)             
'(1 . 2)                 
> (cons "banana" "split")
'("banana" . "split")    
```

Thus, a value produced by `cons` is not always a list. In general, the
result of `cons` is a _pair_. The more traditional name for the `cons?`
function is `pair?`, and we’ll use the traditional name from now on.

The name `rest` also makes less sense for non-list pairs; the more
traditional names for `first` and `rest` are `car` and `cdr`,
respectively. \(Granted, the traditional names are also nonsense. Just
remember that “a” comes before “d,” and `cdr` is pronounced
“could-er.”\)

Examples:

```racket
> (car (cons 1 2))    
1                     
> (cdr (cons 1 2))    
2                     
> (pair? empty)       
#f                    
> (pair? (cons 1 2))  
#t                    
> (pair? (list 1 2 3))
#t                    
```

Racket’s pair datatype and its relation to lists is essentially a
historical curiosity, along with the dot notation for printing and the
funny names `car` and `cdr`. Pairs are deeply wired into to the culture,
specification, and implementation of Racket, however, so they survive in
the language.

You are perhaps most likely to encounter a non-list pair when making a
mistake, such as accidentally reversing the arguments to `cons`:

```racket
> (cons (list 2 3) 1)
'((2 3) . 1)         
> (cons 1 (list 2 3))
'(1 2 3)             
```

Non-list pairs are used intentionally, sometimes. For example, the
`make-hash` function takes a list of pairs, where the `car` of each pair
is a key and the `cdr` is an arbitrary value.

The only thing more confusing to new Racketeers than non-list pairs is
the printing convention for pairs where the second element _is_ a pair,
but _is not_ a list:

```racket
> (cons 0 (cons 1 2))
'(0 1 . 2)           
```

In general, the rule for printing a pair is as follows: use the dot
notation unless the dot is immediately followed by an open parenthesis.
In that case, remove the dot, the open parenthesis, and the matching
close parenthesis. Thus, `'(0 . (1 . 2))` becomes `'(0 1 . 2)`, and `'(1
. (2 . (3 . ())))` becomes `'(1 2 3)`.

#### 2.4.1. Quoting Pairs and Symbols with `quote`

A list prints with a quote mark before it, but if an element of a list
is itself a list, then no quote mark is printed for the inner list:

```racket
> (list (list 1) (list 2 3) (list 4))
'((1) (2 3) (4))                     
```

For nested lists, especially, the `quote` form lets you write a list as
an expression in essentially the same way that the list prints:

```racket
> (quote ("red" "green" "blue"))
'("red" "green" "blue")         
> (quote ((1) (2 3) (4)))       
'((1) (2 3) (4))                
> (quote ())                    
'()                             
```

The `quote` form works with the dot notation, too, whether the quoted
form is normalized by the dot-parenthesis elimination rule or not:

```racket
> (quote (1 . 2))      
'(1 . 2)               
> (quote (0 . (1 . 2)))
'(0 1 . 2)             
```

Naturally, lists of any kind can be nested:

```racket
> (list (list 1 2 3) 5 (list "a" "b" "c"))
'((1 2 3) 5 ("a" "b" "c"))                
> (quote ((1 2 3) 5 ("a" "b" "c")))       
'((1 2 3) 5 ("a" "b" "c"))                
```

If you wrap an identifier with `quote`, then you get output that looks
like an identifier, but with a `'` prefix:

```racket
> (quote jane-doe)
'jane-doe         
```

A value that prints like a quoted identifier is a _symbol_. In the same
way that parenthesized output should not be confused with expressions, a
printed symbol should not be confused with an identifier. In particular,
the symbol `(quote map)` has nothing to do with the `map` identifier or
the predefined function that is bound to `map`, except that the symbol
and the identifier happen to be made up of the same letters.

Indeed, the intrinsic value of a symbol is nothing more than its
character content. In this sense, symbols and strings are almost the
same thing, and the main difference is how they print. The functions
`symbol->string` and `string->symbol` convert between them.

Examples:

```racket
> map                         
#<procedure:map>              
> (quote map)                 
'map                          
> (symbol? (quote map))       
#t                            
> (symbol? map)               
#f                            
> (procedure? map)            
#t                            
> (string->symbol "map")      
'map                          
> (symbol->string (quote map))
"map"                         
```

In the same way that `quote` for a list automatically applies itself to
nested lists, `quote` on a parenthesized sequence of identifiers
automatically applies itself to the identifiers to create a list of
symbols:

```racket
> (car (quote (road map)))          
'road                               
> (symbol? (car (quote (road map))))
#t                                  
```

When a symbol is inside a list that is printed with `'`, the `'` on the
symbol is omitted, since `'` is doing the job already:

```racket
> (quote (road map))
'(road map)         
```

The `quote` form has no effect on a literal expression such as a number
or string:

```racket
> (quote 42)             
42                       
> (quote "on the record")
"on the record"          
```

#### 2.4.2. Abbreviating `quote` with `'`

As you may have guessed, you can abbreviate a use of `quote` by just
putting `'` in front of a form to quote:

```racket
> '(1 2 3)                     
'(1 2 3)                       
> 'road                        
'road                          
> '((1 2 3) road ("a" "b" "c"))
'((1 2 3) road ("a" "b" "c"))  
```

In the documentation, `'` within an expression is printed in green along
with the form after it, since the combination is an expression that is a
constant. In DrRacket, only the `'` is colored green. DrRacket is more
precisely correct, because the meaning of `quote` can vary depending on
the context of an expression. In the documentation, however, we
routinely assume that standard bindings are in scope, and so we paint
quoted forms in green for extra clarity.

A `'` expands to a `quote` form in quite a literal way. You can see this
if you put a `'` in front of a form that has a `'`:

```racket
> (car ''road)       
'quote               
> (car '(quote road))
'quote               
```

The `'` abbreviation works in output as well as input. The REPL’s
printer recognizes the symbol `'quote` as the first element of a
two-element list when printing output, in which case it uses `’` to
print the output:

```racket
> (quote (quote road))
”road                 
> '(quote road)       
”road                 
> ''road              
”road                 
```

#### 2.4.3. Lists and Racket Syntax

Now that you know the truth about pairs and lists, and now that you’ve
seen `quote`, you’re ready to understand the main way in which we have
been simplifying Racket’s true syntax.

The syntax of Racket is not defined directly in terms of character
streams. Instead, the syntax is determined by two layers:

* a _reader_ layer, which turns a sequence of characters into lists,
  symbols, and other constants; and

* an _expander_ layer, which processes the lists, symbols, and other
  constants to parse them as an expression.

The rules for printing and reading go together. For example, a list is
printed with parentheses, and reading a pair of parentheses produces a
list. Similarly, a non-list pair is printed with the dot notation, and a
dot on input effectively runs the dot-notation rules in reverse to
obtain a pair.

One consequence of the read layer for expressions is that you can use
the dot notation in expressions that are not quoted forms:

```racket
> (+ 1 . (2))
3            
```

This works because `(+ 1 . (2))` is just another way of writing `(+ 1
2)`. It is practically never a good idea to write application
expressions using this dot notation; it’s just a consequence of the way
Racket’s syntax is defined.

Normally, `.` is allowed by the reader only with a parenthesized
sequence, and only before the last element of the sequence. However, a
pair of `.`s can also appear around a single element in a parenthesized
sequence, as long as the element is not first or last. Such a pair
triggers a reader conversion that moves the element between `.`s to the
front of the list. The conversion enables a kind of general infix
notation:

```racket
> (1 . < . 2) 
#t            
> '(1 . < . 2)
'(< 1 2)      
```

This two-dot convention is non-traditional, and it has essentially
nothing to do with the dot notation for non-list pairs. Racket
programmers use the infix convention sparingly—mostly for asymmetric
binary operators such as `<` and `is-a?`.

## 3. Built-In Datatypes

The previous chapter introduced some of Racket’s built-in datatypes:
numbers, booleans, strings, lists, and procedures. This section provides
a more complete coverage of the built-in datatypes for simple forms of
data.

    3.1 Booleans              
    3.2 Numbers               
    3.3 Characters            
    3.4 Strings \(Unicode\)   
    3.5 Bytes and Byte Strings
    3.6 Symbols               
    3.7 Keywords              
    3.8 Pairs and Lists       
    3.9 Vectors               
    3.10 Hash Tables          
    3.11 Boxes                
    3.12 Void and Undefined   

### 3.1. Booleans

Racket has two distinguished constants to represent boolean values: `#t`
for true and `#f` for false. Uppercase `#T` and `#F` are parsed as the
same values, but the lowercase forms are preferred.

The `boolean?` procedure recognizes the two boolean constants. In the
result of a test expression for `if`, `cond`, `and`, `or`, etc.,
however, any value other than `#f` counts as true.

Examples:

```racket
> (= 2 (+ 1 1))  
#t               
> (boolean? #t)  
#t               
> (boolean? #f)  
#t               
> (boolean? "no")
#f               
> (if "no" 1 0)  
1                
```

### 3.2. Numbers

A Racket _number_ is either exact or inexact:

* An _exact_ number is either

  * an arbitrarily large or small integer, such as `5`,
    `99999999999999999`, or `-17`;

  * a rational that is exactly the ratio of two arbitrarily small or
    large integers, such as `1/2`, `99999999999999999/2`, or `-3/4`; or

  * a complex number with exact real and imaginary parts \(where the
    imaginary part is not zero\), such as `1+2i` or `1/2+3/4i`.

* An _inexact_ number is either

  * an IEEE floating-point representation of a number, such as `2.0` or
    `3.14e+87`, where the IEEE infinities and not-a-number are written
    `+inf.0`, `-inf.0`, and `+nan.0` \(or `-nan.0`\); or

  * a complex number with real and imaginary parts that are IEEE
    floating-point representations, such as `2.0+3.0i` or
    `-inf.0+nan.0i`; as a special case, an inexact complex number can
    have an exact zero real part with an inexact imaginary part.

Inexact numbers print with a decimal point or exponent specifier, and
exact numbers print as integers and fractions.  The same conventions
apply for reading number constants, but `#e` or `#i` can prefix a number
to force its parsing as an exact or inexact number. The prefixes `#b`,
`#o`, and `#x` specify binary, octal, and hexadecimal interpretation of
digits.

> +\[missing\] in \[missing\] documents the fine points of the syntax of
> numbers.

Examples:

```racket
> 0.5   
0.5     
> #e0.5 
1/2     
> #x03BB
955     
```

Computations that involve an inexact number produce inexact results, so
that inexactness acts as a kind of taint on numbers. Beware, however,
that Racket offers no “inexact booleans,” so computations that branch on
the comparison of inexact numbers can nevertheless produce exact
results. The procedures `exact->inexact` and `inexact->exact` convert
between the two types of numbers.

Examples:

```racket
> (/ 1 2)                         
1/2                               
> (/ 1 2.0)                       
0.5                               
> (if (= 3.0 2.999) 1 2)          
2                                 
> (inexact->exact 0.1)            
3602879701896397/36028797018963968
```

Inexact results are also produced by procedures such as `sqrt`, `log`,
and `sin` when an exact result would require representing real numbers
that are not rational. Racket can represent only rational numbers and
complex numbers with rational parts.

Examples:

```racket
> (sin 0)   ; rational...    
0                            
> (sin 1/2) ; not rational...
0.479425538604203            
```

In terms of performance, computations with small integers are typically
the fastest, where “small” means that the number fits into one bit less
than the machine’s word-sized representation for signed numbers.
Computation with very large exact integers or with non-integer exact
numbers can be much more expensive than computation with inexact
numbers.

```racket                                             
(define (sigma f a b)                                 
  (if (= a b)                                         
      0                                               
      (+ (f a) (sigma f (+ a 1) b))))                 
```                                                   
                                                      
```racket                                             
> (time (round (sigma (lambda (x) (/ 1 x)) 1 2000)))  
cpu time: 64 real time: 64 gc time: 0                 
8                                                     
> (time (round (sigma (lambda (x) (/ 1.0 x)) 1 2000)))
cpu time: 0 real time: 0 gc time: 0                   
8.0                                                   
```                                                   

The number categories _integer_, _rational_, _real_ \(always rational\),
and _complex_ are defined in the usual way, and are recognized by the
procedures `integer?`, `rational?`, `real?`, and `complex?`, in addition
to the generic `number?`. A few mathematical procedures accept only real
numbers, but most implement standard extensions to complex numbers.

Examples:

```racket
> (integer? 5)                        
#t                                    
> (complex? 5)                        
#t                                    
> (integer? 5.0)                      
#t                                    
> (integer? 1+2i)                     
#f                                    
> (complex? 1+2i)                     
#t                                    
> (complex? 1.0+2.0i)                 
#t                                    
> (abs -5)                            
5                                     
> (abs -5+2i)                         
abs: contract violation               
  expected: real?                     
  given: -5+2i                        
> (sin -5+2i)                         
3.6076607742131563+1.0288031496599337i
```

The `=` procedure compares numbers for numerical equality. If it is
given both inexact and exact numbers to compare, it essentially converts
the inexact numbers to exact before comparing. The `eqv?` \(and
therefore `equal?`\) procedure, in contrast, compares numbers
considering both exactness and numerical equality.

Examples:

```racket
> (= 1 1.0)   
#t            
> (eqv? 1 1.0)
#f            
```

Beware of comparisons involving inexact numbers, which by their nature
can have surprising behavior. Even apparently simple inexact numbers may
not mean what you think they mean; for example, while a base-2 IEEE
floating-point number can represent `1/2` exactly, it can only
approximate `1/10`:

Examples:

```racket
> (= 1/2 0.5)                     
#t                                
> (= 1/10 0.1)                    
#f                                
> (inexact->exact 0.1)            
3602879701896397/36028797018963968
```

> +\[missing\] in \[missing\] provides more on numbers and number
> procedures.

### 3.3. Characters

A Racket _character_ corresponds to a Unicode _scalar value_. Roughly, a
scalar value is an unsigned integer whose representation fits into 21
bits, and that maps to some notion of a natural-language character or
piece of a character. Technically, a scalar value is a simpler notion
than the concept called a “character” in the Unicode standard, but it’s
an approximation that works well for many purposes. For example, any
accented Roman letter can be represented as a scalar value, as can any
common Chinese character.

Although each Racket character corresponds to an integer, the character
datatype is separate from numbers. The `char->integer` and
`integer->char` procedures convert between scalar-value numbers and the
corresponding character.

A printable character normally prints as `#\` followed by the
represented character. An unprintable character normally prints as `#\u`
followed by the scalar value as hexadecimal number. A few characters are
printed specially; for example, the space and linefeed characters print
as `#\space` and `#\newline`, respectively.

> +\[missing\] in \[missing\] documents the fine points of the syntax of
> characters.

Examples:

```racket
> (integer->char 65)     
#\A                      
> (char->integer #\A)    
65                       
> #\λ                    
#\λ                      
> #\u03BB                
#\λ                      
> (integer->char 17)     
#\u0011                  
> (char->integer #\space)
32                       
```

The `display` procedure directly writes a character to the current
output port \(see Input and Output\), in contrast to the
character-constant syntax used to print a character result.

Examples:

```racket
> #\A          
#\A            
> (display #\A)
A              
```

Racket provides several classification and conversion procedures on
characters. Beware, however, that conversions on some Unicode characters
work as a human would expect only when they are in a string \(e.g.,
upcasing “ß” or downcasing “Σ”\).

Examples:

```racket
> (char-alphabetic? #\A)      
#t                            
> (char-numeric? #\0)         
#t                            
> (char-whitespace? #\newline)
#t                            
> (char-downcase #\A)         
#\a                           
> (char-upcase #\ß)           
#\ß                           
```

The `char=?` procedure compares two or more characters, and `char-ci=?`
compares characters ignoring case. The `eqv?` and `equal?` procedures
behave the same as `char=?` on characters; use `char=?` when you want to
more specifically declare that the values being compared are characters.

Examples:

```racket
> (char=? #\a #\A)   
#f                   
> (char-ci=? #\a #\A)
#t                   
> (eqv? #\a #\A)     
#f                   
```

> +\[missing\] in \[missing\] provides more on characters and character
> procedures.

### 3.4. Strings \(Unicode\)

A _string_ is a fixed-length array of characters. It prints using
doublequotes, where doublequote and backslash characters within the
string are escaped with backslashes. Other common string escapes are
supported, including `\n` for a linefeed, `\r` for a carriage return,
octal escapes using `\` followed by up to three octal digits, and
hexadecimal escapes with `\u` \(up to four digits\).  Unprintable
characters in a string are normally shown with `\u` when the string is
printed.

> +\[missing\] in \[missing\] documents the fine points of the syntax of
> strings.

The `display` procedure directly writes the characters of a string to
the current output port \(see Input and Output\), in contrast to the
string-constant syntax used to print a string result.

Examples:

```racket
> "Apple"                       
"Apple"                         
> "\u03BB"                      
"λ"                             
> (display "Apple")             
Apple                           
> (display "a \"quoted\" thing")
a "quoted" thing                
> (display "two\nlines")        
two                             
lines                           
> (display "\u03BB")            
λ                               
```

A string can be mutable or immutable; strings written directly as
expressions are immutable, but most other strings are mutable. The
`make-string` procedure creates a mutable string given a length and
optional fill character. The `string-ref` procedure accesses a character
from a string \(with 0-based indexing\); the `string-set!`  procedure
changes a character in a mutable string.

Examples:

```racket
> (string-ref "Apple" 0)        
#\A                             
> (define s (make-string 5 #\.))
> s                             
"....."                         
> (string-set! s 2 #\λ)         
> s                             
"..λ.."                         
```

String ordering and case operations are generally _locale-independent_;
that is, they work the same for all users. A few _locale-dependent_
operations are provided that allow the way that strings are case-folded
and sorted to depend on the end-user’s locale. If you’re sorting
strings, for example, use `string<?` or `string-ci<?` if the sort result
should be consistent across machines and users, but use
`string-locale<?` or `string-locale-ci<?` if the sort is purely to order
strings for an end user.

Examples:

```racket
> (string<? "apple" "Banana")         
#f                                    
> (string-ci<? "apple" "Banana")      
#t                                    
> (string-upcase "Straße")            
"STRASSE"                             
> (parameterize ([current-locale "C"])
    (string-locale-upcase "Straße"))  
"STRAßE"                              
```

For working with plain ASCII, working with raw bytes, or
encoding/decoding Unicode strings as bytes, use byte strings.

> +\[missing\] in \[missing\] provides more on strings and string
> procedures.

### 3.5. Bytes and Byte Strings

A _byte_ is an exact integer between `0` and `255`, inclusive. The
`byte?` predicate recognizes numbers that represent bytes.

Examples:

```racket
> (byte? 0)  
#t           
> (byte? 256)
#f           
```

A _byte string_ is similar to a string—see Strings \(Unicode\)—but its
content is a sequence of bytes instead of characters. Byte strings can
be used in applications that process pure ASCII instead of Unicode text.
The printed form of a byte string supports such uses in particular,
because a byte string prints like the ASCII decoding of the byte string,
but prefixed with a `#`. Unprintable ASCII characters or non-ASCII bytes
in the byte string are written with octal notation.

> +\[missing\] in \[missing\] documents the fine points of the syntax of
> byte strings.

Examples:

```racket
> #"Apple"                   
#"Apple"                     
> (bytes-ref #"Apple" 0)     
65                           
> (make-bytes 3 65)          
#"AAA"                       
> (define b (make-bytes 2 0))
> b                          
#"\0\0"                      
> (bytes-set! b 0 1)         
> (bytes-set! b 1 255)       
> b                          
#"\1\377"                    
```

The `display` form of a byte string writes its raw bytes to the current
output port \(see Input and Output\). Technically, `display` of a normal
\(i.e,. character\) string prints the UTF-8 encoding of the string to
the current output port, since output is ultimately defined in terms of
bytes; `display` of a byte string, however, writes the raw bytes with no
encoding. Along the same lines, when this documentation shows output, it
technically shows the UTF-8-decoded form of the output.

Examples:

```racket
> (display #"Apple")                         
Apple                                        
> (display "\316\273")  ; same as "λ"       
λ                                           
> (display #"\316\273") ; UTF-8 encoding of λ
λ                                            
```

For explicitly converting between strings and byte strings, Racket
supports three kinds of encodings directly: UTF-8, Latin-1, and the
current locale’s encoding. General facilities for byte-to-byte
conversions \(especially to and from UTF-8\) fill the gap to support
arbitrary string encodings.

Examples:

```racket
> (bytes->string/utf-8 #"\316\273")                               
"λ"                                                               
> (bytes->string/latin-1 #"\316\273")                             
"λ"                                                              
> (parameterize ([current-locale "C"])  ; C locale supports ASCII,
    (bytes->string/locale #"\316\273")) ; only, so...             
bytes->string/locale: byte string is not a valid encoding         
for the current locale                                            
  byte string: #"\316\273"                                        
> (let ([cvt (bytes-open-converter "cp1253" ; Greek code page     
                                   "UTF-8")]                      
        [dest (make-bytes 2)])                                    
    (bytes-convert cvt #"\353" 0 1 dest)                          
    (bytes-close-converter cvt)                                   
    (bytes->string/utf-8 dest))                                   
"λ"                                                               
```

> +\[missing\] in \[missing\] provides more on byte strings and
> byte-string procedures.

### 3.6. Symbols

A _symbol_ is an atomic value that prints like an identifier preceded
with `'`.  An expression that starts with `'` and continues with an
identifier produces a symbol value.

Examples:

```racket
> 'a          
'a            
> (symbol? 'a)
#t            
```

For any sequence of characters, exactly one corresponding symbol is
_interned_; calling the `string->symbol` procedure, or `read`ing a
syntactic identifier, produces an interned symbol. Since interned
symbols can be cheaply compared with `eq?` \(and thus `eqv?` or
`equal?`\), they serve as a convenient values to use for tags and
enumerations.

Symbols are case-sensitive. By using a `#ci` prefix or in other ways,
the reader can be made to case-fold character sequences to arrive at a
symbol, but the reader preserves case by default.

Examples:

```racket
> (eq? 'a 'a)                  
#t                             
> (eq? 'a (string->symbol "a"))
#t                             
> (eq? 'a 'b)                  
#f                             
> (eq? 'a 'A)                  
#f                             
> #ci'A                        
'a                             
```

Any string \(i.e., any character sequence\) can be supplied to
`string->symbol` to obtain the corresponding symbol. For reader input,
any character can appear directly in an identifier, except for
whitespace and the following special characters:

   `(` `)` `[` `]` `{` `}` `"` `,` `'` ` `;` `#` `|` `\`

Actually, `#` is disallowed only at the beginning of a symbol, and then
only if not followed by `%`; otherwise, `#` is allowed, too. Also, `.`
by itself is not a symbol.

Whitespace or special characters can be included in an identifier by
quoting them with `|` or `\`. These quoting mechanisms are used in the
printed form of identifiers that contain special characters or that
might otherwise look like numbers.

Examples:

```racket
> (string->symbol "one, two")
'|one, two|                  
> (string->symbol "6")       
'|6|                         
```

> +\[missing\] in \[missing\] documents the fine points of the syntax of
> symbols.

The `write` function prints a symbol without a `'` prefix. The `display`
form of a symbol is the same as the corresponding string.

Examples:

```racket
> (write 'Apple)  
Apple             
> (display 'Apple)
Apple             
> (write '|6|)    
|6|               
> (display '|6|)  
6                 
```

The `gensym` and `string->uninterned-symbol` procedures generate fresh
_uninterned_ symbols that are not equal \(according to `eq?`\) to any
previously interned or uninterned symbol. Uninterned symbols are useful
as fresh tags that cannot be confused with any other value.

Examples:

```racket
> (define s (gensym))                     
> s                                       
'g42                                      
> (eq? s 'g42)                            
#f                                        
> (eq? 'a (string->uninterned-symbol "a"))
#f                                        
```

> +\[missing\] in \[missing\] provides more on symbols.

### 3.7. Keywords

A _keyword_ value is similar to a symbol \(see Symbols\), but its
printed form is prefixed with `#:`.

> +\[missing\] in \[missing\] documents the fine points of the syntax of
> keywords.

Examples:

```racket
> (string->keyword "apple")               
'#:apple                                  
> '#:apple                                
'#:apple                                  
> (eq? '#:apple (string->keyword "apple"))
#t                                        
```

More precisely, a keyword is analogous to an identifier; in the same way
that an identifier can be quoted to produce a symbol, a keyword can be
quoted to produce a value. The same term “keyword” is used in both
cases, but we sometimes use _keyword value_ to refer more specifically
to the result of a quote-keyword expression or of `string->keyword`. An
unquoted keyword is not an expression, just as an unquoted identifier
does not produce a symbol:

Examples:

```racket
> not-a-symbol-expression                            
not-a-symbol-expression: undefined;                  
 cannot reference an identifier before its definition
  in module: top-level                               
> #:not-a-keyword-expression                         
eval:2:0: #%datum: keyword misused as an expression  
  at: #:not-a-keyword-expression                     
```

Despite their similarities, keywords are used in a different way than
identifiers or symbols. Keywords are intended for use \(unquoted\) as
special markers in argument lists and in certain syntactic forms.  For
run-time flags and enumerations, use symbols instead of keywords.  The
example below illustrates the distinct roles of keywords and symbols.

Examples:

```racket
> (define dir (find-system-path 'temp-dir)) ; not '#:temp-dir   
> (with-output-to-file (build-path dir "stuff.txt")             
    (lambda () (printf "example\n"))                            
    ; optional #:mode argument can be 'text or 'binary          
    #:mode 'text                                                
    ; optional #:exists argument can be 'replace, 'truncate, ...
    #:exists 'replace)                                          
```

### 3.8. Pairs and Lists

A _pair_ joins two arbitrary values. The `cons` procedure constructs
pairs, and the `car` and `cdr` procedures extract the first and second
elements of the pair, respectively. The `pair?` predicate recognizes
pairs.

Some pairs print by wrapping parentheses around the printed forms of the
two pair elements, putting a `'` at the beginning and a `.` between the
elements.

Examples:

```racket
> (cons 1 2)         
'(1 . 2)             
> (cons (cons 1 2) 3)
'((1 . 2) . 3)       
> (car (cons 1 2))   
1                    
> (cdr (cons 1 2))   
2                    
> (pair? (cons 1 2)) 
#t                   
```

A _list_ is a combination of pairs that creates a linked list. More
precisely, a list is either the empty list `null`, or it is a pair whose
first element is a list element and whose second element is a list. The
`list?` predicate recognizes lists. The `null?`  predicate recognizes
the empty list.

A list normally prints as a `'` followed by a pair of parentheses
wrapped around the list elements.

Examples:

```racket
> null                           
'()                              
> (cons 0 (cons 1 (cons 2 null)))
'(0 1 2)                         
> (list? null)                   
#t                               
> (list? (cons 1 (cons 2 null))) 
#t                               
> (list? (cons 1 2))             
#f                               
```

A list or pair prints using `list` or `cons` when one of its elements
cannot be written as a `quote`d value. For example, a value constructed
with `srcloc` cannot be written using `quote`, and it prints using
`srcloc`:

```racket
> (srcloc "file.rkt" 1 0 1 (+ 4 4))              
(srcloc "file.rkt" 1 0 1 8)                      
> (list 'here (srcloc "file.rkt" 1 0 1 8) 'there)
(list 'here (srcloc "file.rkt" 1 0 1 8) 'there)  
> (cons 1 (srcloc "file.rkt" 1 0 1 8))           
(cons 1 (srcloc "file.rkt" 1 0 1 8))             
> (cons 1 (cons 2 (srcloc "file.rkt" 1 0 1 8)))  
(list* 1 2 (srcloc "file.rkt" 1 0 1 8))          
```

> See also `list*`.

As shown in the last example, `list*` is used to abbreviate a series of
`cons`es that cannot be abbreviated using `list`.

The `write` and `display` functions print a pair or list without a
leading `'`, `cons`, `list`, or `list*`. There is no difference between
`write` and `display` for a pair or list, except as they apply to
elements of the list:

Examples:

```racket
> (write (cons 1 2))      
(1 . 2)                   
> (display (cons 1 2))    
(1 . 2)                   
> (write null)            
()                        
> (display null)          
()                        
> (write (list 1 2 "3"))  
(1 2 "3")                 
> (display (list 1 2 "3"))
(1 2 3)                   
```

Among the most important predefined procedures on lists are those that
iterate through the list’s elements:

```racket
> (map (lambda (i) (/ 1 i))                                
       '(1 2 3))                                           
'(1 1/2 1/3)                                               
> (andmap (lambda (i) (i . < . 3))                         
         '(1 2 3))                                         
#f                                                         
> (ormap (lambda (i) (i . < . 3))                          
         '(1 2 3))                                         
#t                                                         
> (filter (lambda (i) (i . < . 3))                         
          '(1 2 3))                                        
'(1 2)                                                     
> (foldl (lambda (v i) (+ v i))                            
         10                                                
         '(1 2 3))                                         
16                                                         
> (for-each (lambda (i) (display i))                       
            '(1 2 3))                                      
123                                                        
> (member "Keys"                                           
          '("Florida" "Keys" "U.S.A."))                    
'("Keys" "U.S.A.")                                         
> (assoc 'where                                            
         '((when "3:30") (where "Florida") (who "Mickey")))
'(where "Florida")                                         
```

> +\[missing\] in \[missing\] provides more on pairs and lists.

Pairs are immutable \(contrary to Lisp tradition\), and `pair?` and
`list?` recognize immutable pairs and lists, only. The `mcons` procedure
creates a _mutable pair_, which works with `set-mcar!` and `set-mcdr!`,
as well as `mcar` and `mcdr`. A mutable pair prints using `mcons`, while
`write` and `display` print mutable pairs with `{` and `}`:

Examples:

```racket
> (define p (mcons 1 2))
> p                     
(mcons 1 2)             
> (pair? p)             
#f                      
> (mpair? p)            
#t                      
> (set-mcar! p 0)       
> p                     
(mcons 0 2)             
> (write p)             
{0 . 2}                 
```

> +\[missing\] in \[missing\] provides more on mutable pairs.

### 3.9. Vectors

A _vector_ is a fixed-length array of arbitrary values. Unlike a list, a
vector supports constant-time access and update of its elements.

A vector prints similar to a list—as a parenthesized sequence of its
elements—but a vector is prefixed with `#` after `'`, or it uses
`vector` if one of its elements cannot be expressed with `quote`.

For a vector as an expression, an optional length can be supplied. Also,
a vector as an expression implicitly `quote`s the forms for its content,
which means that identifiers and parenthesized forms in a vector
constant represent symbols and lists.

> +\[missing\] in \[missing\] documents the fine points of the syntax of
> vectors.

Examples:

```racket
> #("a" "b" "c")                    
'#("a" "b" "c")                     
> #(name (that tune))               
'#(name (that tune))                
> #4(baldwin bruce)                 
'#(baldwin bruce bruce bruce)       
> (vector-ref #("a" "b" "c") 1)     
"b"                                 
> (vector-ref #(name (that tune)) 1)
'(that tune)                        
```

Like strings, a vector is either mutable or immutable, and vectors
written directly as expressions are immutable.

Vectors can be converted to lists and vice versa via `vector->list` and
`list->vector`; such conversions are particularly useful in combination
with predefined procedures on lists. When allocating extra lists seems
too expensive, consider using looping forms like `for/fold`, which
recognize vectors as well as lists.

Example:

```racket
> (list->vector (map string-titlecase                          
                     (vector->list #("three" "blind" "mice"))))
'#("Three" "Blind" "Mice")                                     
```

> +\[missing\] in \[missing\] provides more on vectors and vector
> procedures.

### 3.10. Hash Tables

A _hash table_ implements a mapping from keys to values, where both keys
and values can be arbitrary Racket values, and access and update to the
table are normally constant-time operations. Keys are compared using
`equal?`, `eqv?`, or `eq?`, depending on whether the hash table is
created with `make-hash`, `make-hasheqv`, or `make-hasheq`.

Examples:

```racket
> (define ht (make-hash))               
> (hash-set! ht "apple" '(red round))   
> (hash-set! ht "banana" '(yellow long))
> (hash-ref ht "apple")                 
'(red round)                            
> (hash-ref ht "coconut")               
hash-ref: no value found for key        
  key: "coconut"                        
> (hash-ref ht "coconut" "not there")   
"not there"                             
```

The `hash`, `hasheqv`, and `hasheq` functions create immutable hash
tables from an initial set of keys and values, in which each value is
provided as an argument after its key. Immutable hash tables can be
extended with `hash-set`, which produces a new immutable hash table in
constant time.

Examples:

```racket
> (define ht (hash "apple" 'red "banana" 'yellow))
> (hash-ref ht "apple")                           
'red                                              
> (define ht2 (hash-set ht "coconut" 'brown))     
> (hash-ref ht "coconut")                         
hash-ref: no value found for key                  
  key: "coconut"                                  
> (hash-ref ht2 "coconut")                        
'brown                                            
```

A literal immutable hash table can be written as an expression by using
`#hash` \(for an `equal?`-based table\), `#hasheqv` \(for an
`eqv?`-based table\), or `#hasheq` \(for an `eq?`-based table\). A
parenthesized sequence must immediately follow `#hash`, `#hasheq`, or
`#hasheqv`, where each element is a dotted key–value pair. The `#hash`,
etc. forms implicitly `quote` their key and value sub-forms.

Examples:

```racket
> (define ht #hash(("apple" . red)      
                   ("banana" . yellow)))
> (hash-ref ht "apple")                 
'red                                    
```

> +\[missing\] in \[missing\] documents the fine points of the syntax of
> hash table literals.

Both mutable and immutable hash tables print like immutable hash tables,
using a quoted `#hash`, `#hasheqv`, or `#hasheq` form if all keys and
values can be expressed with `quote` or using `hash`, `hasheq`, or
`hasheqv` otherwise:

Examples:

```racket
> #hash(("apple" . red)                     
        ("banana" . yellow))                
'#hash(("banana" . yellow) ("apple" . red)) 
> (hash 1 (srcloc "file.rkt" 1 0 1 (+ 4 4)))
(hash 1 (srcloc "file.rkt" 1 0 1 8))        
```

A mutable hash table can optionally retain its keys _weakly_, so each
mapping is retained only so long as the key is retained elsewhere.

Examples:

```racket
> (define ht (make-weak-hasheq))           
> (hash-set! ht (gensym) "can you see me?")
> (collect-garbage)                        
> (hash-count ht)                          
0                                          
```

Beware that even a weak hash table retains its values strongly, as long
as the corresponding key is accessible. This creates a catch-22
dependency when a value refers back to its key, so that the mapping is
retained permanently. To break the cycle, map the key to an _ephemeron_
that pairs the value with its key \(in addition to the implicit pairing
of the hash table\).

> +\[missing\] in \[missing\] documents the fine points of using
> ephemerons.

Examples:

```racket
> (define ht (make-weak-hasheq))
> (let ([g (gensym)])           
    (hash-set! ht g (list g)))  
> (collect-garbage)             
> (hash-count ht)               
1                               
```

```racket
> (define ht (make-weak-hasheq))                 
> (let ([g (gensym)])                            
    (hash-set! ht g (make-ephemeron g (list g))))
> (collect-garbage)                              
> (hash-count ht)                                
0                                                
```

> +\[missing\] in \[missing\] provides more on hash tables and hash-table
> procedures.

### 3.11. Boxes

A _box_ is like a single-element vector. It can print as a quoted `#&`
followed by the printed form of the boxed value. A `#&` form can also be
used as an expression, but since the resulting box is constant, it has
practically no use.

Examples:

```racket
> (define b (box "apple"))   
> b                          
'#&"apple"                   
> (unbox b)                  
"apple"                      
> (set-box! b '(banana boat))
> b                          
'#&(banana boat)             
```

> +\[missing\] in \[missing\] provides more on boxes and box procedures.

### 3.12. Void and Undefined

Some procedures or expression forms have no need for a result value. For
example, the `display` procedure is called only for the side-effect of
writing output. In such cases the result value is normally a special
constant that prints as `#<void>`.  When the result of an expression is
simply `#<void>`, the REPL does not print anything.

The `void` procedure takes any number of arguments and returns
`#<void>`. \(That is, the identifier `void` is bound to a procedure that
returns `#<void>`, instead of being bound directly to `#<void>`.\)

Examples:

```racket
> (void)       
> (void 1 2 3) 
> (list (void))
'(#<void>)     
```

The `undefined` constant, which prints as `#<undefined>`, is sometimes
used as the result of a reference whose value is not yet available. In
previous versions of Racket \(before version 6.1\), referencing a local
binding too early produced `#<undefined>`; too-early references now
raise an exception, instead.

> The `undefined` result can still be produced in some cases by the
> `shared` form.

```racket                        
(define (fails)                  
  (define x x)                   
  x)                             
```                              
                                 
```racket                        
> (fails)                        
x: undefined;                    
 cannot use before initialization
```                              

## 4. Expressions and Definitions

The Racket Essentials chapter introduced some of Racket’s syntactic
forms: definitions, procedure applications, conditionals, and so on.
This section provides more details on those forms, plus a few additional
basic forms.

    4.1 Notation                                         
    4.2 Identifiers and Binding                          
    4.3 Function Calls \(Procedure Applications\)        
      4.3.1 Evaluation Order and Arity                   
      4.3.2 Keyword Arguments                            
      4.3.3 The `apply` Function                         
    4.4 Functions \(Procedures\): `lambda`               
      4.4.1 Declaring a Rest Argument                    
      4.4.2 Declaring Optional Arguments                 
      4.4.3 Declaring Keyword Arguments                  
      4.4.4 Arity-Sensitive Functions: `case-lambda`     
    4.5 Definitions: `define`                            
      4.5.1 Function Shorthand                           
      4.5.2 Curried Function Shorthand                   
      4.5.3 Multiple Values and `define-values`          
      4.5.4 Internal Definitions                         
    4.6 Local Binding                                    
      4.6.1 Parallel Binding: `let`                      
      4.6.2 Sequential Binding: `let*`                   
      4.6.3 Recursive Binding: `letrec`                  
      4.6.4 Named `let`                                  
      4.6.5 Multiple Values: `let-values`, `let*-values`,
`letrec-values`                                          
    4.7 Conditionals                                     
      4.7.1 Simple Branching: `if`                       
      4.7.2 Combining Tests: `and` and `or`              
      4.7.3 Chaining Tests: `cond`                       
    4.8 Sequencing                                       
      4.8.1 Effects Before: `begin`                      
      4.8.2 Effects After: `begin0`                      
      4.8.3 Effects If...: `when` and `unless`           
    4.9 Assignment: `set!`                               
      4.9.1 Guidelines for Using Assignment              
      4.9.2 Multiple Values: `set!-values`               
    4.10 Quoting: `quote` and `'`                        
    4.11 Quasiquoting: `quasiquote` and `‘`              
    4.12 Simple Dispatch: `case`                         
    4.13 Dynamic Binding: `parameterize`                 

### 4.1. Notation

This chapter \(and the rest of the documentation\) uses a slightly
different notation than the character-based grammars of the Racket
Essentials chapter. The grammar for a use of a syntactic form
`something` is shown like this:

```racket
(something [id ...+] an-expr ...)
```

The italicized meta-variables in this specification, such as `id` and
`an-expr`, use the syntax of Racket identifiers, so `an-expr` is one
meta-variable. A naming convention implicitly defines the meaning of
many meta-variables:

* A meta-variable that ends in `id` stands for an identifier, such as
  `x` or `my-favorite-martian`.

* A meta-identifier that ends in `keyword` stands for a keyword, such as
  `#:tag`.

* A meta-identifier that ends with `expr` stands for any sub-form, and
  it will be parsed as an expression.

* A meta-identifier that ends with `body` stands for any sub-form; it
  will be parsed as either a local definition or an expression. A `body`
  can parse as a definition only if it is not preceded by any
  expression, and the last `body` must be an expression; see also
  Internal Definitions.

Square brackets in the grammar indicate a parenthesized sequence of
forms, where square brackets are normally used \(by convention\). That
is, square brackets _do not_ mean optional parts of the syntactic form.

A `...` indicates zero or more repetitions of the preceding form, and
`...+` indicates one or more repetitions of the preceding datum.
Otherwise, non-italicized identifiers stand for themselves.

Based on the above grammar, then, here are a few conforming uses of
`something`:

```racket
(something [x])                                 
(something [x] (+ 1 2))                         
(something [x my-favorite-martian x] (+ 1 2) #f)
```

Some syntactic-form specifications refer to meta-variables that are not
implicitly defined and not previously defined. Such meta-variables are
defined after the main form, using a BNF-like format for alternatives:

```racket
(something-else [thing ...+] an-expr ...)
                                         
thing = thing-id                         
      | thing-keyword                    
```

The above example says that, within a `something-else` form, a `thing`
is either an identifier or a keyword.

### 4.2. Identifiers and Binding

The context of an expression determines the meaning of identifiers that
appear in the expression. In particular, starting a module with the
language `racket`, as in

`#lang` `racket`

means that, within the module, the identifiers described in this guide
start with the meaning described here: `cons` refers to the function
that creates a pair, `car` refers to the function that extracts the
first element of a pair, and so on.

> +Symbols introduces the syntax of identifiers.

Forms like `define`, `lambda`, and `let` associate a meaning with one or
more identifiers; that is, they _bind_ identifiers. The part of the
program for which the binding applies is the _scope_ of the binding. The
set of bindings in effect for a given expression is the expression’s
_environment_.

For example, in

```racket
#lang racket    
                
(define f       
  (lambda (x)   
    (let ([y 5])
      (+ x y))))
                
(f 10)          
```

the `define` is a binding of `f`, the `lambda` has a binding for `x`,
and the `let` has a binding for `y`. The scope of the binding for `f` is
the entire module; the scope of the `x` binding is `(let ([y 5]) (+ x
y))`; and the scope of the `y` binding is just `(+ x y)`. The
environment of `(+ x y)` includes bindings for `y`, `x`, and `f`, as
well as everything in `racket`.

A module-level `define` can bind only identifiers that are not already
defined or `require`d into the module. A local `define` or other binding
forms, however, can give a new local binding for an identifier that
already has a binding; such a binding _shadows_ the existing binding.

Examples:

```racket
(define f                                    
  (lambda (append)                           
    (define cons (append "ugly" "confusing"))
    (let ([append 'this-was])                
      (list append cons))))                  
                                             
> (f list)                                   
'(this-was ("ugly" "confusing"))             
```

Similarly, a module-level `define` can shadow a binding from the
module’s language. For example, `(define cons 1)` in a `racket` module
shadows the `cons` that is provided by `racket`. Intentionally shadowing
a language binding is rarely a good idea—especially for widely used
bindings like `cons`—but shadowing relieves a programmer from having to
avoid every obscure binding that is provided by a language.

Even identifiers like `define` and `lambda` get their meanings from
bindings, though they have _transformer_ bindings \(which means that
they indicate syntactic forms\) instead of value bindings. Since
`define` has a transformer binding, the identifier `define` cannot be
used by itself to get a value. However, the normal binding for `define`
can be shadowed.

Examples:

```racket
> define                    
eval:1:0: define: bad syntax
  in: define                
> (let ([define 5]) define) 
5                           
```

Again, shadowing standard bindings in this way is rarely a good idea,
but the possibility is an inherent part of Racket’s flexibility.

### 4.3. Function Calls \(Procedure Applications\)

An expression of the form

```racket
(proc-expr arg-expr ...)
```

is a function call—also known as a _procedure application_—when
`proc-expr` is not an identifier that is bound as a syntax transformer
\(such as `if` or `define`\).

#### 4.3.1. Evaluation Order and Arity

A function call is evaluated by first evaluating the `proc-expr` and all
`arg-expr`s in order \(left to right\). Then, if `proc-expr` produces a
function that accepts as many arguments as supplied `arg-expr`s, the
function is called. Otherwise, an exception is raised.

Examples:

```racket
> (cons 1 null)                                           
'(1)                                                      
> (+ 1 2 3)                                               
6                                                         
> (cons 1 2 3)                                            
cons: arity mismatch;                                     
 the expected number of arguments does not match the given
number                                                    
  expected: 2                                             
  given: 3                                                
  arguments...:                                           
   1                                                      
   2                                                      
   3                                                      
> (1 2 3)                                                 
application: not a procedure;                             
 expected a procedure that can be applied to arguments    
  given: 1                                                
  arguments...:                                           
   2                                                      
   3                                                      
```

Some functions, such as `cons`, accept a fixed number of arguments. Some
functions, such as `+` or `list`, accept any number of arguments. Some
functions accept a range of argument counts; for example `substring`
accepts either two or three arguments. A function’s _arity_ is the
number of arguments that it accepts.

#### 4.3.2. Keyword Arguments

Some functions accept _keyword arguments_ in addition to by-position
arguments. For that case, an `arg` can be an `arg-keyword arg-expr`
sequence instead of just a `arg-expr`:

> +Keywords introduces keywords.

```racket
(proc-expr arg ...)       
                          
arg = arg-expr            
    | arg-keyword arg-expr
```

For example,

`(go` `"super.rkt"` `#:mode` `'fast)`

calls the function bound to `go` with `"super.rkt"` as a by-position
argument, and with `'fast` as an argument associated with the `#:mode`
keyword. A keyword is implicitly paired with the expression that follows
it.

Since a keyword by itself is not an expression, then

`(go` `"super.rkt"` `#:mode` `#:fast)`

is a syntax error. The `#:mode` keyword must be followed by an
expression to produce an argument value, and `#:fast` is not an
expression.

The order of keyword `arg`s determines the order in which `arg-expr`s
are evaluated, but a function accepts keyword arguments independent of
their position in the argument list. The above call to `go` can be
equivalently written

`(go` `#:mode` `'fast` `"super.rkt")`

> +\[missing\] in \[missing\] provides more on procedure applications.

#### 4.3.3. The `apply` Function

The syntax for function calls supports any number of arguments, but a
specific call always specifies a fixed number of arguments. As a result,
a function that takes a list of arguments cannot directly apply a
function like `+` to all of the items in a list:

```racket                          
(define (avg lst) ; doesn’t work...
  (/ (+ lst) (length lst)))        
```                                
                                   
```racket                          
> (avg '(1 2 3))                   
+: contract violation              
  expected: number?                
  given: '(1 2 3)                  
```                                

```racket                                                  
(define (avg lst) ; doesn’t always work...                 
  (/ (+ (list-ref lst 0) (list-ref lst 1) (list-ref lst 2))
     (length lst)))                                        
```                                                        
                                                           
```racket                                                  
> (avg '(1 2 3))                                           
2                                                          
> (avg '(1 2))                                             
list-ref: index too large for list                         
  index: 2                                                 
  in: '(1 2)                                               
```                                                        

The `apply` function offers a way around this restriction. It takes a
function and a _list_ argument, and it applies the function to the
values in the list:

```racket                        
(define (avg lst)                
  (/ (apply + lst) (length lst)))
```                              
                                 
```racket                        
> (avg '(1 2 3))                 
2                                
> (avg '(1 2))                   
3/2                              
> (avg '(1 2 3 4))               
5/2                              
```                              

As a convenience, the `apply` function accepts additional arguments
between the function and the list. The additional arguments are
effectively `cons`ed onto the argument list:

```racket             
(define (anti-sum lst)
  (apply - 0 lst))    
```                   
                      
```racket             
> (anti-sum '(1 2 3)) 
-6                    
```                   

The `apply` function accepts keyword arguments, too, and it passes them
along to the called function:

```racket
(apply go #:mode 'fast '("super.rkt"))
(apply go '("super.rkt") #:mode 'fast)
```

Keywords that are included in `apply`’s list argument do not count as
keyword arguments for the called function; instead, all arguments in
this list are treated as by-position arguments. To pass a list of
keyword arguments to a function, use the `keyword-apply` function, which
accepts a function to apply and three lists. The first two lists are in
parallel, where the first list contains keywords \(sorted by
`keyword<?`\), and the second list contains a corresponding argument for
each keyword. The third list contains by-position function arguments, as
for `apply`.

```racket
(keyword-apply go             
               '(#:mode)      
               '(fast)        
               '("super.rkt"))
```

### 4.4. Functions \(Procedures\): `lambda`

A `lambda` expression creates a function. In the simplest case, a
`lambda` expression has the form

```racket
(lambda (arg-id ...)
  body ...+)        
```

A `lambda` form with _n_ `arg-id`s accepts _n_ arguments:

```racket
> ((lambda (x) x)                                         
   1)                                                     
1                                                         
> ((lambda (x y) (+ x y))                                 
   1 2)                                                   
3                                                         
> ((lambda (x y) (+ x y))                                 
   1)                                                     
#<procedure>: arity mismatch;                             
 the expected number of arguments does not match the given
number                                                    
  expected: 2                                             
  given: 1                                                
  arguments...:                                           
   1                                                      
```

#### 4.4.1. Declaring a Rest Argument

A `lambda` expression can also have the form

```racket
(lambda rest-id
  body ...+)   
```

That is, a `lambda` expression can have a single `rest-id` that is not
surrounded by parentheses. The resulting function accepts any number of
arguments, and the arguments are put into a list bound to `rest-id`.

Examples:

```racket
> ((lambda x x)      
   1 2 3)            
'(1 2 3)             
> ((lambda x x))     
'()                  
> ((lambda x (car x))
   1 2 3)            
1                    
```

Functions with a `rest-id` often use `apply` to call another function
that accepts any number of arguments.

> +The `apply` Function describes `apply`.

Examples:

```racket
(define max-mag                       
  (lambda nums                        
    (apply max (map magnitude nums))))
                                      
> (max 1 -2 0)                        
1                                     
> (max-mag 1 -2 0)                    
2                                     
```

The `lambda` form also supports required arguments combined with a
`rest-id`:

```racket
(lambda (arg-id ...+ . rest-id)
  body ...+)                   
```

The result of this form is a function that requires at least as many
arguments as `arg-id`s, and also accepts any number of additional
arguments.

Examples:

```racket
(define max-mag                                           
  (lambda (num . nums)                                    
    (apply max (map magnitude (cons num nums)))))         
                                                          
> (max-mag 1 -2 0)                                        
2                                                         
> (max-mag)                                               
max-mag: arity mismatch;                                  
 the expected number of arguments does not match the given
number                                                    
  expected: at least 1                                    
  given: 0                                                
```

A `rest-id` variable is sometimes called a _rest argument_, because it
accepts the “rest” of the function arguments.

#### 4.4.2. Declaring Optional Arguments

Instead of just an identifier, an argument \(other than a rest
argument\) in a `lambda` form can be specified with an identifier and a
default value:

```racket
(lambda gen-formals                
  body ...+)                       
                                   
gen-formals = (arg ...)            
            | rest-id              
            | (arg ...+ . rest-id) 
                                   
arg         = arg-id               
            | [arg-id default-expr]
```

An argument of the form `[arg-id default-expr]` is optional. When the
argument is not supplied in an application, `default-expr` produces the
default value. The `default-expr` can refer to any preceding `arg-id`,
and every following `arg-id` must have a default as well.

Examples:

```racket
(define greet                                    
  (lambda (given [surname "Smith"])              
    (string-append "Hello, " given " " surname)))
                                                 
> (greet "John")                                 
"Hello, John Smith"                              
> (greet "John" "Doe")                           
"Hello, John Doe"                                
```

```racket                                          
(define greet                                      
  (lambda (given [surname (if (equal? given "John")
                              "Doe"                
                              "Smith")])           
    (string-append "Hello, " given " " surname)))  
```                                                
                                                   
```racket                                          
> (greet "John")                                   
"Hello, John Doe"                                  
> (greet "Adam")                                   
"Hello, Adam Smith"                                
```                                                

#### 4.4.3. Declaring Keyword Arguments

A `lambda` form can declare an argument to be passed by keyword, instead
of position. Keyword arguments can be mixed with by-position arguments,
and default-value expressions can be supplied for either kind of
argument:

> +Keyword Arguments introduces function calls with keywords.

```racket
(lambda gen-formals                            
  body ...+)                                   
                                               
gen-formals = (arg ...)                        
            | rest-id                          
            | (arg ...+ . rest-id)             
                                               
arg         = arg-id                           
            | [arg-id default-expr]            
            | arg-keyword arg-id               
            | arg-keyword [arg-id default-expr]
```

An argument specified as `arg-keyword arg-id` is supplied by an
application using the same `arg-keyword`.  The position of the
keyword–identifier pair in the argument list does not matter for
matching with arguments in an application, because it will be matched to
an argument value by keyword instead of by position.

```racket                                        
(define greet                                    
  (lambda (given #:last surname)                 
    (string-append "Hello, " given " " surname)))
```                                              
                                                 
```racket                                        
> (greet "John" #:last "Smith")                  
"Hello, John Smith"                              
> (greet #:last "Doe" "John")                    
"Hello, John Doe"                                
```                                              

An `arg-keyword [arg-id default-expr]` argument specifies a
keyword-based argument with a default value.

Examples:

```racket
(define greet                                               
  (lambda (#:hi [hi "Hello"] given #:last [surname "Smith"])
    (string-append hi ", " given " " surname)))             
                                                            
> (greet "John")                                            
"Hello, John Smith"                                         
> (greet "Karl" #:last "Marx")                              
"Hello, Karl Marx"                                          
> (greet "John" #:hi "Howdy")                               
"Howdy, John Smith"                                         
> (greet "Karl" #:last "Marx" #:hi "Guten Tag")             
"Guten Tag, Karl Marx"                                      
```

The `lambda` form does not directly support the creation of a function
that accepts “rest” keywords. To construct a function that accepts all
keyword arguments, use `make-keyword-procedure`. The function supplied
to `make-keyword-procedure` receives keyword arguments through parallel
lists in the first two \(by-position\) arguments, and then all
by-position arguments from an application as the remaining by-position
arguments.

> +The `apply` Function introduces `keyword-apply`.

Examples:

```racket
(define (trace-wrap f)                                 
  (make-keyword-procedure                              
   (lambda (kws kw-args . rest)                        
     (printf "Called with ~s ~s ~s\n" kws kw-args rest)
     (keyword-apply f kws kw-args rest))))             
                                                       
> ((trace-wrap greet) "John" #:hi "Howdy")             
Called with (#:hi) ("Howdy") ("John")                  
"Howdy, John Smith"                                    
```

> +\[missing\] in \[missing\] provides more on function expressions.

#### 4.4.4. Arity-Sensitive Functions: `case-lambda`

The `case-lambda` form creates a function that can have completely
different behaviors depending on the number of arguments that are
supplied. A case-lambda expression has the form

```racket
(case-lambda                     
  [formals body ...+]            
  ...)                           
                                 
formals = (arg-id ...)           
        | rest-id                
        | (arg-id ...+ . rest-id)
```

where each `[formals body ...+]` is analogous to `(lambda formals body
...+)`. Applying a function produced by `case-lambda` is like applying a
`lambda` for the first case that matches the number of given arguments.

Examples:

```racket
(define greet                                                      
  (case-lambda                                                     
    [(name) (string-append "Hello, " name)]                        
    [(given surname) (string-append "Hello, " given " " surname)]))
                                                                   
> (greet "John")                                                   
"Hello, John"                                                      
> (greet "John" "Smith")                                           
"Hello, John Smith"                                                
> (greet)                                                          
greet: arity mismatch;                                             
 the expected number of arguments does not match the given         
number                                                             
  given: 0                                                         
```

A `case-lambda` function cannot directly support optional or keyword
arguments.

### 4.5. Definitions: `define`

A basic definition has the form

```racket
(define id expr)
```

in which case `id` is bound to the result of `expr`.

Examples:

```racket
(define salutation (list-ref '("Hi" "Hello") (random 2)))
                                                         
> salutation                                             
"Hello"                                                  
```

#### 4.5.1. Function Shorthand

The `define` form also supports a shorthand for function definitions:

```racket
(define (id arg ...) body ...+)
```

which is a shorthand for

`(define` `id` `(lambda` `(arg` `...)` `body` `...+))`

Examples:

```racket
(define (greet name)                   
  (string-append salutation ", " name))
                                       
> (greet "John")                       
"Hello, John"                          
```

```racket                                                   
(define (greet first [surname "Smith"] #:hi [hi salutation])
  (string-append hi ", " first " " surname))                
```                                                         
                                                            
```racket                                                   
> (greet "John")                                            
"Hello, John Smith"                                         
> (greet "John" #:hi "Hey")                                 
"Hey, John Smith"                                           
> (greet "John" "Doe")                                      
"Hello, John Doe"                                           
```                                                         

The function shorthand via `define` also supports a rest argument
\(i.e., a final argument to collect extra arguments in a list\):

```racket
(define (id arg ... . rest-id) body ...+)
```

which is a shorthand

`(define` `id` `(lambda` `(arg` `...` `. rest-id)` `body` `...+))`

Examples:

```racket
(define (avg . l)            
  (/ (apply + l) (length l)))
                             
> (avg 1 2 3)                
2                            
```

#### 4.5.2. Curried Function Shorthand

Consider the following `make-add-suffix` function that takes a string
and returns another function that takes a string:

```racket                              
(define make-add-suffix                
  (lambda (s2)                         
    (lambda (s) (string-append s s2))))
```                                    
                                       
                                       

Although it’s not common, result of `make-add-suffix` could be called
directly, like this:

```racket
> ((make-add-suffix "!") "hello")
"hello!"                         
```

In a sense, `make-add-suffix` is a function takes two arguments, but it
takes them one at a time. A function that takes some of its arguments
and returns a function to consume more is sometimes called a _curried
function_.

Using the function-shorthand form of `define`, `make-add-suffix` can be
written equivalently as

```racket
(define (make-add-suffix s2)        
  (lambda (s) (string-append s s2)))
```

This shorthand reflects the shape of the function call `(make-add-suffix
"!")`. The `define` form further supports a shorthand for defining
curried functions that reflects nested function calls:

```racket                        
(define ((make-add-suffix s2) s) 
  (string-append s s2))          
```                              
                                 
```racket                        
> ((make-add-suffix "!") "hello")
"hello!"                         
```                              

```racket                               
(define louder (make-add-suffix "!"))   
(define less-sure (make-add-suffix "?"))
```                                     
                                        
```racket                               
> (less-sure "really")                  
"really?"                               
> (louder "really")                     
"really!"                               
```                                     

The full syntax of the function shorthand for `define` is as follows:

```racket
(define (head args) body ...+)
                              
head = id                     
     | (head args)            
                              
args = arg ...                
     | arg ... . rest-id      
```

The expansion of this shorthand has one nested `lambda` form for each
`head` in the definition, where the innermost `head` corresponds to the
outermost `lambda`.

#### 4.5.3. Multiple Values and `define-values`

A Racket expression normally produces a single result, but some
expressions can produce multiple results. For example, `quotient` and
`remainder` each produce a single value, but `quotient/remainder`
produces the same two values at once:

```racket
> (quotient 13 3)          
4                          
> (remainder 13 3)         
1                          
> (quotient/remainder 13 3)
4                          
1                          
```

As shown above, the REPL prints each result value on its own line.

Multiple-valued functions can be implemented in terms of the `values`
function, which takes any number of values and returns them as the
results:

```racket
> (values 1 2 3)
1               
2               
3               
```

```racket                                             
(define (split-name name)                             
  (let ([parts (regexp-split " " name)])              
    (if (= (length parts) 2)                          
        (values (list-ref parts 0) (list-ref parts 1))
        (error "not a <first> <last> name"))))        
```                                                   
                                                      
```racket                                             
> (split-name "Adam Smith")                           
"Adam"                                                
"Smith"                                               
```                                                   

The `define-values` form binds multiple identifiers at once to multiple
results produced from a single expression:

```racket
(define-values (id ...) expr)
```

The number of results produced by the `expr` must match the number of
`id`s.

Examples:

```racket
(define-values (given surname) (split-name "Adam Smith"))
                                                         
> given                                                  
"Adam"                                                   
> surname                                                
"Smith"                                                  
```

A `define` form \(that is not a function shorthand\) is equivalent to a
`define-values` form with a single `id`.

> +\[missing\] in \[missing\] provides more on definitions.

#### 4.5.4. Internal Definitions

When the grammar for a syntactic form specifies `body`, then the
corresponding form can be either a definition or an expression. A
definition as a `body` is an _internal definition_.

Expressions and internal definitions in a `body` sequence can be mixed,
as long as the last `body` is an expression.

For example, the syntax of `lambda` is

```racket
(lambda gen-formals
  body ...+)       
```

so the following are valid instances of the grammar:

```racket
(lambda (f)                ; no definitions 
  (printf "running\n")                      
  (f 0))                                    
                                            
(lambda (f)                ; one definition 
  (define (log-it what)                     
    (printf "~a\n" what))                   
  (log-it "running")                        
  (f 0)                                     
  (log-it "done"))                          
                                            
(lambda (f n)              ; two definitions
  (define (call n)                          
    (if (zero? n)                           
        (log-it "done")                     
        (begin                              
          (log-it "running")                
          (f n)                             
          (call (- n 1)))))                 
  (define (log-it what)                     
    (printf "~a\n" what))                   
  (call n))                                 
```

Internal definitions in a particular `body` sequence are mutually
recursive; that is, any definition can refer to any other definition—as
long as the reference isn’t actually evaluated before its definition
takes place. If a definition is referenced too early, an error occurs.

Examples:

```racket
(define (weird)                  
  (define x x)                   
  x)                             
                                 
> (weird)                        
x: undefined;                    
 cannot use before initialization
```

A sequence of internal definitions using just `define` is easily
translated to an equivalent `letrec` form \(as introduced in the next
section\). However, other definition forms can appear as a `body`,
including `define-values`, `struct` \(see Programmer-Defined Datatypes\)
or `define-syntax` \(see Macros\).

> +\[missing\] in \[missing\] documents the fine points of internal
> definitions.

### 4.6. Local Binding

Although internal `define`s can be used for local binding, Racket
provides three forms that give the programmer more control over
bindings: `let`, `let*`, and `letrec`.

#### 4.6.1. Parallel Binding: `let`

> +\[missing\] in \[missing\] also documents `let`.

A `let` form binds a set of identifiers, each to the result of some
expression, for use in the `let` body:

```racket
(let ([id expr] ...) body ...+)
```

The `id`s are bound “in parallel.” That is, no `id` is bound in the
right-hand side `expr` for any `id`, but all are available in the
`body`. The `id`s must be different from each other.

Examples:

```racket
> (let ([me "Bob"])                      
    me)                                  
"Bob"                                    
> (let ([me "Bob"]                       
        [myself "Robert"]                
        [I "Bobby"])                     
    (list me myself I))                  
'("Bob" "Robert" "Bobby")                
> (let ([me "Bob"]                       
        [me "Robert"])                   
    me)                                  
eval:3:0: let: duplicate identifier      
  at: me                                 
  in: (let ((me "Bob") (me "Robert")) me)
```

The fact that an `id`’s `expr` does not see its own binding is often
useful for wrappers that must refer back to the old value:

```racket
> (let ([+ (lambda (x y)                     
             (if (string? x)                 
                 (string-append x y)         
                 (+ x y)))]) ; use original +
    (list (+ 1 2)                            
          (+ "see" "saw")))                  
'(3 "seesaw")                                
```

Occasionally, the parallel nature of `let` bindings is convenient for
swapping or rearranging a set of bindings:

```racket
> (let ([me "Tarzan"]
        [you "Jane"])
    (let ([me you]   
          [you me])  
      (list me you)))
'("Jane" "Tarzan")   
```

The characterization of `let` bindings as “parallel” is not meant to
imply concurrent evaluation. The `expr`s are evaluated in order, even
though the bindings are delayed until all `expr`s are evaluated.

#### 4.6.2. Sequential Binding: `let*`

> +\[missing\] in \[missing\] also documents `let*`.

The syntax of `let*` is the same as `let`:

```racket
(let* ([id expr] ...) body ...+)
```

The difference is that each `id` is available for use in later `expr`s,
as well as in the `body`. Furthermore, the `id`s need not be distinct,
and the most recent binding is the visible one.

Examples:

```racket
> (let* ([x (list "Burroughs")]                                   
         [y (cons "Rice" x)]                                      
         [z (cons "Edgar" y)])                                    
    (list x y z))                                                 
'(("Burroughs") ("Rice" "Burroughs") ("Edgar" "Rice" "Burroughs"))
> (let* ([name (list "Burroughs")]                                
         [name (cons "Rice" name)]                                
         [name (cons "Edgar" name)])                              
    name)                                                         
'("Edgar" "Rice" "Burroughs")                                     
```

In other words, a `let*` form is equivalent to nested `let` forms, each
with a single binding:

```racket
> (let ([name (list "Burroughs")])     
    (let ([name (cons "Rice" name)])   
      (let ([name (cons "Edgar" name)])
        name)))                        
'("Edgar" "Rice" "Burroughs")          
```

#### 4.6.3. Recursive Binding: `letrec`

> +\[missing\] in \[missing\] also documents `letrec`.

The syntax of `letrec` is also the same as `let`:

```racket
(letrec ([id expr] ...) body ...+)
```

While `let` makes its bindings available only in the `body`s, and `let*`
makes its bindings available to any later binding `expr`, `letrec` makes
its bindings available to all other `expr`s—even earlier ones. In other
words, `letrec` bindings are recursive.

The `expr`s in a `letrec` form are most often `lambda` forms for
recursive and mutually recursive functions:

```racket
> (letrec ([swing                               
            (lambda (t)                         
              (if (eq? (car t) 'tarzan)         
                  (cons 'vine                   
                        (cons 'tarzan (cddr t)))
                  (cons (car t)                 
                        (swing (cdr t)))))])    
    (swing '(vine tarzan vine vine)))           
'(vine vine tarzan vine)                        
```

```racket
> (letrec ([tarzan-near-top-of-tree?                                     
            (lambda (name path depth)                                    
              (or (equal? name "tarzan")                                 
                  (and (directory-exists? path)                          
                       (tarzan-in-directory? path depth))))]             
           [tarzan-in-directory?                                         
            (lambda (dir depth)                                          
              (cond                                                      
                [(zero? depth) #f]                                       
                [else                                                    
                 (ormap                                                  
                  (λ (elem)                                              
                    (tarzan-near-top-of-tree? (path-element->string elem)
                                              (build-path dir elem)      
                                              (- depth 1)))              
                  (directory-list dir))]))])                             
    (tarzan-near-top-of-tree? "tmp"                                      
                              (find-system-path 'temp-dir)               
                              4))                                        
#f                                                                       
```

While the `expr`s of a `letrec` form are typically `lambda` expressions,
they can be any expression. The expressions are evaluated in order, and
after each value is obtained, it is immediately associated with its
corresponding `id`. If an `id` is referenced before its value is ready,
an error is raised, just as for internal definitions.

```racket
> (letrec ([quicksand quicksand])
    quicksand)                   
quicksand: undefined;            
 cannot use before initialization
```

#### 4.6.4. Named `let`

A named `let` is an iteration and recursion form. It uses the same
syntactic keyword `let` as for local binding, but an identifier after
the `let` \(instead of an immediate open parenthesis\) triggers a
different parsing.

```racket
(let proc-id ([arg-id init-expr] ...)
  body ...+)                         
```

A named `let` form is equivalent to

```racket
(letrec ([proc-id (lambda (arg-id ...)
                     body ...+)])     
  (proc-id init-expr ...))            
```

That is, a named `let` binds a function identifier that is visible only
in the function’s body, and it implicitly calls the function with the
values of some initial expressions.

Examples:

```racket
(define (duplicate pos lst)                             
  (let dup ([i 0]                                       
            [lst lst])                                  
   (cond                                                
    [(= i pos) (cons (car lst) lst)]                    
    [else (cons (car lst) (dup (+ i 1) (cdr lst)))])))  
                                                        
> (duplicate 1 (list "apple" "cheese burger!" "banana"))
'("apple" "cheese burger!" "cheese burger!" "banana")   
```

#### 4.6.5. Multiple Values: `let-values`, `let*-values`, `letrec-values`

> +\[missing\] in \[missing\] also documents multiple-value binding forms.

In the same way that `define-values` binds multiple results in a
definition \(see Multiple Values and `define-values`\), `let-values`,
`let*-values`, and `letrec-values` bind multiple results locally.

```racket
(let-values ([(id ...) expr] ...)
  body ...+)                     
```

```racket
(let*-values ([(id ...) expr] ...)
  body ...+)                      
```

```racket
(letrec-values ([(id ...) expr] ...)
  body ...+)                        
```

Each `expr` must produce as many values as corresponding `id`s. The
binding rules are the same for the forms without `-values` forms: the
`id`s of `let-values` are bound only in the `body`s, the `id`s of
`let*-values`s are bound in `expr`s of later clauses, and the `id`s of
`letrec-value`s are bound for all `expr`s.

Example:

```racket
> (let-values ([(q r) (quotient/remainder 14 3)])
    (list q r))                                  
'(4 2)                                           
```

### 4.7. Conditionals

Most functions used for branching, such as `<` and `string?`, produce
either `#t` or `#f`. Racket’s branching forms, however, treat any value
other than `#f` as true. We say a _true value_ to mean any value other
than `#f`.

This convention for “true value” meshes well with protocols where `#f`
can serve as failure or to indicate that an optional value is not
supplied. \(Beware of overusing this trick, and remember that an
exception is usually a better mechanism to report failure.\)

For example, the `member` function serves double duty; it can be used to
find the tail of a list that starts with a particular item, or it can be
used to simply check whether an item is present in a list:

```racket
> (member "Groucho" '("Harpo" "Zeppo"))              
#f                                                   
> (member "Groucho" '("Harpo" "Groucho" "Zeppo"))    
'("Groucho" "Zeppo")                                 
> (if (member "Groucho" '("Harpo" "Zeppo"))          
      'yep                                           
      'nope)                                         
'nope                                                
> (if (member "Groucho" '("Harpo" "Groucho" "Zeppo"))
      'yep                                           
      'nope)                                         
'yep                                                 
```

#### 4.7.1. Simple Branching: `if`

> +\[missing\] in \[missing\] also documents `if`.

In an `if` form,

```racket
(if test-expr then-expr else-expr)
```

the `test-expr` is always evaluated. If it produces any value other than
`#f`, then `then-expr` is evaluated. Otherwise, `else-expr` is
evaluated.

An `if` form must have both a `then-expr` and an `else-expr`; the latter
is not optional. To perform \(or skip\) side-effects based on a
`test-expr`, use `when` or `unless`, which we describe later in
Sequencing.

#### 4.7.2. Combining Tests: `and` and `or`

> +\[missing\] in \[missing\] also documents `and` and `or`.

Racket’s `and` and `or` are syntactic forms, rather than functions.
Unlike a function, the `and` and `or` forms can skip evaluation of later
expressions if an earlier one determines the answer.

```racket
(and expr ...)
```

An `and` form produces `#f` if any of its `expr`s produces `#f`.
Otherwise, it produces the value of its last `expr`. As a special case,
`(and)` produces `#t`.

```racket
(or expr ...)
```

The `or` form produces `#f` if all of its `expr`s produce `#f`.
Otherwise, it produces the first non-`#f` value from its `expr`s.  As a
special case, `(or)` produces `#f`.

Examples:

```racket
> (define (got-milk? lst)                                    
    (and (not (null? lst))                                   
         (or (eq? 'milk (car lst))                           
             (got-milk? (cdr lst))))) ; recurs only if needed
> (got-milk? '(apple banana))                                
#f                                                           
> (got-milk? '(apple milk banana))                           
#t                                                           
```

If evaluation reaches the last `expr` of an `and` or `or` form, then the
`expr`’s value directly determines the `and` or `or` result. Therefore,
the last `expr` is in tail position, which means that the above
`got-milk?` function runs in constant space.

> +Tail Recursion introduces tail calls and tail positions.

#### 4.7.3. Chaining Tests: `cond`

The `cond` form chains a series of tests to select a result expression.
To a first approximation, the syntax of `cond` is as follows:

> +\[missing\] in \[missing\] also documents `cond`.

```racket
(cond [test-expr body ...+]
      ...)                 
```

Each `test-expr` is evaluated in order. If it produces `#f`, the
corresponding `body`s are ignored, and evaluation proceeds to the next
`test-expr`. As soon as a `test-expr` produces a true value, its `body`s
are evaluated to produce the result for the `cond` form, and no further
`test-expr`s are evaluated.

The last `test-expr` in a `cond` can be replaced by `else`. In terms of
evaluation, `else` serves as a synonym for `#t`, but it clarifies that
the last clause is meant to catch all remaining cases. If `else` is not
used, then it is possible that no `test-expr`s produce a true value; in
that case, the result of the `cond` expression is `#<void>`.

Examples:

```racket
> (cond                       
   [(= 2 3) (error "wrong!")] 
   [(= 2 2) 'ok])             
'ok                           
> (cond                       
   [(= 2 3) (error "wrong!")])
> (cond                       
   [(= 2 3) (error "wrong!")] 
   [else 'ok])                
'ok                           
```

```racket                         
(define (got-milk? lst)           
  (cond                           
    [(null? lst) #f]              
    [(eq? 'milk (car lst)) #t]    
    [else (got-milk? (cdr lst))]))
```                               
                                  
```racket                         
> (got-milk? '(apple banana))     
#f                                
> (got-milk? '(apple milk banana))
#t                                
```                               

The full syntax of `cond` includes two more kinds of clauses:

```racket
(cond cond-clause ...)                  
                                        
cond-clause = [test-expr then-body ...+]
            | [else then-body ...+]     
            | [test-expr => proc-expr]  
            | [test-expr]               
```

The `=>` variant captures the true result of its `test-expr` and passes
it to the result of the `proc-expr`, which must be a function of one
argument.

Examples:

```racket
> (define (after-groucho lst)                 
    (cond                                     
      [(member "Groucho" lst) => cdr]         
      [else (error "not there")]))            
> (after-groucho '("Harpo" "Groucho" "Zeppo"))
'("Zeppo")                                    
> (after-groucho '("Harpo" "Zeppo"))          
not there                                     
```

A clause that includes only a `test-expr` is rarely used. It captures
the true result of the `test-expr`, and simply returns the result for
the whole `cond` expression.

### 4.8. Sequencing

Racket programmers prefer to write programs with as few side-effects as
possible, since purely functional code is more easily tested and
composed into larger programs. Interaction with the external
environment, however, requires sequencing, such as when writing to a
display, opening a graphical window, or manipulating a file on disk.

#### 4.8.1. Effects Before: `begin`

> +\[missing\] in \[missing\] also documents `begin`.

A `begin` expression sequences expressions:

```racket
(begin expr ...+)
```

The `expr`s are evaluated in order, and the result of all but the last
`expr` is ignored. The result from the last `expr` is the result of the
`begin` form, and it is in tail position with respect to the `begin`
form.

Examples:

```racket
(define (print-triangle height)           
  (if (zero? height)                      
      (void)                              
      (begin                              
        (display (make-string height #\*))
        (newline)                         
        (print-triangle (sub1 height))))) 
                                          
> (print-triangle 4)                      
****                                      
***                                       
**                                        
*                                         
```

Many forms, such as `lambda` or `cond` support a sequence of expressions
even without a `begin`. Such positions are sometimes said to have an
_implicit begin_.

Examples:

```racket
(define (print-triangle height)        
  (cond                                
    [(positive? height)                
     (display (make-string height #\*))
     (newline)                         
     (print-triangle (sub1 height))])) 
                                       
> (print-triangle 4)                   
****                                   
***                                    
**                                     
*                                      
```

The `begin` form is special at the top level, at module level, or as a
`body` after only internal definitions. In those positions, instead of
forming an expression, the content of `begin` is spliced into the
surrounding context.

Example:

```racket
> (let ([curly 0])             
    (begin                     
      (define moe (+ 1 curly)) 
      (define larry (+ 1 moe)))
    (list larry curly moe))    
'(2 0 1)                       
```

This splicing behavior is mainly useful for macros, as we discuss later
in Macros.

#### 4.8.2. Effects After: `begin0`

> +\[missing\] in \[missing\] also documents `begin0`.

A `begin0` expression has the same syntax as a `begin` expression:

```racket
(begin0 expr ...+)
```

The difference is that `begin0` returns the result of the first `expr`,
instead of the result of the last `expr`. The `begin0` form is useful
for implementing side-effects that happen after a computation,
especially in the case where the computation produces an unknown number
of results.

Examples:

```racket
(define (log-times thunk)                                  
  (printf "Start: ~s\n" (current-inexact-milliseconds))    
  (begin0                                                  
    (thunk)                                                
    (printf "End..: ~s\n" (current-inexact-milliseconds))))
                                                           
> (log-times (lambda () (sleep 0.1) 0))                    
Start: 1548117375450.803                                   
End..: 1548117375553.018                                   
0                                                          
> (log-times (lambda () (values 1 2)))                     
Start: 1548117375553.624                                   
End..: 1548117375553.65                                    
1                                                          
2                                                          
```

#### 4.8.3. Effects If...: `when` and `unless`

> +\[missing\] in \[missing\] also documents `when` and `unless`.

The `when` form combines an `if`-style conditional with sequencing for
the “then” clause and no “else” clause:

```racket
(when test-expr then-body ...+)
```

If `test-expr` produces a true value, then all of the `then-body`s are
evaluated. The result of the last `then-body` is the result of the
`when` form. Otherwise, no `then-body`s are evaluated and the result is
`#<void>`.

The `unless` form is similar:

```racket
(unless test-expr then-body ...+)
```

The difference is that the `test-expr` result is inverted: the
`then-body`s are evaluated only if the `test-expr` result is `#f`.

Examples:

```racket
(define (enumerate lst)               
  (if (null? (cdr lst))               
      (printf "~a.\n" (car lst))      
      (begin                          
        (printf "~a, " (car lst))     
        (when (null? (cdr (cdr lst))) 
          (printf "and "))            
        (enumerate (cdr lst)))))      
                                      
> (enumerate '("Larry" "Curly" "Moe"))
Larry, Curly, and Moe.                
```

```racket                             
(define (print-triangle height)       
  (unless (zero? height)              
    (display (make-string height #\*))
    (newline)                         
    (print-triangle (sub1 height))))  
```                                   
                                      
```racket                             
> (print-triangle 4)                  
****                                  
***                                   
**                                    
*                                     
```                                   

### 4.9. Assignment: `set!`

> +\[missing\] in \[missing\] also documents `set!`.

Assign to a variable using `set!`:

```racket
(set! id expr)
```

A `set!` expression evaluates `expr` and changes `id` \(which must be
bound in the enclosing environment\) to the resulting value. The result
of the `set!`  expression itself is `#<void>`.

Examples:

```racket
(define greeted null)               
                                    
(define (greet name)                
  (set! greeted (cons name greeted))
  (string-append "Hello, " name))   
                                    
> (greet "Athos")                   
"Hello, Athos"                      
> (greet "Porthos")                 
"Hello, Porthos"                    
> (greet "Aramis")                  
"Hello, Aramis"                     
> greeted                           
'("Aramis" "Porthos" "Athos")       
```

```racket                         
(define (make-running-total)      
  (let ([n 0])                    
    (lambda ()                    
      (set! n (+ n 1))            
      n)))                        
(define win (make-running-total)) 
(define lose (make-running-total))
```                               
                                  
```racket                         
> (win)                           
1                                 
> (win)                           
2                                 
> (lose)                          
1                                 
> (win)                           
3                                 
```                               

#### 4.9.1. Guidelines for Using Assignment

Although using `set!` is sometimes appropriate, Racket style generally
discourages the use of `set!`. The following guidelines may help explain
when using `set!` is appropriate.

* As in any modern language, assigning to a shared identifier is no
  substitute for passing an argument to a procedure or getting  its
  result.

  **_Really awful_** example:

  ```racket                                      
  (define name "unknown")                        
  (define result "unknown")                      
  (define (greet)                                
    (set! result (string-append "Hello, " name)))
  ```                                            
                                                 
  ```racket                                      
  > (set! name "John")                           
  > (greet)                                      
  > result                                       
  "Hello, John"                                  
  ```                                            

  Ok example:

  ```racket                        
  (define (greet name)             
    (string-append "Hello, " name))
  ```                              
                                   
  ```racket                        
  > (greet "John")                 
  "Hello, John"                    
  > (greet "Anna")                 
  "Hello, Anna"                    
  ```                              

* A sequence of assignments to a local variable is far inferior to
  nested bindings.

  **Bad** example:

  ```racket
> (let ([tree 0])                           
      (set! tree (list tree 1 tree))          
      (set! tree (list tree 2 tree))          
      (set! tree (list tree 3 tree))          
      tree)                                   
  '(((0 1 0) 2 (0 1 0)) 3 ((0 1 0) 2 (0 1 0)))
```

  Ok example:

  ```racket
> (let* ([tree 0]                           
           [tree (list tree 1 tree)]          
           [tree (list tree 2 tree)]          
           [tree (list tree 3 tree)])         
      tree)                                   
  '(((0 1 0) 2 (0 1 0)) 3 ((0 1 0) 2 (0 1 0)))
```

* Using assignment to accumulate results from an iteration is bad style.
  Accumulating through a loop argument is better.

  Somewhat bad example:

  ```racket                                  
  (define (sum lst)                          
    (let ([s 0])                             
      (for-each (lambda (i) (set! s (+ i s)))
                lst)                         
      s))                                    
  ```                                        
                                             
  ```racket                                  
  > (sum '(1 2 3))                           
  6                                          
  ```                                        

  Ok example:

  ```racket                                  
  (define (sum lst)                          
    (let loop ([lst lst] [s 0])              
      (if (null? lst)                        
          s                                  
          (loop (cdr lst) (+ s (car lst))))))
  ```                                        
                                             
  ```racket                                  
  > (sum '(1 2 3))                           
  6                                          
  ```                                        

  Better \(use an existing function\) example:

  ```racket        
  (define (sum lst)
    (apply + lst)) 
  ```              
                   
  ```racket        
  > (sum '(1 2 3)) 
  6                
  ```              

  Good \(a general approach\) example:

  ```racket                      
  (define (sum lst)              
    (for/fold ([s 0])            
              ([i (in-list lst)])
      (+ s i)))                  
  ```                            
                                 
  ```racket                      
  > (sum '(1 2 3))               
  6                              
  ```                            

* For cases where stateful objects are necessary or appropriate, then
  implementing the object’s state with `set!` is fine.

  Ok example:

  ```racket              
  (define next-number!   
    (let ([n 0])         
      (lambda ()         
        (set! n (add1 n))
        n)))             
  ```                    
                         
  ```racket              
  > (next-number!)       
  1                      
  > (next-number!)       
  2                      
  > (next-number!)       
  3                      
  ```                    

All else being equal, a program that uses no assignments or mutation is
always preferable to one that uses assignments or mutation. While side
effects are to be avoided, however, they should be used if the resulting
code is significantly more readable or if it implements a significantly
better algorithm.

The use of mutable values, such as vectors and hash tables, raises fewer
suspicions about the style of a program than using `set!` directly.
Nevertheless, simply replacing `set!`s in a program with `vector-set!`s
obviously does not improve the style of the program.

#### 4.9.2. Multiple Values: `set!-values`

> +\[missing\] in \[missing\] also documents `set!-values`.

The `set!-values` form assigns to multiple variables at once, given an
expression that produces an appropriate number of values:

```racket
(set!-values (id ...) expr)
```

This form is equivalent to using `let-values` to receive multiple
results from `expr`, and then assigning the results individually to the
`id`s using `set!`.

Examples:

```racket
(define game                                
  (let ([w 0]                               
        [l 0])                              
    (lambda (win?)                          
      (if win?                              
          (set! w (+ w 1))                  
          (set! l (+ l 1)))                 
      (begin0                               
        (values w l)                        
        ; swap sides...                     
        (set!-values (w l) (values l w))))))
                                            
> (game #t)                                 
1                                           
0                                           
> (game #t)                                 
1                                           
1                                           
> (game #f)                                 
1                                           
2                                           
```

### 4.10. Quoting: `quote` and `'`

> +\[missing\] in \[missing\] also documents `quote`.

The `quote` form produces a constant:

```racket
(quote datum)
```

The syntax of a `datum` is technically specified as anything that the
`read` function parses as a single element. The value of the `quote`
form is the same value that `read` would produce given `datum`.

The `datum` can be a symbol, a boolean, a number, a \(character or
byte\) string, a character, a keyword, an empty list, a pair \(or list\)
containing more such values, a vector containing more such values, a
hash table containing more such values, or a box containing another such
value.

Examples:

```racket
> (quote apple)                       
'apple                                
> (quote #t)                          
#t                                    
> (quote 42)                          
42                                    
> (quote "hello")                     
"hello"                               
> (quote ())                          
'()                                   
> (quote ((1 2 3) #("z" x) . the-end))
'((1 2 3) #("z" x) . the-end)         
> (quote (1 2 . (3)))                 
'(1 2 3)                              
```

As the last example above shows, the `datum` does not have to match the
normalized printed form of a value. A `datum` cannot be a printed
representation that starts with `#<`, so it cannot be `#<void>`,
`#<undefined>`, or a procedure.

The `quote` form is rarely used for a `datum` that is a boolean, number,
or string by itself, since the printed forms of those values can already
be used as constants. The `quote` form is more typically used for
symbols and lists, which have other meanings \(identifiers, function
calls, etc.\) when not quoted.

An expression

```racket
'datum
```

is a shorthand for

`(quote` `datum)`

and this shorthand is almost always used instead of `quote`. The
shorthand applies even within the `datum`, so it can produce a list
containing `quote`.

> +\[missing\] in \[missing\] provides more on the `'` shorthand.

Examples:

```racket
> 'apple                  
'apple                    
> '"hello"                
"hello"                   
> '(1 2 3)                
'(1 2 3)                  
> (display '(you can 'me))
(you can (quote me))      
```

### 4.11. Quasiquoting: `quasiquote` and `‘`

> +\[missing\] in \[missing\] also documents `quasiquote`.

The `quasiquote` form is similar to `quote`:

```racket
(quasiquote datum)
```

However, for each `(unquote expr)` that appears within the `datum`, the
`expr` is evaluated to produce a value that takes the place of the
`unquote` sub-form.

Example:

```racket
> (quasiquote (1 2 (unquote (+ 1 2)) (unquote (- 5 1))))
'(1 2 3 4)                                              
```

This form can be used to write functions that build lists according to
certain patterns.

Examples:

```racket
> (define (deep n)                                           
    (cond                                                    
      [(zero? n) 0]                                          
      [else                                                  
       (quasiquote ((unquote n) (unquote (deep (- n 1)))))]))
> (deep 8)                                                   
'(8 (7 (6 (5 (4 (3 (2 (1 0))))))))                           
```

Or even to cheaply construct expressions programmatically. \(Of course,
9 times out of 10, you should be using a macro to do this \(the 10th
time being when you’re working through a textbook like
[PLAI](http://www.cs.brown.edu/~sk/Publications/Books/ProgLangs/)\).\)

Examples:

```racket
> (define (build-exp n)                                       
    (add-lets n (make-sum n)))                                
> (define (add-lets n body)                                   
    (cond                                                     
      [(zero? n) body]                                        
      [else                                                   
       (quasiquote                                            
        (let ([(unquote (n->var n)) (unquote n)])             
          (unquote (add-lets (- n 1) body))))]))              
> (define (make-sum n)                                        
    (cond                                                     
      [(= n 1) (n->var 1)]                                    
      [else                                                   
       (quasiquote (+ (unquote (n->var n))                    
                      (unquote (make-sum (- n 1)))))]))       
> (define (n->var n) (string->symbol (format "x~a" n)))       
> (build-exp 3)                                               
'(let ((x3 3)) (let ((x2 2)) (let ((x1 1)) (+ x3 (+ x2 x1)))))
```

The `unquote-splicing` form is similar to `unquote`, but its `expr` must
produce a list, and the `unquote-splicing` form must appear in a context
that produces either a list or a vector. As the name suggests, the
resulting list is spliced into the context of its use.

Example:

```racket
> (quasiquote (1 2 (unquote-splicing (list (+ 1 2) (- 5 1))) 5))
'(1 2 3 4 5)                                                    
```

Using splicing we can revise the construction of our example expressions
above to have just a single `let` expression and a single `+`
expression.

Examples:

```racket
> (define (build-exp n)                                          
    (add-lets                                                    
     n                                                           
     (quasiquote (+ (unquote-splicing                            
                     (build-list                                 
                      n                                          
                      (λ (x) (n->var (+ x 1)))))))))             
> (define (add-lets n body)                                      
    (quasiquote                                                  
     (let (unquote                                               
           (build-list                                           
            n                                                    
            (λ (n)                                               
              (quasiquote                                        
               [(unquote (n->var (+ n 1))) (unquote (+ n 1))]))))
       (unquote body))))                                         
> (define (n->var n) (string->symbol (format "x~a" n)))          
> (build-exp 3)                                                  
'(let ((x1 1) (x2 2) (x3 3)) (+ x1 x2 x3))                       
```

If a `quasiquote` form appears within an enclosing `quasiquote` form,
then the inner `quasiquote` effectively cancels one layer of `unquote`
and `unquote-splicing` forms, so that a second `unquote` or
`unquote-splicing` is needed.

Examples:

```racket
> (quasiquote (1 2 (quasiquote (unquote (+ 1 2)))))                            
'(1 2 (quasiquote (unquote (+ 1 2))))                                          
> (quasiquote (1 2 (quasiquote (unquote (unquote (+ 1 2))))))                  
'(1 2 (quasiquote (unquote 3)))                                                
>                                                                              
(quasiquote (1 2 (quasiquote ((unquote (+ 1 2)) (unquote (unquote (- 5 1)))))))
'(1 2 (quasiquote ((unquote (+ 1 2)) (unquote 4))))                            
```

The evaluations above will not actually print as shown. Instead, the
shorthand form of `quasiquote` and `unquote` will be used: ` \(i.e., a
backquote\) and `,` \(i.e., a comma\). The same shorthands can be used
in expressions:

Example:

```racket
> `(1 2 `(,(+ 1 2) ,,(- 5 1)))
'(1 2 `(,(+ 1 2) ,4))         
```

The shorthand form of `unquote-splicing` is `,@`:

Example:

```racket
> `(1 2 ,@(list (+ 1 2) (- 5 1)))
'(1 2 3 4)                       
```

### 4.12. Simple Dispatch: `case`

The `case` form dispatches to a clause by matching the result of an
expression to the values for the clause:

```racket
(case expr                
  [(datum ...+) body ...+]
  ...)                    
```

Each `datum` will be compared to the result of `expr` using `equal?`,
and then the corresponding `body`s are evaluated. The `case` form can
dispatch to the correct clause in _O_\(_log N_\)__ time for _N_
`datum`s.

Multiple `datum`s can be supplied for each clause, and the corresponding
`body`s are evaluated if any of the `datum`s match.

Example:

```racket
> (let ([v (random 6)])
    (printf "~a\n" v)  
    (case v            
      [(0) 'zero]      
      [(1) 'one]       
      [(2) 'two]       
      [(3 4 5) 'many]))
3                      
'many                  
```

The last clause of a `case` form can use `else`, just like `cond`:

Example:

```racket
> (case (random 6)
    [(0) 'zero]   
    [(1) 'one]    
    [(2) 'two]    
    [else 'many]) 
'one              
```

For more general pattern matching \(but without the dispatch-time
guarantee\), use `match`, which is introduced in Pattern Matching.

### 4.13. Dynamic Binding: `parameterize`

> +\[missing\] in \[missing\] also documents `parameterize`.

The `parameterize` form associates a new value with a _parameter_ during
the evaluation of `body` expressions:

```racket
(parameterize ([parameter-expr value-expr] ...)
  body ...+)                                   
```

> The term “parameter” is sometimes used to refer to the arguments of a
> function, but “parameter” in Racket has the more specific meaning
> described here.

For example, the `error-print-width` parameter controls how many
characters of a value are printed in an error message:

```racket
> (parameterize ([error-print-width 5]) 
    (car (expt 10 1024)))               
car: contract violation                 
  expected: pair?                       
  given: 10...                          
> (parameterize ([error-print-width 10])
    (car (expt 10 1024)))               
car: contract violation                 
  expected: pair?                       
  given: 1000000...                     
```

More generally, parameters implement a kind of dynamic binding. The
`make-parameter` function takes any value and returns a new parameter
that is initialized to the given value. Applying the parameter as a
function returns its current value:

```racket
> (define location (make-parameter "here"))
> (location)                               
"here"                                     
```

In a `parameterize` form, each `parameter-expr` must produce a
parameter. During the evaluation of the `body`s, each specified
parameter is given the result of the corresponding `value-expr`. When
control leaves the `parameterize` form—either through a normal return,
an exception, or some other escape—the parameter reverts to its earlier
value:

```racket
> (parameterize ([location "there"])               
    (location))                                    
"there"                                            
> (location)                                       
"here"                                             
> (parameterize ([location "in a house"])          
    (list (location)                               
          (parameterize ([location "with a mouse"])
            (location))                            
          (location)))                             
'("in a house" "with a mouse" "in a house")        
> (parameterize ([location "in a box"])            
    (car (location)))                              
car: contract violation                            
  expected: pair?                                  
  given: "in a box"                                
> (location)                                       
"here"                                             
```

The `parameterize` form is not a binding form like `let`; each use of
`location` above refers directly to the original definition. A
`parameterize` form adjusts the value of a parameter during the whole
time that the `parameterize` body is evaluated, even for uses of the
parameter that are textually outside of the `parameterize` body:

```racket
> (define (would-you-could-you?)            
    (and (not (equal? (location) "here"))   
         (not (equal? (location) "there"))))
> (would-you-could-you?)                    
#f                                          
> (parameterize ([location "on a bus"])     
    (would-you-could-you?))                 
#t                                          
```

If a use of a parameter is textually inside the body of a `parameterize`
but not evaluated before the `parameterize` form produces a value, then
the use does not see the value installed by the `parameterize` form:

```racket
> (let ([get (parameterize ([location "with a fox"])
               (lambda () (location)))])            
    (get))                                          
"here"                                              
```

The current binding of a parameter can be adjusted imperatively by
calling the parameter as a function with a value. If a `parameterize`
has adjusted the value of the parameter, then directly applying the
parameter procedure affects only the value associated with the active
`parameterize`:

```racket
> (define (try-again! where)             
    (location where))                    
> (location)                             
"here"                                   
> (parameterize ([location "on a train"])
    (list (location)                     
          (begin (try-again! "in a boat")
                 (location))))           
'("on a train" "in a boat")              
> (location)                             
"here"                                   
```

Using `parameterize` is generally preferable to updating a parameter
value imperatively—for much the same reasons that binding a fresh
variable with `let` is preferable to using `set!`  \(see Assignment:
`set!`\).

It may seem that variables and `set!` can solve many of the same
problems that parameters solve. For example, `lokation` could be defined
as a string, and `set!` could be used to adjust its value:

```racket
> (define lokation "here")                
> (define (would-ya-could-ya?)            
    (and (not (equal? lokation "here"))   
         (not (equal? lokation "there"))))
> (set! lokation "on a bus")              
> (would-ya-could-ya?)                    
#t                                        
```

Parameters, however, offer several crucial advantages over `set!`:

* The `parameterize` form helps automatically reset the value of a
  parameter when control escapes due to an exception. Adding exception
  handlers and other forms to rewind a `set!` is relatively tedious.

* Parameters work nicely with tail calls \(see Tail Recursion\). The
  last `body` in a `parameterize` form is in tail position with respect
  to the `parameterize` form.

* Parameters work properly with threads \(see \[missing\]\). The
  `parameterize` form adjusts the value of a parameter only for
  evaluation in the current thread, which avoids race conditions with
  other threads.

## 5. Programmer-Defined Datatypes

> +\[missing\] in \[missing\] also documents structure types.

New datatypes are normally created with the `struct` form, which is the
topic of this chapter. The class-based object system, which we defer to
Classes and Objects, offers an alternate mechanism for creating new
datatypes, but even classes and objects are implemented in terms of
structure types.

### 5.1. Simple Structure Types: `struct`

> +\[missing\] in \[missing\] also documents `struct`.

To a first approximation, the syntax of `struct` is

```racket
(struct struct-id (field-id ...))
```

Examples:

`(struct` `posn` `(x` `y))`

The `struct` form binds `struct-id` and a number of identifiers that are
built from `struct-id` and the `field-id`s:

* `struct-id` : a _constructor_ function that takes as many arguments as
  the number of `field-id`s, and returns an instance of the structure
  type.

  Example:

  ```racket
> (posn 1 2)
  #<posn>     
```

* `struct-id?` : a _predicate_ function that takes a single argument and
  returns `#t` if it is an instance of the structure type, `#f`
  otherwise.

  Examples:

  ```racket
> (posn? 3)         
  #f                  
  > (posn? (posn 1 2))
  #t                  
```

* `struct-id-field-id` : for each `field-id`, an _accessor_ that
  extracts the value of the corresponding field from an instance of the
  structure type.

  Examples:

  ```racket
> (posn-x (posn 1 2))
  1                    
  > (posn-y (posn 1 2))
  2                    
```

* `struct:struct-id` : a _structure type descriptor_, which is a value
  that represents the structure type as a first-class value \(with
  `#:super`, as discussed later in More Structure Type Options\).

A `struct` form places no constraints on the kinds of values that can
appear for fields in an instance of the structure type. For example,
`(posn "apple" #f)` produces an instance of `posn`, even though
`"apple"` and `#f` are not valid coordinates for the obvious uses of
`posn` instances. Enforcing constraints on field values, such as
requiring them to be numbers, is normally the job of a contract, as
discussed later in Contracts.

### 5.2. Copying and Update

The `struct-copy` form clones a structure and optionally updates
specified fields in the clone. This process is sometimes called a
_functional update_, because the result is a structure with updated
field values. but the original structure is not modified.

```racket
(struct-copy struct-id struct-expr [field-id expr] ...)
```

The `struct-id` that appears after `struct-copy` must be a structure
type name bound by `struct` \(i.e., the name that cannot be used
directly as an expression\). The `struct-expr` must produce an instance
of the structure type. The result is a new instance of the structure
type that is like the old one, except that the field indicated by each
`field-id` gets the value of the corresponding `expr`.

Examples:

```racket
> (define p1 (posn 1 2))                 
> (define p2 (struct-copy posn p1 [x 3]))
> (list (posn-x p2) (posn-y p2))         
'(3 2)                                   
> (list (posn-x p1) (posn-x p2))         
'(1 3)                                   
```

### 5.3. Structure Subtypes

An extended form of `struct` can be used to define a _structure
subtype_, which is a structure type that extends an existing structure
type:

```racket
(struct struct-id super-id (field-id ...))
```

The `super-id` must be a structure type name bound by `struct` \(i.e.,
the name that cannot be used directly as an expression\).

Examples:

```racket
(struct posn (x y))      
(struct 3d-posn posn (z))
```

A structure subtype inherits the fields of its supertype, and the
subtype constructor accepts the values for the subtype fields after
values for the supertype fields. An instance of a structure subtype can
be used with the predicate and accessors of the supertype.

Examples:

```racket
> (define p (3d-posn 1 2 3))                                   
> p                                                            
#<3d-posn>                                                     
> (posn? p)                                                    
#t                                                             
> (3d-posn-z p)                                                
3                                                              
; a 3d-posn has an x field, but there is no 3d-posn-x selector:
> (3d-posn-x p)                                                
3d-posn-x: undefined;                                          
 cannot reference an identifier before its definition          
  in module: top-level                                         
; use the supertype's posn-x selector to access the x field:   
> (posn-x p)                                                   
1                                                              
```

### 5.4. Opaque versus Transparent Structure Types

With a structure type definition like

`(struct` `posn` `(x` `y))`

an instance of the structure type prints in a way that does not show any
information about the fields’ values. That is, structure types by
default are _opaque_. If the accessors and mutators of a structure type
are kept private to a module, then no other module can rely on the
representation of the type’s instances.

To make a structure type _transparent_, use the `#:transparent` keyword
after the field-name sequence:

```racket             
(struct posn (x y)    
        #:transparent)
```                   
                      
```racket             
> (posn 1 2)          
(posn 1 2)            
```                   

An instance of a transparent structure type prints like a call to the
constructor, so that it shows the structures field values. A transparent
structure type also allows reflective operations, such as `struct?` and
`struct-info`, to be used on its instances \(see Reflection and Dynamic
Evaluation\).

Structure types are opaque by default, because opaque structure
instances provide more encapsulation guarantees. That is, a library can
use an opaque structure to encapsulate data, and clients of the library
cannot manipulate the data in the structure except as allowed by the
library.

### 5.5. Structure Comparisons

A generic `equal?` comparison automatically recurs on the fields of a
transparent structure type, but `equal?` defaults to mere instance
identity for opaque structure types:

`(struct` `glass` `(width` `height)` `#:transparent)`
                                                     
```racket                                            
> (equal? (glass 1 2) (glass 1 2))                   
#t                                                   
```                                                  

`(struct` `lead` `(width` `height))`
                                    
```racket                           
> (define slab (lead 1 2))          
> (equal? slab slab)                
#t                                  
> (equal? slab (lead 1 2))          
#f                                  
```                                 

To support instances comparisons via `equal?` without making the
structure type transparent, you can use the `#:methods` keyword,
`gen:equal+hash`, and implement three methods:

```racket                                                 
(struct lead (width height)                               
  #:methods                                               
  gen:equal+hash                                          
  [(define (equal-proc a b equal?-recur)                  
     ; compare a and b                                    
     (and (equal?-recur (lead-width a) (lead-width b))    
          (equal?-recur (lead-height a) (lead-height b))))
   (define (hash-proc a hash-recur)                       
     ; compute primary hash code of a                     
     (+ (hash-recur (lead-width a))                       
        (* 3 (hash-recur (lead-height a)))))              
   (define (hash2-proc a hash2-recur)                     
     ; compute secondary hash code of a                   
     (+ (hash2-recur (lead-width a))                      
             (hash2-recur (lead-height a))))])            
```                                                       
                                                          
```racket                                                 
> (equal? (lead 1 2) (lead 1 2))                          
#t                                                        
```                                                       

The first function in the list implements the `equal?` test on two
`lead`s; the third argument to the function is used instead of `equal?`
for recursive equality testing, so that data cycles can be handled
correctly. The other two functions compute primary and secondary hash
codes for use with hash tables:

```racket
> (define h (make-hash))        
> (hash-set! h (lead 1 2) 3)    
> (hash-ref h (lead 1 2))       
3                               
> (hash-ref h (lead 2 1))       
hash-ref: no value found for key
  key: #<lead>                  
```

The first function provided with `gen:equal+hash` is not required to
recursively compare the fields of the structure. For example, a
structure type representing a set might implement equality by checking
that the members of the set are the same, independent of the order of
elements in the internal representation. Just take care that the hash
functions produce the same value for any two structure types that are
supposed to be equivalent.

### 5.6. Structure Type Generativity

Each time that a `struct` form is evaluated, it generates a structure
type that is distinct from all existing structure types, even if some
other structure type has the same name and fields.

This generativity is useful for enforcing abstractions and implementing
programs such as interpreters, but beware of placing a `struct` form in
positions that are evaluated multiple times.

Examples:

```racket
(define (add-bigger-fish lst)                            
  (struct fish (size) #:transparent) ; new every time    
  (cond                                                  
   [(null? lst) (list (fish 1))]                         
   [else (cons (fish (* 2 (fish-size (car lst))))        
               lst)]))                                   
                                                         
> (add-bigger-fish null)                                 
(list (fish 1))                                          
> (add-bigger-fish (add-bigger-fish null))               
fish-size: contract violation;                           
 given value instantiates a different structure type with
the same name                                            
  expected: fish?                                        
  given: (fish 1)                                        
```

```racket                                        
(struct fish (size) #:transparent)               
(define (add-bigger-fish lst)                    
  (cond                                          
   [(null? lst) (list (fish 1))]                 
   [else (cons (fish (* 2 (fish-size (car lst))))
               lst)]))                           
```                                              
                                                 
```racket                                        
> (add-bigger-fish (add-bigger-fish null))       
(list (fish 2) (fish 1))                         
```                                              

### 5.7. Prefab Structure Types

Although a transparent structure type prints in a way that shows its
content, the printed form of the structure cannot be used in an
expression to get the structure back, unlike the printed form of a
number, string, symbol, or list.

A _prefab_ \(“previously fabricated”\) structure type is a built-in type
that is known to the Racket printer and expression reader. Infinitely
many such types exist, and they are indexed by name, field count,
supertype, and other such details. The printed form of a prefab
structure is similar to a vector, but it starts `#s` instead of just
`#`, and the first element in the printed form is the prefab structure
type’s name.

The following examples show instances of the `sprout` prefab structure
type that has one field. The first instance has a field value `'bean`,
and the second has field value `'alfalfa`:

```racket
> '#s(sprout bean)   
'#s(sprout bean)     
> '#s(sprout alfalfa)
'#s(sprout alfalfa)  
```

Like numbers and strings, prefab structures are “self-quoting,” so the
quotes above are optional:

```racket
> #s(sprout bean)
'#s(sprout bean) 
```

When you use the `#:prefab` keyword with `struct`, instead of generating
a new structure type, you obtain bindings that work with the existing
prefab structure type:

```racket
> (define lunch '#s(sprout bean))
> (struct sprout (kind) #:prefab)
> (sprout? lunch)                
#t                               
> (sprout-kind lunch)            
'bean                            
> (sprout 'garlic)               
'#s(sprout garlic)               
```

The field name `kind` above does not matter for finding the prefab
structure type; only the name `sprout` and the number of fields matters.
At the same time, the prefab structure type `sprout` with three fields
is a different structure type than the one with a single field:

```racket
> (sprout? #s(sprout bean #f 17))                        
#f                                                       
> (struct sprout (kind yummy? count) #:prefab) ; redefine
> (sprout? #s(sprout bean #f 17))                        
#t                                                       
> (sprout? lunch)                                        
#f                                                       
```

A prefab structure type can have another prefab structure type as its
supertype, it can have mutable fields, and it can have auto fields.
Variations in any of these dimensions correspond to different prefab
structure types, and the printed form of the structure type’s name
encodes all of the relevant details.

```racket
> (struct building (rooms [location #:mutable]) #:prefab)
> (struct house building ([occupied #:auto]) #:prefab    
    #:auto-value 'no)                                    
> (house 5 'factory)                                     
'#s((house (1 no) building 2 #(1)) 5 factory no)         
```

Every prefab structure type is transparent—but even less abstract than a
transparent type, because instances can be created without any access to
a particular structure-type declaration or existing examples. Overall,
the different options for structure types offer a spectrum of
possibilities from more abstract to more convenient:

* Opaque \(the default\) : Instances cannot be inspected or forged
  without access to the structure-type declaration. As discussed in the
  next section, constructor guards and properties can be attached to the
  structure type to further protect or to specialize the behavior of its
  instances.

* Transparent : Anyone can inspect or create an instance without access
  to the structure-type declaration, which means that the value printer
  can show the content of an instance. All instance creation passes
  through a constructor guard, however, so that the content of an
  instance can be controlled, and the behavior of instances can be
  specialized through properties. Since the structure type is generated
  by its definition, instances cannot be manufactured simply through the
  name of the structure type, and therefore cannot be generated
  automatically by the expression reader.

* Prefab : Anyone can inspect or create an instance at any time, without
  prior access to a structure-type declaration or an example instance.
  Consequently, the expression reader can manufacture instances
  directly. The instance cannot have a constructor guard or properties.

Since the expression reader can generate prefab instances, they are
useful when convenient serialization is more important than abstraction.
Opaque and transparent structures also can be serialized, however, if
they are defined with `serializable-struct` as described in Datatypes
and Serialization.

### 5.8. More Structure Type Options

The full syntax of `struct` supports many options, both at the
structure-type level and at the level of individual fields:

```racket
(struct struct-id maybe-super (field ...)
        struct-option ...)               
                                         
maybe-super =                            
            | super-id                   
                                         
field       = field-id                   
            | [field-id field-option ...]
```

A `struct-option` always starts with a keyword:

```racket
#:mutable
```
Causes all fields of the structure to be mutable, and introduces  for
each `field-id` a _mutator_   `set-struct-id-field-id!`  that sets the
value of the corresponding field in an instance of  the structure type.
Examples:

```racket
> (struct dot (x y) #:mutable)
(define d (dot 1 2))          
                              
> (dot-x d)                   
1                             
> (set-dot-x! d 10)           
> (dot-x d)                   
10                            
```
The `#:mutable` option can also be used as a `field-option`, in which
case it makes an individual field mutable.
Examples:

```racket
> (struct person (name [age #:mutable]))             
(define friend (person "Barney" 5))                  
                                                     
> (set-person-age! friend 6)                         
> (set-person-name! friend "Mary")                   
set-person-name!: undefined;                         
 cannot reference an identifier before its definition
  in module: top-level                               
```

```racket
#:transparent
```
Controls reflective access to structure instances, as discussed in a
previous section, Opaque versus Transparent Structure Types.

```racket
#:inspector inspector-expr
```
Generalizes `#:transparent` to support more controlled access to
reflective operations.

```racket
#:prefab
```
Accesses a built-in structure type, as discussed in a previous section,
Prefab Structure Types.

```racket
#:auto-value auto-expr
```
Specifies a value to be used for all automatic fields in the structure
type, where an automatic field is indicated by the `#:auto` field
option. The constructor procedure does not accept arguments for
automatic fields. Automatic fields are implicitly mutable \(via
reflective operations\), but mutator functions are bound only if
`#:mutable` is also specified.
Examples:

```racket
> (struct posn (x y [z #:auto])
               #:transparent   
               #:auto-value 0) 
> (posn 1 2)                   
(posn 1 2 0)                   
```

```racket
#:guard guard-expr
```
Specifies a  _constructor guard_ procedure to be called whenever an
instance of the structure type is created. The guard takes as many
arguments as non-automatic fields in the structure type, plus one  more
for the name of the instantiated type \(in case a sub-type is
instantiated, in which case it’s best to report an error using the
sub-type’s name\). The guard should return the same number of values  as
given, minus the name argument. The guard can raise an exception  if one
of the given arguments is unacceptable, or it can convert an  argument.
Examples:

```racket
> (struct thing (name)                                      
          #:transparent                                     
          #:guard (lambda (name type-name)                  
                    (cond                                   
                      [(string? name) name]                 
                      [(symbol? name) (symbol->string name)]
                      [else (error type-name                
                                   "bad name: ~e"           
                                   name)])))                
> (thing "apple")                                           
(thing "apple")                                             
> (thing 'apple)                                            
(thing "apple")                                             
> (thing 1/2)                                               
thing: bad name: 1/2                                        
```
The guard is called even when subtype instances are created. In that
case, only the fields accepted by the constructor are provided to  the
guard \(but the subtype’s guard gets both the original fields and
fields added by the subtype\).
Examples:

```racket
> (struct person thing (age)                               
          #:transparent                                    
          #:guard (lambda (name age type-name)             
                    (if (negative? age)                    
                        (error type-name "bad age: ~e" age)
                        (values name age))))               
> (person "John" 10)                                       
(person "John" 10)                                         
> (person "Mary" -1)                                       
person: bad age: -1                                        
> (person 10 10)                                           
person: bad name: 10                                       
```

```racket
#:methods interface-expr [body ...]
```
Associates method definitions for the structure type that correspond to
a _generic interface_.  For example, implementing the methods for
`gen:dict` allows instances of a structure type to be used as
dictionaries. Implementing the methods for `gen:custom-write` allows the
customization of how an instance of a structure type is `display`ed.
Examples:

```racket
> (struct cake (candles)                             
          #:methods gen:custom-write                 
          [(define (write-proc cake port mode)       
             (define n (cake-candles cake))          
             (show "   ~a   ~n" n #\. port)          
             (show " .-~a-. ~n" n #\| port)          
             (show " | ~a | ~n" n #\space port)      
             (show "---~a---~n" n #\- port))         
           (define (show fmt n ch port)              
             (fprintf port fmt (make-string n ch)))])
> (display (cake 5))                                 
   .....                                             
 .-|||||-.                                           
 |       |                                           
-----------                                          
```

```racket
#:property prop-expr val-expr
```
Associates a _property_ and value with the structure type.   For
example, the `prop:procedure` property allows a   structure instance to
be used as a function; the property value   determines how a call is
implemented when using the structure as a   function.
Examples:

```racket
> (struct greeter (name)                               
          #:property prop:procedure                    
                     (lambda (self other)              
                       (string-append                  
                        "Hi " other                    
                        ", I'm " (greeter-name self))))
(define joe-greet (greeter "Joe"))                     
                                                       
> (greeter-name joe-greet)                             
"Joe"                                                  
> (joe-greet "Mary")                                   
"Hi Mary, I'm Joe"                                     
> (joe-greet "John")                                   
"Hi John, I'm Joe"                                     
```

```racket
#:super super-expr
```
An alternative to supplying a `super-id` next to `struct-id`. Instead of
the name of a structure type \(which is not an expression\),
`super-expr` should produce a structure type descriptor value. An
advantage of `#:super` is that structure type descriptors are values, so
they can be passed to procedures.
Examples:

```racket
(define (raven-constructor super-type)                 
  (struct raven ()                                     
          #:super super-type                           
          #:transparent                                
          #:property prop:procedure (lambda (self)     
                                      'nevermore))     
  raven)                                               
                                                       
> (let ([r ((raven-constructor struct:posn) 1 2)])     
    (list r (r)))                                      
(list (raven 1 2) 'nevermore)                          
> (let ([r ((raven-constructor struct:thing) "apple")])
    (list r (r)))                                      
(list (raven "apple") 'nevermore)                      
```

> +\[missing\] in \[missing\] provides more on structure types.

## 6. Modules

Modules let you organize Racket code into multiple files and reusable
libraries.

    6.1 Module Basics               
      6.1.1 Organizing Modules      
      6.1.2 Library Collections     
      6.1.3 Packages and Collections
      6.1.4 Adding Collections      
    6.2 Module Syntax               
      6.2.1 The `module` Form       
      6.2.2 The `#lang` Shorthand   
      6.2.3 Submodules              
      6.2.4 Main and Test Submodules
    6.3 Module Paths                
    6.4 Imports: `require`          
    6.5 Exports: `provide`          
    6.6 Assignment and Redefinition 
    6.7 Modules and Macros          

### 6.1. Module Basics

Each Racket module typically resides in its own file. For example,
suppose the file `"cake.rkt"` contains the following module:

`"cake.rkt"`
```racket
#lang racket                     
                                 
(provide print-cake)             
                                 
; draws a cake with n candles    
(define (print-cake n)           
  (show "   ~a   " n #\.)        
  (show " .-~a-. " n #\|)        
  (show " | ~a | " n #\space)    
  (show "---~a---" n #\-))       
                                 
(define (show fmt n ch)          
  (printf fmt (make-string n ch))
  (newline))                     
```

Then, other modules can import `"cake.rkt"` to use the `print-cake`
function, since the `provide` line in `"cake.rkt"` explicitly exports
the definition `print-cake`. The `show` function is private to
`"cake.rkt"` \(i.e., it cannot be used from other modules\), since
`show` is not exported.

The following `"random-cake.rkt"` module imports `"cake.rkt"`:

`"random-cake.rkt"`
```racket
#lang racket            
                        
(require "cake.rkt")    
                        
(print-cake (random 30))
```

The relative reference `"cake.rkt"` in the import `(require "cake.rkt")`
works if the `"cake.rkt"` and `"random-cake.rkt"` modules are in the
same directory. Unix-style relative paths are used for relative module
references on all platforms, much like relative URLs in HTML pages.

#### 6.1.1. Organizing Modules

The `"cake.rkt"` and `"random-cake.rkt"` example demonstrates the most
common way to organize a program into modules: put all module files in a
single directory \(perhaps with subdirectories\), and then have the
modules reference each other through relative paths. A directory of
modules can act as a project, since it can be moved around on the
filesystem or copied to other machines, and relative paths preserve the
connections among modules.

As another example, if you are building a candy-sorting program, you
might have a main `"sort.rkt"` module that uses other modules to access
a candy database and a control sorting machine. If the candy-database
module itself is organized into sub-modules that handle barcode and
manufacturer information, then the database module could be
`"db/lookup.rkt"` that uses helper modules `"db/barcodes.rkt"` and
`"db/makers.rkt"`.  Similarly, the sorting-machine driver
`"machine/control.rkt"` might use helper modules `"machine/sensors.rkt"`
and `"machine/actuators.rkt"`.

\#<pict>

The `"sort.rkt"` module uses the relative paths `"db/lookup.rkt"` and
`"machine/control.rkt"` to import from the database and machine-control
libraries:

`"sort.rkt"`
```racket
#lang racket                                   
(require "db/lookup.rkt" "machine/control.rkt")
....                                           
```

The `"db/lookup.rkt"` module similarly uses paths relative to its own
source to access the `"db/barcodes.rkt"` and `"db/makers.rkt"` modules:

`"db/lookup.rkt"`
```racket
#lang racket                        
(require "barcode.rkt" "makers.rkt")
....                                
```

Ditto for `"machine/control.rkt"`:

`"machine/control.rkt"`
```racket
#lang racket                           
(require "sensors.rkt" "actuators.rkt")
....                                   
```

Racket tools all work automatically with relative paths. For example,

  `racket sort.rkt`

on the command line runs the `"sort.rkt"` program and automatically
loads and compiles required modules. With a large enough program,
compilation from source can take too long, so use

  `raco make sort.rkt`

> See \[missing\] for more information on `raco make`.

to compile `"sort.rkt"` and all its dependencies to bytecode files.
Running `racket sort.rkt` will automatically use bytecode files when
they are present.

#### 6.1.2. Library Collections

A _collection_ is a hierarchical grouping of installed library modules.
A module in a collection is referenced through an unquoted, suffixless
path. For example, the following module refers to the `"date.rkt"`
library that is part of the `"racket"` collection:

```racket
#lang racket                                             
                                                         
(require racket/date)                                    
                                                         
(printf "Today is ~s\n"                                  
        (date->string (seconds->date (current-seconds))))
```

When you search the online Racket documentation, the search results
indicate the module that provides each binding. Alternatively, if you
reach a binding’s documentation by clicking on hyperlinks, you can hover
over the binding name to find out which modules provide it.

A module reference like `racket/date` looks like an identifier, but it
is not treated in the same way as `printf` or `date->string`. Instead,
when `require` sees a module reference that is unquoted, it converts the
reference to a collection-based module path:

* First, if the unquoted path contains no `/`, then `require`
  automatically adds a `"/main"` to the reference. For example,
  `(require slideshow)` is equivalent to `(require slideshow/main)`.

* Second, `require` implicitly adds a `".rkt"` suffix to the path.

* Finally, `require` resolves the path by searching among installed
  collections, instead of treating the path as relative to the enclosing
  module’s path.

To a first approximation, a collection is implemented as a filesystem
directory. For example, the `"racket"` collection is mostly located in a
`"racket"` directory within the Racket installation’s `"collects"`
directory, as reported by

```racket
#lang racket                                               
                                                           
(require setup/dirs)                                       
                                                           
(build-path (find-collects-dir) ; main collection directory
            "racket")                                      
```

The Racket installation’s `"collects"` directory, however, is only one
place that `require` looks for collection directories. Other places
include the user-specific directory reported by
`(find-user-collects-dir)` and directories configured through the
`PLTCOLLECTS` search path. Finally, and most typically, collections are
found through installed packages.

#### 6.1.3. Packages and Collections

A _package_ is a set of libraries that are installed through the Racket
package manager \(or included as pre-installed in a Racket
distribution\). For example, the `racket/gui` library is provided by the
`"gui"` package, while `parser-tools/lex` is provided by the
`"parser-tools"` library.

> More precisely, `racket/gui` is provided by `"gui-lib"`,
> `parser-tools/lex` is provided by `"parser-tools-lib"`, and the `"gui"`
> and `"parser-tools"` packages extend `"gui-lib"` and
> `"parser-tools-lib"` with documentation.

Racket programs do not refer to packages directly. Instead, programs
refer to libraries via collections, and adding or removing a package
changes the set of collection-based libraries that are available. A
single package can supply libraries in multiple collections, and two
different packages can supply libraries in the same collection \(but not
the same libraries, and the package manager ensures that installed
packages do not conflict at that level\).

For more information about packages, see \[missing\].

#### 6.1.4. Adding Collections

Looking back at the candy-sorting example of Organizing Modules, suppose
that modules in `"db/"` and `"machine/"` need a common set of helper
functions. Helper functions could be put in a `"utils/"` directory, and
modules in `"db/"` or `"machine/"` could access utility modules with
relative paths that start `"../utils/"`. As long as a set of modules
work together in a single project, it’s best to stick with relative
paths. A programmer can follow relative-path references without knowing
about your Racket configuration.

Some libraries are meant to be used across multiple projects, so that
keeping the library source in a directory with its uses does not make
sense. In that case, the best option is add a new collection. After the
library is in a collection, it can be referenced with an unquoted path,
just like libraries that are included with the Racket distribution.

You could add a new collection by placing files in the Racket
installation or one of the directories reported by
`(get-collects-search-dirs)`. Alternatively, you could add to the list
of searched directories by setting the `PLTCOLLECTS` environment
variable.If you set `PLTCOLLECTS`, include an empty path in by starting
the value with a colon \(Unix and Mac OS\) or semicolon \(Windows\) so
that the original search paths are preserved. The best option, however,
is to add a package.

Creating a package _does not_ mean that you have to register with a
package server or perform a bundling step that copies your source code
into an archive format. Creating a package can simply mean using the
package manager to make your libraries locally accessible as a
collection from their current source locations.

For example, suppose you have a directory `"/usr/molly/bakery"` that
contains the `"cake.rkt"` module \(from the beginning of this section\)
and other related modules. To make the modules available as a `"bakery"`
collection, either

* Use the `raco pkg` command-line tool:

    `raco pkg install --link /usr/molly/bakery`

  where the `--link` flag is not actually needed when the provided path
  includes a directory separator.

* Use DrRacket’s Package Manager item from the File menu. In the Do What
  I Mean panel, click Browse..., choose the `"/usr/molly/bakery"`
  directory, and click Install.

Afterward, `(require bakery/cake)` from any module will import the
`print-cake` function from `"/usr/molly/bakery/cake.rkt"`.

By default, the name of the directory that you install is used both as
the package name and as the collection that is provided by the package.
Also, the package manager normally defaults to installation only for the
current user, as opposed to all users of a Racket installation. See
\[missing\] for more information.

If you intend to distribute your libraries to others, choose collection
and package names carefully. The collection namespace is hierarchical,
but top-level collection names are global, and the package namespace is
flat. Consider putting one-off libraries under some top-level name like
`"molly"` that identifies the producer.  Use a collection name like
`"bakery"` when producing the definitive collection of baked-goods
libraries.

After your libraries are put in a collection you can still use `raco
make` to compile the library sources, but it’s better and more
convenient to use `raco setup`. The `raco setup` command takes a
collection name \(as opposed to a file name\) and compiles all libraries
within the collection. In addition, `raco setup` can build documentation
for the collection and add it to the documentation index, as specified
by a `"info.rkt"` module in the collection. See \[missing\] for more
information on `raco setup`.

### 6.2. Module Syntax

The `#lang` at the start of a module file begins a shorthand for a
`module` form, much like `'` is a shorthand for a `quote` form. Unlike
`'`, the `#lang` shorthand does not work well in a REPL, in part because
it must be terminated by an end-of-file, but also because the longhand
expansion of `#lang` depends on the name of the enclosing file.

#### 6.2.1. The `module` Form

The longhand form of a module declaration, which works in a REPL as well
as a file, is

```racket
(module name-id initial-module-path
  decl ...)                        
```

where the `name-id` is a name for the module, `initial-module-path` is
an initial import, and each `decl` is an import, export, definition, or
expression.  In the case of a file, `name-id` normally matches the name
of the containing file, minus its directory path or file extension, but
`name-id` is ignored when the module is `require`d through its file’s
path.

The `initial-module-path` is needed because even the `require` form must
be imported for further use in the module body. In other words, the
`initial-module-path` import bootstraps the syntax that is available in
the body. The most commonly used `initial-module-path` is `racket`,
which supplies most of the bindings described in this guide, including
`require`, `define`, and `provide`. Another commonly used
`initial-module-path` is `racket/base`, which provides less
functionality, but still much of the most commonly needed functions and
syntax.

For example, the `"cake.rkt"` example of the previous section could be
written as

```racket
(module cake racket                
  (provide print-cake)             
                                   
  (define (print-cake n)           
    (show "   ~a   " n #\.)        
    (show " .-~a-. " n #\|)        
    (show " | ~a | " n #\space)    
    (show "---~a---" n #\-))       
                                   
  (define (show fmt n ch)          
    (printf fmt (make-string n ch))
    (newline)))                    
```

Furthermore, this `module` form can be evaluated in a REPL to declare a
`cake` module that is not associated with any file. To refer to such an
unassociated module, quote the module name:

Examples:

```racket
> (require 'cake)
> (print-cake 3) 
   ...           
 .-|||-.         
 |     |         
---------        
```

Declaring a module does not immediately evaluate the body definitions
and expressions of the module. The module must be explicitly `require`d
at the top level to trigger evaluation. After evaluation is triggered
once, later `require`s do not re-evaluate the module body.

Examples:

```racket
> (module hi racket    
    (printf "Hello\n"))
> (require 'hi)        
Hello                  
> (require 'hi)        
```

#### 6.2.2. The `#lang` Shorthand

The body of a `#lang` shorthand has no specific syntax, because the
syntax is determined by the language name that follows `#lang`.

In the case of `#lang` `racket`, the syntax is

```racket
#lang racket
decl ...    
```

which reads the same as

```racket
(module name racket
  decl ...)        
```

where `name` is derived from the name of the file that contains the
`#lang` form.

The `#lang` `racket/base` form has the same syntax as `#lang` `racket`,
except that the longhand expansion uses `racket/base` instead of
`racket`. The `#lang` `scribble/manual` form, in contrast, has a
completely different syntax that doesn’t even look like Racket, and
which we do not attempt to describe in this guide.

Unless otherwise specified, a module that is documented as a “language”
using the `#lang` notation will expand to `module` in the same way as
`#lang` `racket`. The documented language name can be used directly with
`module` or `require`, too.

#### 6.2.3. Submodules

A `module` form can be nested within a module, in which case the nested
`module` form declares a _submodule_. Submodules can be referenced
directly by the enclosing module using a quoted name. The following
example prints `"Tony"` by importing `tiger` from the `zoo` submodule:

`"park.rkt"`
```racket
#lang racket            
                        
(module zoo racket      
  (provide tiger)       
  (define tiger "Tony"))
                        
(require 'zoo)          
                        
tiger                   
```

Running a module does not necessarily run its submodules. In the above
example, running `"park.rkt"` runs its submodule `zoo` only because the
`"park.rkt"` module `require`s the `zoo` submodule. Otherwise, a module
and each of its submodules can be run independently. Furthermore, if
`"park.rkt"` is compiled to a bytecode file \(via `raco make`\), then
the code for `"park.rkt"` or the code for `zoo` can be loaded
independently.

Submodules can be nested within submodules, and a submodule can be
referenced directly by a module other than its enclosing module by using
a submodule path.

A `module*` form is similar to a nested `module` form:

```racket
(module* name-id initial-module-path-or-#f
  decl ...)                               
```

The `module*` form differs from `module` in that it inverts the
possibilities for reference between the submodule and enclosing module:

* A submodule declared with `module` can be `require`d by its enclosing
  module, but the submodule cannot `require` the enclosing module or
  lexically reference the enclosing module’s bindings.

* A submodule declared with `module*` can `require` its enclosing
  module, but the enclosing module cannot `require` the submodule.

In addition, a `module*` form can specify `#f` in place of an
`initial-module-path`, in which case the submodule sees all of the
enclosing module’s bindings—including bindings that are not exported via
`provide`.

One use of submodules declared with `module*` and `#f` is to export
additional bindings through a submodule that are not normally exported
from the module:

`"cake.rkt"`
```racket
#lang racket                     
                                 
(provide print-cake)             
                                 
(define (print-cake n)           
  (show "   ~a   " n #\.)        
  (show " .-~a-. " n #\|)        
  (show " | ~a | " n #\space)    
  (show "---~a---" n #\-))       
                                 
(define (show fmt n ch)          
  (printf fmt (make-string n ch))
  (newline))                     
                                 
(module* extras #f               
  (provide show))                
```

In this revised `"cake.rkt"` module, `show` is not imported by a module
that uses `(require "cake.rkt")`, since most clients of `"cake.rkt"`
will not want the extra function.  A module can require the `extra`
submodule using `(require (submod "cake.rkt" extras))` to access the
otherwise hidden `show` function.See submodule paths for more
information on `submod`.

#### 6.2.4. Main and Test Submodules

The following variant of `"cake.rkt"` includes a `main` submodule that
calls `print-cake`:

`"cake.rkt"`
```racket
#lang racket                     
                                 
(define (print-cake n)           
  (show "   ~a   " n #\.)        
  (show " .-~a-. " n #\|)        
  (show " | ~a | " n #\space)    
  (show "---~a---" n #\-))       
                                 
(define (show fmt n ch)          
  (printf fmt (make-string n ch))
  (newline))                     
                                 
(module* main #f                 
  (print-cake 10))               
```

Running a module does not run its `module*`-defined submodules.
Nevertheless, running the above module via `racket` or DrRacket prints a
cake with 10 candles, because the `main` submodule is a special case.

When a module is provided as a program name to the `racket` executable
or run directly within DrRacket, if the module has a `main` submodule,
the `main` submodule is run after its enclosing module. Declaring a
`main` submodule thus specifies extra actions to be performed when a
module is run directly, instead of `require`d as a library within a
larger program.

A `main` submodule does not have to be declared with `module*`. If the
`main` module does not need to use bindings from its enclosing module,
it can be declared with `module`. More commonly, `main` is declared
using `module+`:

```racket
(module+ name-id
  decl ...)     
```

A submodule declared with `module+` is like one declared with `module*`
using `#f` as its `initial-module-path`.  In addition, multiple
`module+` forms can specify the same submodule name, in which case the
bodies of the `module+` forms are combined to create a single submodule.

The combining behavior of `module+` is particularly useful for defining
a `test` submodule, which can be conveniently run using `raco test` in
much the same way that `main` is conveniently run with `racket`. For
example, the following `"physics.rkt"` module exports `drop` and
`to-energy` functions, and it defines a `test` module to hold unit
tests:

`"physics.rkt"`
```racket
#lang racket                          
(module+ test                         
  (require rackunit)                  
  (define ε 1e-10))                   
                                      
(provide drop                         
         to-energy)                   
                                      
(define (drop t)                      
  (* 1/2 9.8 t t))                    
                                      
(module+ test                         
  (check-= (drop 0) 0 ε)              
  (check-= (drop 10) 490 ε))          
                                      
(define (to-energy m)                 
  (* m (expt 299792458.0 2)))         
                                      
(module+ test                         
  (check-= (to-energy 0) 0 ε)         
  (check-= (to-energy 1) 9e+16 1e+15))
```

Importing `"physics.rkt"` into a larger program does not run the `drop`
and `to-energy` tests—or even trigger the loading of the test code, if
the module is compiled—but running `raco test physics.rkt` at a command
line runs the tests.

The above `"physics.rkt"` module is equivalent to using `module*`:

`"physics.rkt"`
```racket
#lang racket                          
                                      
(provide drop                         
         to-energy)                   
                                      
(define (drop t)                      
  (* 1/2 49/5 t t))                   
                                      
(define (to-energy m)                 
  (* m (expt 299792458 2)))           
                                      
(module* test #f                      
  (require rackunit)                  
  (define ε 1e-10)                    
  (check-= (drop 0) 0 ε)              
  (check-= (drop 10) 490 ε)           
  (check-= (to-energy 0) 0 ε)         
  (check-= (to-energy 1) 9e+16 1e+15))
```

Using `module+` instead of `module*` allows tests to be interleaved with
function definitions.

The combining behavior of `module+` is also sometimes helpful for a
`main` module. Even when combining is not needed, `(module+ main ....)`
is preferred as it is more readable than `(module* main #f ....)`.

### 6.3. Module Paths

A _module path_ is a reference to a module, as used with `require` or as
the `initial-module-path` in a `module` form. It can be any of several
forms:

```racket
(quote id)
```
A module path that is a quoted identifier refers to a non-file `module`
declaration using the identifier. This form of module reference makes
the most sense in a REPL.
Examples:

```racket
> (module m racket                             
    (provide color)                            
    (define color "blue"))                     
> (module n racket                             
    (require 'm)                               
    (printf "my favorite color is ~a\n" color))
> (require 'n)                                 
my favorite color is blue                      
```

```racket
rel-string
```
A string module path is a relative path using Unix-style conventions:
`/` is the path separator, `..` refers to the parent directory, and `.`
refers to the same directory. The `rel-string` must not start or end
with a path separator. If the path has no suffix, `".rkt"` is added
automatically.
The path is relative to the enclosing file, if any, or it is relative to
the current directory. \(More precisely, the path is relative to the
value of `(current-load-relative-directory)`, which is set while loading
a file.\)
Module Basics shows examples using relative paths.
If a relative path ends with a `".ss"` suffix, it is converted to
`".rkt"`. If the file that implements the referenced module actually
ends in `".ss"`, the suffix will be changed back when attempting to load
the file \(but a `".rkt"` suffix takes precedence\). This two-way
conversion provides compatibility with older versions of Racket.

```racket
id
```
A module path that is an unquoted identifier refers to an installed
library. The `id` is constrained to contain only ASCII letters, ASCII
numbers, `+`, `-`, `_`, and `/`, where `/` separates path elements
within the identifier. The elements refer to collections and
sub-collections, instead of directories and sub-directories.
An example of this form is `racket/date`. It refers to the module whose
source is the `"date.rkt"` file in the `"racket"` collection, which is
installed as part of Racket. The `".rkt"` suffix is added automatically.
Another example of this form is `racket`, which is commonly used at the
initial import. The path `racket` is shorthand for `racket/main`; when
an `id` has no `/`, then `/main` is automatically added to the end.
Thus, `racket` or `racket/main` refers to the module whose source is the
`"main.rkt"` file in the `"racket"` collection.
Examples:

```racket
> (module m racket                                            
    (require racket/date)                                     
                                                              
    (printf "Today is ~s\n"                                   
            (date->string (seconds->date (current-seconds)))))
> (require 'm)                                                
Today is "Monday, January 21st, 2019"                         
```
When the full path of a module ends with `".rkt"`, if no such file
exists but one does exist with the `".ss"` suffix, then the `".ss"`
suffix is substituted automatically. This transformation provides
compatibility with older versions of Racket.

```racket
(lib rel-string)
```
Like an unquoted-identifier path, but expressed as a string instead of
an identifier. Also, the `rel-string` can end with a file suffix, in
which case `".rkt"` is not automatically added.
Example of this form include `(lib "racket/date.rkt")` and `(lib
"racket/date")`, which are equivalent to `racket/date`. Other examples
include `(lib "racket")`, `(lib "racket/main")`, and `(lib
"racket/main.rkt")`, which are all equivalent to `racket`.
Examples:

```racket
> (module m (lib "racket")                                    
    (require (lib "racket/date.rkt"))                         
                                                              
    (printf "Today is ~s\n"                                   
            (date->string (seconds->date (current-seconds)))))
> (require 'm)                                                
Today is "Monday, January 21st, 2019"                         
```

```racket
(planet id)
```
Accesses a third-party library that is distributed through the PLaneT
server. The library is downloaded the first time that it is needed, and
then the local copy is used afterward.
The `id` encodes several pieces of information separated by a `/`: the
package owner, then package name with optional version information, and
an optional path to a specific library with the package. Like `id` as
shorthand for a `lib` path, a `".rkt"` suffix is added automatically,
and `/main` is used as the path if no sub-path element is supplied.
Examples:

```racket
> (module m (lib "racket")                                   
    ; Use "schematics"'s "random.plt" 1.0, file "random.rkt":
    (require (planet schematics/random:1/random))            
    (display (random-gaussian)))                             
> (require 'm)                                               
0.9050686838895684                                           
```
As with other forms, an implementation file ending with `".ss"` can be
substituted automatically if no implementation file ending with `".rkt"`
exists.

```racket
(planet package-string)
```
Like the symbol form of a `planet`, but using a string instead of an
identifier. Also, the `package-string` can end with a file suffix, in
which case `".rkt"` is not added.
As with other forms, an `".ss"` extension is converted to `".rkt"`,
while an implementation file ending with `".ss"` can be substituted
automatically if no implementation file ending with `".rkt"` exists.

```racket
(planet rel-string (user-string pkg-string vers ...))
                                                     
vers = nat                                           
     | (nat nat)                                     
     | (= nat)                                       
     | (+ nat)                                       
     | (- nat)                                       
```
A more general form to access a library from the PLaneT server. In this
general form, a PLaneT reference starts like a `lib` reference with a
relative path, but the path is followed by information about the
producer, package, and version of the library. The specified package is
downloaded and installed on demand.
The `vers`es specify a constraint on the acceptable version of the
package, where a version number is a sequence of non-negative integers,
and the constraints determine the allowable values for each element in
the sequence. If no constraint is provided for a particular element,
then any version is allowed; in particular, omitting all `vers`es means
that any version is acceptable. Specifying at least one `vers` is
strongly recommended.
For a version constraint, a plain `nat` is the same as `(+ nat)`, which
matches `nat` or higher for the corresponding element of the version
number.  A `(start-nat end-nat)` matches any number in the range
`start-nat` to `end-nat`, inclusive. A `(= nat)` matches only exactly
`nat`. A `(- nat)` matches `nat` or lower.
Examples:

```racket
> (module m (lib "racket")                                         
    (require (planet "random.rkt" ("schematics" "random.plt" 1 0)))
    (display (random-gaussian)))                                   
> (require 'm)                                                     
0.9050686838895684                                                 
```
The automatic `".ss"` and `".rkt"` conversions apply as with other
forms.

```racket
(file string)
```
Refers to a file, where `string` is a relative or absolute path using
the current platform’s conventions. This form is not portable, and it
should _not_ be used when a plain, portable `rel-string` suffices.
The automatic `".ss"` and `".rkt"` conversions apply as with other
forms.

```racket
(submod base element ...+)
                          
base    = module-path     
        | "."             
        | ".."            
                          
element = id              
        | ".."            
```
Refers to a submodule of `base`. The sequence of `element`s within
`submod` specify a path of submodule names to reach the final submodule.
Examples:

```racket
> (module zoo racket                    
    (module monkey-house racket         
      (provide monkey)                  
      (define monkey "Curious George")))
> (require (submod 'zoo monkey-house))  
> monkey                                
"Curious George"                        
```
Using `"."` as `base` within `submod` stands for the enclosing module.
Using `".."` as `base` is equivalent to using `"."` followed by an extra
`".."`. When a path of the form `(quote id)` refers to a submodule, it
is equivalent to `(submod "." id)`.
Using `".."` as an `element` cancels one submodule step, effectively
referring to the enclosing module. For example, `(submod "..")` refers
to the enclosing module of the submodule in which the path appears.
Examples:

```racket
> (module zoo racket                      
    (module monkey-house racket           
      (provide monkey)                    
      (define monkey "Curious George"))   
    (module crocodile-house racket        
      (require (submod ".." monkey-house))
      (provide dinner)                    
      (define dinner monkey)))            
> (require (submod 'zoo crocodile-house)) 
> dinner                                  
"Curious George"                          
```

### 6.4. Imports: `require`

The `require` form imports from another module. A `require` form can
appear within a module, in which case it introduces bindings from the
specified module into importing module. A `require` form can also appear
at the top level, in which case it both imports bindings and
_instantiates_ the specified module; that is, it evaluates the body
definitions and expressions of the specified module, if they have not
been evaluated already.

A single `require` can specify multiple imports at once:

```racket
(require require-spec ...)
```

Specifying multiple `require-spec`s in a single `require` is essentially
the same as using multiple `require`s, each with a single
`require-spec`. The difference is minor, and confined to the top-level:
a single `require` can import a given identifier at most once, whereas a
separate `require` can replace the bindings of a previous `require`
\(both only at the top level, outside of a module\).

The allowed shape of a `require-spec` is defined recursively:

```racket
module-path
```
In its simplest form, a `require-spec` is a `module-path` \(as defined
in the previous section, Module Paths\). In this case, the bindings
introduced by `require` are determined by `provide` declarations within
each module referenced by each `module-path`.
Examples:

```racket
> (module m racket        
    (provide color)       
    (define color "blue"))
> (module n racket        
    (provide size)        
    (define size 17))     
> (require 'm 'n)         
> (list color size)       
'("blue" 17)              
```

```racket
(only-in require-spec id-maybe-renamed ...)
                                           
id-maybe-renamed = id                      
                 | [orig-id bind-id]       
```
An `only-in` form limits the set of bindings that would be introduced by
a base `require-spec`. Also, `only-in` optionally renames each binding
that is preserved: in a `[orig-id bind-id]` form, the `orig-id` refers
to a binding implied by `require-spec`, and `bind-id` is the name that
will be bound in the importing context instead of `orig-id`.
Examples:

```racket
> (module m (lib "racket")                           
    (provide tastes-great?                           
             less-filling?)                          
    (define tastes-great? #t)                        
    (define less-filling? #t))                       
> (require (only-in 'm tastes-great?))               
> tastes-great?                                      
#t                                                   
> less-filling?                                      
less-filling?: undefined;                            
 cannot reference an identifier before its definition
  in module: top-level                               
> (require (only-in 'm [less-filling? lite?]))       
> lite?                                              
#t                                                   
```

```racket
(except-in require-spec id ...)
```
This form is the complement of `only-in`: it excludes specific bindings
from the set specified by `require-spec`.

```racket
(rename-in require-spec [orig-id bind-id] ...)
```
This form supports renaming like `only-in`, but leaving alone
identifiers from `require-spec` that are not mentioned as an `orig-id`.

```racket
(prefix-in prefix-id require-spec)
```
This is a shorthand for renaming, where `prefix-id` is added to the
front of each identifier specified by `require-spec`.

The `only-in`, `except-in`, `rename-in`, and `prefix-in` forms can be
nested to implement more complex manipulations of imported bindings. For
example,

`(require` `(prefix-in` `m:` `(except-in` `'m` `ghost)))`

imports all bindings that `m` exports, except for the `ghost` binding,
and with local names that are prefixed with `m:`.

Equivalently, the `prefix-in` could be applied before `except-in`, as
long as the omission with `except-in` is specified using the `m:`
prefix:

`(require` `(except-in` `(prefix-in` `m:` `'m)` `m:ghost))`

### 6.5. Exports: `provide`

By default, all of a module’s definitions are private to the module. The
`provide` form specifies definitions to be made available where the
module is `require`d.

```racket
(provide provide-spec ...)
```

A `provide` form can only appear at module level \(i.e., in the
immediate body of a `module`\).  Specifying multiple `provide-spec`s in
a single `provide` is exactly the same as using multiple `provide`s each
with a single `provide-spec`.

Each identifier can be exported at most once from a module across all
`provide`s within the module. More precisely, the external name for each
export must be distinct; the same internal binding can be exported
multiple times with different external names.

The allowed shape of a `provide-spec` is defined recursively:

```racket
identifier
```
In its simplest form, a `provide-spec` indicates a binding within its
module to be exported. The binding can be from either a local
definition, or from an import.

```racket
(rename-out [orig-id export-id] ...)
```
A `rename-out` form is similar to just specifying an identifier, but the
exported binding `orig-id` is given a different name, `export-id`, to
importing modules.

```racket
(struct-out struct-id)
```
A `struct-out` form exports the bindings created by `(struct struct-id
....)`.
> +See Programmer-Defined Datatypes for information on `define-struct`.

```racket
(all-defined-out)
```
The `all-defined-out` shorthand exports all bindings that are defined
within the exporting module \(as opposed to imported\).
Use of the `all-defined-out` shorthand is generally discouraged, because
it makes less clear the actual exports for a module, and because Racket
programmers get into the habit of thinking that definitions can be added
freely to a module without affecting its public interface \(which is not
the case when `all-defined-out` is used\).

```racket
(all-from-out module-path)
```
The `all-from-out` shorthand exports all bindings in the module that
were imported using a `require-spec` that is based on `module-path`.
Although different `module-path`s could refer to the same file-based
module, re-exporting with `all-from-out` is based specifically on the
`module-path` reference, and not the module that is actually referenced.

```racket
(except-out provide-spec id ...)
```
Like `provide-spec`, but omitting the export of each `id`, where `id` is
the external name of the binding to omit.

```racket
(prefix-out prefix-id provide-spec)
```
Like `provide-spec`, but adding `prefix-id` to the beginning of the
external name for each exported binding.

### 6.6. Assignment and Redefinition

The use of `set!` on variables defined within a module is limited to the
body of the defining module. That is, a module is allowed to change the
value of its own definitions, and such changes are visible to importing
modules. However, an importing context is not allowed to change the
value of an imported binding.

Examples:

```racket
> (module m racket                            
    (provide counter increment!)              
    (define counter 0)                        
    (define (increment!)                      
      (set! counter (add1 counter))))         
> (require 'm)                                
> counter                                     
0                                             
> (increment!)                                
> counter                                     
1                                             
> (set! counter -1)                           
set!: cannot mutate module-required identifier
  at: counter                                 
  in: (set! counter -1)                       
```

As the above example illustrates, a module can always grant others the
ability to change its exports by providing a mutator function, such as
`increment!`.

The prohibition on assignment of imported variables helps support
modular reasoning about programs. For example, in the module,

```racket
(module m racket                 
  (provide rx:fish fishy-string?)
  (define rx:fish #rx"fish")     
  (define (fishy-string? s)      
    (regexp-match? rx:fish s)))  
```

the function `fishy-string?` will always match strings that contain
“fish”, no matter how other modules use the `rx:fish` binding.  For
essentially the same reason that it helps programmers, the prohibition
on assignment to imports also allows many programs to be executed more
efficiently.

Along the same lines, when a module contains no `set!` of a particular
identifier that is defined within the module, then the identifier is
considered a _constant_ that cannot be changed—not even by re-declaring
the module.

Consequently, re-declaration of a module is not generally allowed. For
file-based modules, simply changing the file does not lead to a
re-declaration in any case, because file-based modules are loaded on
demand, and the previously loaded declarations satisfy future requests.
It is possible to use Racket’s reflection support to re-declare a
module, however, and non-file modules can be re-declared in the REPL; in
such cases, the re-declaration may fail if it involves the re-definition
of a previously constant binding.

```racket
> (module m racket                   
    (define pie 3.141597))           
> (require 'm)                       
> (module m racket                   
    (define pie 3))                  
define-values: assignment disallowed;
 cannot re-define a constant         
  constant: pie                      
  in module: 'm                      
```

For exploration and debugging purposes, the Racket reflective layer
provides a `compile-enforce-module-constants` parameter to disable the
enforcement of constants.

```racket
> (compile-enforce-module-constants #f)
> (module m2 racket                    
    (provide pie)                      
    (define pie 3.141597))             
> (require 'm2)                        
> (module m2 racket                    
    (provide pie)                      
    (define pie 3))                    
> (compile-enforce-module-constants #t)
> pie                                  
3                                      
```

### 6.7. Modules and Macros

Racket’s module system cooperates closely with Racket’s macro system for
adding new syntactic forms to Racket. For example, in the same way that
importing `racket/base` introduces syntax for `require` and `lambda`,
importing other modules can introduce new syntactic forms \(in addition
to more traditional kinds of imports, such as functions or constants\).

We introduce macros in more detail later, in Macros, but here’s a simple
example of a module that defines a pattern-based macro:

```racket
(module noisy racket                                   
  (provide define-noisy)                               
                                                       
  (define-syntax-rule (define-noisy (id arg ...) body) 
    (define (id arg ...)                               
      (show-arguments 'id  (list arg ...))             
      body))                                           
                                                       
  (define (show-arguments name args)                   
    (printf "calling ~s with arguments ~e" name args)))
```

The `define-noisy` binding provided by this module is a macro that acts
like `define` for a function, but it causes each call to the function to
print the arguments that are provided to the function:

```racket
> (require 'noisy)             
> (define-noisy (f x y)        
    (+ x y))                   
> (f 1 2)                      
calling f with arguments '(1 2)
3                              
```

Roughly, the `define-noisy` form works by replacing

```racket
(define-noisy (f x y)
  (+ x y))           
```

with

```racket
(define (f x y)                 
  (show-arguments 'f (list x y))
  (+ x y))                      
```

Since `show-arguments` isn’t provided by the `noisy` module, however,
this literal textual replacement is not quite right. The actual
replacement correctly tracks the origin of identifiers like
`show-arguments`, so they can refer to other definitions in the place
where the macro is defined—even if those identifiers are not available
at the place where the macro is used.

There’s more to the macro and module interaction than identifier
binding. The `define-syntax-rule` form is itself a macro, and it expands
to compile-time code that implements the transformation from
`define-noisy` into `define`. The module system keeps track of which
code needs to run at compile and which needs to run normally, as
explained more in Compile and Run-Time Phases and Module Instantiations
and Visits.

## 7. Contracts

This chapter provides a gentle introduction to Racket’s contract system.

> +\[missing\] in \[missing\] provides more on contracts.

    7.1 Contracts and Boundaries                         
      7.1.1 Contract Violations                          
      7.1.2 Experimenting with Contracts and Modules     
      7.1.3 Experimenting with Nested Contract Boundaries
    7.2 Simple Contracts on Functions                    
      7.2.1 Styles of `->`                               
      7.2.2 Using `define/contract` and `->`             
      7.2.3 `any` and `any/c`                            
      7.2.4 Rolling Your Own Contracts                   
      7.2.5 Contracts on Higher-order Functions          
      7.2.6 Contract Messages with “???”                 
      7.2.7 Dissecting a contract error message          
    7.3 Contracts on Functions in General                
      7.3.1 Optional Arguments                           
      7.3.2 Rest Arguments                               
      7.3.3 Keyword Arguments                            
      7.3.4 Optional Keyword Arguments                   
      7.3.5 Contracts for `case-lambda`                  
      7.3.6 Argument and Result Dependencies             
      7.3.7 Checking State Changes                       
      7.3.8 Multiple Result Values                       
      7.3.9 Fixed but Statically Unknown Arities         
    7.4 Contracts: A Thorough Example                    
    7.5 Contracts on Structures                          
      7.5.1 Guarantees for a Specific Value              
      7.5.2 Guarantees for All Values                    
      7.5.3 Checking Properties of Data Structures       
    7.6 Abstract Contracts using `#:exists` and `#:∃`    
    7.7 Additional Examples                              
      7.7.1 A Customer-Manager Component                 
      7.7.2 A Parameteric \(Simple\) Stack               
      7.7.3 A Dictionary                                 
      7.7.4 A Queue                                      
    7.8 Building New Contracts                           
      7.8.1 Contract Struct Properties                   
      7.8.2 With all the Bells and Whistles              
    7.9 Gotchas                                          
      7.9.1 Contracts and `eq?`                          
      7.9.2 Contract boundaries and `define/contract`    
      7.9.3 Exists Contracts and Predicates              
      7.9.4 Defining Recursive Contracts                 
      7.9.5 Mixing `set!` and `contract-out`             

### 7.1. Contracts and Boundaries

Like a contract between two business partners, a software contract is an
agreement between two parties. The agreement specifies obligations and
guarantees for each “product” \(or value\) that is handed from one party
to the other.

A contract thus establishes a boundary between the two parties. Whenever
a value crosses this boundary, the contract monitoring system performs
contract checks, making sure the partners abide by the established
contract.

In this spirit, Racket encourages contracts mainly at module boundaries.
Specifically, programmers may attach contracts to `provide` clauses and
thus impose constraints and promises on the use of exported values. For
example, the export specification

```racket
#lang racket                               
                                           
(provide (contract-out [amount positive?]))
                                           
(define amount ...)                        
```

promises to all clients of the above module that the value of `amount`
will always be a positive number. The contract system monitors the
module’s obligation carefully. Every time a client refers to `amount`,
the monitor checks that the value of `amount` is indeed a positive
number.

The contracts library is built into the Racket language, but if you wish
to use `racket/base`, you can explicitly require the contracts library
like this:

```racket
#lang racket/base                                     
(require racket/contract) ; now we can write contracts
                                                      
(provide (contract-out [amount positive?]))           
                                                      
(define amount ...)                                   
```

#### 7.1.1. Contract Violations

If we bind `amount` to a number that is not positive,

```racket
#lang racket                               
                                           
(provide (contract-out [amount positive?]))
                                           
(define amount 0)                          
```

then, when the module is required, the monitoring system signals a
violation of the contract and blames the module for breaking its
promises.

An even bigger mistake would be to bind `amount` to a non-number value:

```racket
#lang racket                               
                                           
(provide (contract-out [amount positive?]))
                                           
(define amount 'amount)                    
```

In this case, the monitoring system will apply `positive?` to a symbol,
but `positive?` reports an error, because its domain is only numbers. To
make the contract capture our intentions for all Racket values, we can
ensure that the value is both a number and is positive, combining the
two contracts with `and/c`:

`(provide` `(contract-out` `[amount` `(and/c` `number?` `positive?)]))`

#### 7.1.2. Experimenting with Contracts and Modules

All of the contracts and modules in this chapter \(excluding those just
following\) are written using the standard `#lang` syntax for describing
modules. Since modules serve as the boundary between parties in a
contract, examples involve multiple modules.

To experiment with multiple modules within a single module or within
DrRacket’s definitions area, use Racket’s submodules. For example, try
the example earlier in this section like this:

```racket
#lang racket                                                 
                                                             
(module+ server                                              
  (provide (contract-out [amount (and/c number? positive?)]))
  (define amount 150))                                       
                                                             
(module+ main                                                
  (require (submod ".." server))                             
  (+ amount 10))                                             
```

Each of the modules and their contracts are wrapped in parentheses with
the `module+` keyword at the front. The first form after `module` is the
name of the module to be used in a subsequent `require` statement
\(where each reference through a `require` prefixes the name with
`".."`\).

#### 7.1.3. Experimenting with Nested Contract Boundaries

In many cases, it makes sense to attach contracts at module boundaries.
It is often convenient, however, to be able to use contracts at a finer
granularity than modules. The `define/contract` form enables this kind
of use:

```racket
#lang racket               
                           
(define/contract amount    
  (and/c number? positive?)
  150)                     
                           
(+ amount 10)              
```

In this example, the `define/contract` form establishes a contract
boundary between the definition of `amount` and its surrounding context.
In other words, the two parties here are the definition and the module
that contains it.

Forms that create these _nested contract boundaries_ can sometimes be
subtle to use because they may have unexpected performance implications
or blame a party that may seem unintuitive. These subtleties are
explained in Using `define/contract` and `->` and Contract boundaries
and `define/contract`.

### 7.2. Simple Contracts on Functions

A mathematical function has a _domain_ and a _range_. The domain
indicates the kind of values that the function can accept as arguments,
and the range indicates the kind of values that it produces. The
conventional notation for describing a function with its domain and
range is

`f` `:` `A` `->` `B`

where `A` is the domain of the function and `B` is the range.

Functions in a programming language have domains and ranges, too, and a
contract can ensure that a function receives only values in its domain
and produces only values in its range. A `->` creates such a contract
for a function. The forms after a `->` specify contracts for the domains
and finally a contract for the range.

Here is a module that might represent a bank account:

```racket
#lang racket                                   
                                               
(provide (contract-out                         
          [deposit (-> number? any)]           
          [balance (-> number?)]))             
                                               
(define amount 0)                              
(define (deposit a) (set! amount (+ amount a)))
(define (balance) amount)                      
```

The module exports two functions:

* `deposit`, which accepts a number and returns some value that is not
  specified in the contract, and

* `balance`, which returns a number indicating the current balance of
  the account.

When a module exports a function, it establishes two channels of
communication between itself as a “server” and the “client” module that
imports the function. If the client module calls the function, it sends
a value into the server module. Conversely, if such a function call ends
and the function returns a value, the server module sends a value back
to the client module. This client–server distinction is important,
because when something goes wrong, one or the other of the parties is to
blame.

If a client module were to apply `deposit` to `'millions`, it would
violate the contract.  The contract-monitoring system would catch this
violation and blame the client for breaking the contract with the above
module. In contrast, if the `balance` function were to return `'broke`,
the contract-monitoring system would blame the server module.

A `->` by itself is not a contract; it is a _contract combinator_, which
combines other contracts to form a contract.

#### 7.2.1. Styles of `->`

If you are used to mathematical functions, you may prefer a contract
arrow to appear between the domain and the range of a function, not   at
the beginning. If you have read _[How to Design
Programs](http://www.htdp.org)_, you have seen this many   times.
Indeed, you may have seen contracts such as these in other   people’s
code:

```racket
(provide (contract-out                    
          [deposit (number? . -> . any)]))
```

If a Racket S-expression contains two dots with a symbol in the middle,
the reader re-arranges the S-expression and place the symbol at the
front, as described in Lists and Racket Syntax.  Thus,

`(number?` `. -> .` `any)`

is just another way of writing

`(->` `number?` `any)`

#### 7.2.2. Using `define/contract` and `->`

The `define/contract` form introduced in Experimenting with Nested
Contract Boundaries can also be used to define functions that come with
a contract. For example,

```racket
(define/contract (deposit amount)
  (-> number? any)               
  ; implementation goes here     
  ....)                          
```

which defines the `deposit` function with the contract from earlier.
Note that this has two potentially important impacts on the use of
`deposit`:

* The contract will be checked on any call to `deposit` that is outside
  of the definition of `deposit` – even those inside the module in which
  it is defined. Because there may be many calls inside the module, this
  checking may cause the contract to be checked too often, which could
  lead to a performance degradation. This is especially true if the
  function is called repeatedly from a loop.

* In some situations, a function may be written to accept a more lax set
  of inputs when called by other code in the same module. For such use
  cases, the contract boundary established by `define/contract` is too
  strict.

#### 7.2.3. `any` and `any/c`

The `any` contract used for `deposit` matches any kind of result, and it
can only be used in the range position of a function contract.  Instead
of `any` above, we could use the more specific contract `void?`, which
says that the function will always return the `(void)` value. The
`void?` contract, however, would require the contract monitoring system
to check the return value every time the function is called, even though
the “client” module can’t do much with the value. In contrast, `any`
tells the monitoring system _not_ to check the return value, it tells a
potential client that the “server” module _makes no promises at all_
about the function’s return value, even whether it is a single value or
multiple values.

The `any/c` contract is similar to `any`, in that it makes no demands on
a value. Unlike `any`, `any/c` indicates a single value, and it is
suitable for use as an argument contract. Using `any/c` as a range
contract imposes a check that the function produces a single value. That
is,

`(->` `integer?` `any)`

describes a function that accepts an integer and returns any number of
values, while

`(->` `integer?` `any/c)`

describes a function that accepts an integer and produces a single
result \(but does not say anything more about the result\). The function

`(define` `(f` `x)` `(values` `(+` `x` `1)` `(-` `x` `1)))`

matches `(-> integer? any)`, but not `(-> integer? any/c)`.

Use `any/c` as a result contract when it is particularly important to
promise a single result from a function. Use `any` when you want to
promise as little as possible \(and incur as little checking as
possible\) for a function’s result.

#### 7.2.4. Rolling Your Own Contracts

The `deposit` function adds the given number to the value of `amount`.
While the function’s contract prevents clients from applying it to
non-numbers, the contract still allows them to apply the function to
complex numbers, negative numbers, or inexact numbers, none of which
sensibly represent amounts of money.

The contract system allows programmers to define their own contracts as
functions:

```racket
#lang racket                                         
                                                     
(define (amount? a)                                  
  (and (number? a) (integer? a) (exact? a) (>= a 0)))
                                                     
(provide (contract-out                               
          ; an amount is a natural number of cents   
          ; is the given number an amount?           
          [deposit (-> amount? any)]                 
          [amount? (-> any/c boolean?)]              
          [balance (-> amount?)]))                   
                                                     
(define amount 0)                                    
(define (deposit a) (set! amount (+ amount a)))      
(define (balance) amount)                            
```

This module defines an `amount?` function and uses it as a contract
within `->` contracts. When a client calls the `deposit` function as
exported with the contract `(-> amount? any)`, it must supply an exact,
nonnegative integer, otherwise the `amount?` function applied to the
argument will return `#f`, which will cause the contract-monitoring
system to blame the client. Similarly, the server module must provide an
exact, nonnegative integer as the result of `balance` to remain
blameless.

Of course, it makes no sense to restrict a channel of communication to
values that the client doesn’t understand. Therefore the module also
exports the `amount?` predicate itself, with a contract saying that it
accepts an arbitrary value and returns a boolean.

In this case, we could also have used `natural-number/c` in place of
`amount?`, since it implies exactly the same check:

```racket
(provide (contract-out                       
          [deposit (-> natural-number/c any)]
          [balance (-> natural-number/c)]))  
```

Every function that accepts one argument can be treated as a predicate
and thus used as a contract. For combining existing checks into a new
one, however, contract combinators such as `and/c` and `or/c` are often
useful. For example, here is yet another way to write the contracts
above:

```racket
(define amount/c                                         
  (and/c number? integer? exact? (or/c positive? zero?)))
                                                         
(provide (contract-out                                   
          [deposit (-> amount/c any)]                    
          [balance (-> amount/c)]))                      
```

Other values also serve double duty as contracts.  For example, if a
function accepts a number or `#f`, `(or/c number? #f)` suffices.
Similarly, the `amount/c` contract could have been written with a `0` in
place of `zero?`. If you use a regular expression as a contract, the
contract accepts strings and byte strings that match the regular
expression.

Naturally, you can mix your own contract-implementing functions with
combinators like `and/c`. Here is a module for creating strings from
banking records:

```racket
#lang racket                                              
                                                          
(define (has-decimal? str)                                
  (define L (string-length str))                          
  (and (>= L 3)                                           
       (char=? #\. (string-ref str (- L 3)))))            
                                                          
(provide (contract-out                                    
          ; convert a random number to a string           
          [format-number (-> number? string?)]            
                                                          
          ; convert an amount into a string with a decimal
          ; point, as in an amount of US currency         
          [format-nat (-> natural-number/c                
                          (and/c string? has-decimal?))]))
```

The contract of the exported function `format-number` specifies that the
function consumes a number and produces a string. The contract of the
exported function `format-nat` is more interesting than the one of
`format-number`.  It consumes only natural numbers. Its range contract
promises a string that has a `.` in the third position from the right.

If we want to strengthen the promise of the range contract for
`format-nat` so that it admits only strings with digits and a single
dot, we could write it like this:

```racket
#lang racket                                              
                                                          
(define (digit-char? x)                                   
  (member x '(#\1 #\2 #\3 #\4 #\5 #\6 #\7 #\8 #\9 #\0)))  
                                                          
(define (has-decimal? str)                                
  (define L (string-length str))                          
  (and (>= L 3)                                           
       (char=? #\. (string-ref str (- L 3)))))            
                                                          
(define (is-decimal-string? str)                          
  (define L (string-length str))                          
  (and (has-decimal? str)                                 
       (andmap digit-char?                                
               (string->list (substring str 0 (- L 3))))  
       (andmap digit-char?                                
               (string->list (substring str (- L 2) L)))))
                                                          
....                                                      
                                                          
(provide (contract-out                                    
          ....                                            
          ; convert an  amount (natural number) of cents  
          ; into a dollar-based string                    
          [format-nat (-> natural-number/c                
                          (and/c string?                  
                                 is-decimal-string?))]))  
```

Alternately, in this case, we could use a regular expression as a
contract:

```racket
#lang racket                                                  
                                                              
(provide                                                      
 (contract-out                                                
  ....                                                        
  ; convert an  amount (natural number) of cents              
  ; into a dollar-based string                                
  [format-nat (-> natural-number/c                            
                  (and/c string? #rx"[0-9]*\\.[0-9][0-9]"))]))
```

#### 7.2.5. Contracts on Higher-order Functions

Function contracts are not just restricted to having simple predicates
on their domains or ranges. Any of the contract combinators discussed
here, including function contracts themselves, can be used as contracts
on the arguments and results of a function.

For example,

`(->` `integer?` `(->` `integer?` `integer?))`

is a contract that describes a curried function. It matches functions
that accept one argument and then return another function accepting a
second argument before finally returning an integer. If a server exports
a function `make-adder` with this contract, and if `make-adder` returns
a value other than a function, then the server is to blame. If
`make-adder` does return a function, but the resulting function is
applied to a value other than an integer, then the client is to blame.

Similarly, the contract

`(->` `(->` `integer?` `integer?)` `integer?)`

describes functions that accept other functions as its input. If a
server exports a function `twice` with this contract and the `twice` is
applied to a value other than a function of one argument, then the
client is to blame. If `twice` is applied to a function of one argument
and `twice` calls the given function on a value other than an integer,
then the server is to blame.

#### 7.2.6. Contract Messages with “???”

You wrote your module. You added contracts. You put them into the
interface so that client programmers have all the information from
interfaces. It’s a piece of art:

```racket
> (module bank-server racket                                 
    (provide                                                 
     (contract-out                                           
      [deposit (-> (λ (x)                                    
                     (and (number? x) (integer? x) (>= x 0)))
                   any)]))                                   
                                                             
    (define total 0)                                         
    (define (deposit a) (set! total (+ a total))))           
```

Several clients used your module. Others used their modules in turn. And
all of a sudden one of them sees this error message:

```racket
> (require 'bank-server)             
> (deposit -10)                      
deposit: contract violation          
  expected: ???                      
  given: -10                         
  in: the 1st argument of            
      (-> ??? any)                   
  contract from: bank-server         
  blaming: top-level                 
   (assuming the contract is correct)
  at: eval:2.0                       
```

What is the `???` doing there?  Wouldn’t it be nice if we had a name for
this class of data much like we have string, number, and so on?

For this situation, Racket provides _flat named contracts_. The use of
“contract” in this term shows that contracts are first-class values. The
“flat” means that the collection of data is a subset of the built-in
atomic classes of data; they are described by a predicate that consumes
all Racket values and produces a boolean. The “named” part says what we
want to do, which is to name the contract so that error messages become
intelligible:

```racket
> (module improved-bank-server racket                          
    (provide                                                   
     (contract-out                                             
      [deposit (-> (flat-named-contract                        
                    'amount                                    
                    (λ (x)                                     
                      (and (number? x) (integer? x) (>= x 0))))
                   any)]))                                     
                                                               
    (define total 0)                                           
    (define (deposit a) (set! total (+ a total))))             
```

With this little change, the error message becomes quite readable:

```racket
> (require 'improved-bank-server)    
> (deposit -10)                      
deposit: contract violation          
  expected: amount                   
  given: -10                         
  in: the 1st argument of            
      (-> amount any)                
  contract from: improved-bank-server
  blaming: top-level                 
   (assuming the contract is correct)
  at: eval:5.0                       
```

#### 7.2.7. Dissecting a contract error message

In general, each contract error message consists of six sections:

* a name for the function or method associated with the contract and
  either the phrase “contract violation” or “broke its contract”
  depending on whether the contract was violated by the client or the
  server; e.g. in the previous example:

  deposit: contract violation

* a description of the precise aspect of the contract that was violated,

  expected: amount
  given: -10      

* the complete contract plus a path into it showing which aspect was
  violated,

  in: the 1st argument of
  \(-> amount any\)      

* the module where the contract was put \(or, more generally, the
  boundary that the contract mediates\),

  contract from: improved-bank-server

* who was blamed,

  blaming: top-level                  
  \(assuming the contract is correct\)

* and the source location where the contract appears.

  at: eval:5.0

### 7.3. Contracts on Functions in General

The `->` contract constructor works for functions that take a fixed
number of arguments and where the result contract is independent of the
input arguments. To support other kinds of functions, Racket supplies
additional contract constructors, notably `->*` and `->i`.

#### 7.3.1. Optional Arguments

Take a look at this excerpt from a string-processing module, inspired by
the [Scheme cookbook](http://schemecookbook.org):

```racket
#lang racket                                            
                                                        
(provide                                                
 (contract-out                                          
  ; pad the given str left and right with               
  ; the (optional) char so that it is centered          
  [string-pad-center (->* (string? natural-number/c)    
                          (char?)                       
                          string?)]))                   
                                                        
(define (string-pad-center str width [pad #\space])     
  (define field-width (min width (string-length str)))  
  (define rmargin (ceiling (/ (- width field-width) 2)))
  (define lmargin (floor (/ (- width field-width) 2)))  
  (string-append (build-string lmargin (λ (x) pad))     
                 str                                    
                 (build-string rmargin (λ (x) pad))))   
```

The module exports `string-pad-center`, a function  that creates a
string of a given `width` with the  given string in the center. The
default fill character is  `#\space`; if the client module wishes to use
a  different character, it may call `string-pad-center`  with a third
argument, a `char`, overwriting the  default.

The function definition uses optional arguments, which is appropriate
for this kind of functionality. The interesting point here is the
formulation of the contract for the `string-pad-center`.

The contract combinator `->*`, demands several groups of contracts:

* The first one is a parenthesized group of contracts for all required
  arguments. In this example, we see two: `string?` and
  `natural-number/c`.

* The second one is a parenthesized group of contracts for all optional
  arguments: `char?`.

* The last one is a single contract: the result of the function.

Note that if a default value does not satisfy a contract, you won’t get
a  contract error for this interface. If you can’t trust yourself to get
the initial value right, you need to communicate the initial value
across a boundary.

#### 7.3.2. Rest Arguments

The `max` operator consumes at least one real number, but it  accepts
any number of additional arguments. You can write other such  functions
using a rest argument, such as in `max-abs`:

> See Declaring a Rest Argument for an introduction to rest arguments.

```racket
(define (max-abs n . rst)                            
  (foldr (lambda (n m) (max (abs n) m)) (abs n) rst))
```

Describing this function through a contract requires a further extension
of `->*`: a `#:rest` keyword specifies a contract on a list of arguments
after the required and optional arguments:

```racket
(provide                                                  
 (contract-out                                            
  [max-abs (->* (real?) () #:rest (listof real?) real?)]))
```

As always for `->*`, the contracts for the required arguments are
enclosed in the first pair of parentheses, which in this case is a
single real number. The empty pair of parenthesis indicates that there
are no optional arguments \(not counting the rest arguments\). The
contract for the rest argument follows `#:rest`; since all additional
arguments must be real numbers, the list of rest arguments must satisfy
the contract `(listof real?)`.

#### 7.3.3. Keyword Arguments

It turns out that the `->` contract constructor also contains support
for keyword arguments. For example, consider this function, which
creates a simple GUI and asks the user a yes-or-no question:

> See Declaring Keyword Arguments for an introduction to keyword
> arguments.

```racket
#lang racket/gui                                             
                                                             
(define (ask-yes-or-no-question question                     
                                #:default answer             
                                #:title title                
                                #:width w                    
                                #:height h)                  
  (define d (new dialog% [label title] [width w] [height h]))
  (define msg (new message% [label question] [parent d]))    
  (define (yes) (set! answer #t) (send d show #f))           
  (define (no) (set! answer #f) (send d show #f))            
  (define yes-b (new button%                                 
                     [label "Yes"] [parent d]                
                     [callback (λ (x y) (yes))]              
                     [style (if answer '(border) '())]))     
  (define no-b (new button%                                  
                    [label "No"] [parent d]                  
                    [callback (λ (x y) (no))]                
                    [style (if answer '() '(border))]))      
  (send d show #t)                                           
  answer)                                                    
                                                             
(provide (contract-out                                       
          [ask-yes-or-no-question                            
           (-> string?                                       
               #:default boolean?                            
               #:title string?                               
               #:width exact-integer?                        
               #:height exact-integer?                       
               boolean?)]))                                  
```

> If you really want to ask a yes-or-no question via a GUI, you should use
> `message-box/custom`. For that matter, it’s usually better to provide
> buttons with more specific answers than “yes” and “no.”

The contract for `ask-yes-or-no-question` uses `->`, and in the same way
that `lambda` \(or `define`-based functions\) allows a keyword to
precede a functions formal argument, `->` allows a keyword to precede a
function contract’s argument contract. In this case, the contract says
that `ask-yes-or-no-question` must receive four keyword arguments, one
for each of the keywords `#:default`, `#:title`, `#:width`, and
`#:height`. As in a function definition, the order of the keywords in
`->` relative to each other does not matter for clients of the function;
only the relative order of argument contracts without keywords matters.

#### 7.3.4. Optional Keyword Arguments

Of course, many of the parameters in `ask-yes-or-no-question` \(from the
previous question\) have reasonable defaults and should be made
optional:

```racket
(define (ask-yes-or-no-question question                    
                                #:default answer            
                                #:title [title "Yes or No?"]
                                #:width [w 400]             
                                #:height [h 200])           
  ...)                                                      
```

To specify this function’s contract, we need to use `->*` again. It
supports keywords just as you might expect in both the optional and
mandatory argument sections. In this case, we have the mandatory keyword
`#:default` and optional keywords `#:title`, `#:width`, and `#:height`.
So, we write the contract like this:

```racket
(provide (contract-out                   
          [ask-yes-or-no-question        
           (->* (string?                 
                 #:default boolean?)     
                (#:title string?         
                 #:width exact-integer?  
                 #:height exact-integer?)
                                         
                boolean?)]))             
```

That is, we put the mandatory keywords in the first section, and we put
the optional ones in the second section.

#### 7.3.5. Contracts for `case-lambda`

A function defined with `case-lambda` might impose different constraints
on its arguments depending on how many are provided. For example, a
`report-cost` function might convert either a pair of numbers or a
string into a new string:

> See Arity-Sensitive Functions: `case-lambda` for an introduction to
> `case-lambda`.

```racket                                         
(define report-cost                               
  (case-lambda                                    
    [(lo hi) (format "between $~a and $~a" lo hi)]
    [(desc) (format "~a of dollars" desc)]))      
```                                               
                                                  
```racket                                         
> (report-cost 5 8)                               
"between $5 and $8"                               
> (report-cost "millions")                        
"millions of dollars"                             
```                                               

The contract for such a function is formed with the `case->`
combinator, which combines as many functional contracts as needed:

```racket
(provide (contract-out                        
          [report-cost                        
           (case->                            
            (integer? integer? . -> . string?)
            (string? . -> . string?))]))      
```

As you can see, the contract for `report-cost` combines two  function
contracts, which is just as many clauses as the explanation  of its
functionality required.

#### 7.3.6. Argument and Result Dependencies

The following is an excerpt from an imaginary numerics module:

```racket
(provide                                                 
 (contract-out                                           
  [real-sqrt (->i ([argument (>=/c 1)])                  
                  [result (argument) (<=/c argument)])]))
```

> The word “indy” is meant to suggest that blame may be assigned to the
> contract itself, because the contract must be considered an independent
> component. The name was chosen in response to two existing labels—“lax”
> and “picky”—for different semantics of function contracts in the
> research literature.

The contract for the exported function `real-sqrt` uses the `->i` rather
than `->*` function contract. The “i” stands for an _indy dependent_
contract, meaning the contract for the function range depends on the
value of the argument. The appearance of `argument` in the line for
`result`’s contract means that the result depends on the argument. In
this particular case, the argument of `real-sqrt` is greater or equal to
1, so a very basic correctness check is that the result is smaller than
the argument.

In general, a dependent function contract looks just like the more
general `->*` contract, but with names added that can be used elsewhere
in the contract.

Going back to the bank-account example, suppose that we generalize the
module to support multiple accounts and that we also include a
withdrawal operation. The improved bank-account module includes an
`account` structure type and the following functions:

```racket
(provide (contract-out                               
          [balance (-> account? amount/c)]           
          [withdraw (-> account? amount/c account?)] 
          [deposit (-> account? amount/c account?)]))
```

Besides requiring that a client provide a valid amount for a withdrawal,
however, the amount should be less than or equal to the specified
account’s balance, and the resulting account will have less money than
it started with. Similarly, the module might promise that a deposit
produces an account with money added to the account. The following
implementation enforces those constraints and guarantees through
contracts:

```racket
#lang racket                                                        
                                                                    
; section 1: the contract definitions                               
(struct account (balance))                                          
(define amount/c natural-number/c)                                  
                                                                    
; section 2: the exports                                            
(provide                                                            
 (contract-out                                                      
  [create   (amount/c . -> . account?)]                             
  [balance  (account? . -> . amount/c)]                             
  [withdraw (->i ([acc account?]                                    
                  [amt (acc) (and/c amount/c (<=/c (balance acc)))])
                 [result (acc amt)                                  
                         (and/c account?                            
                                (lambda (res)                       
                                  (>= (balance res)                 
                                      (- (balance acc) amt))))])]   
  [deposit  (->i ([acc account?]                                    
                  [amt amount/c])                                   
                 [result (acc amt)                                  
                         (and/c account?                            
                                (lambda (res)                       
                                  (>= (balance res)                 
                                      (+ (balance acc) amt))))])])) 
                                                                    
; section 3: the function definitions                               
(define balance account-balance)                                    
                                                                    
(define (create amt) (account amt))                                 
                                                                    
(define (withdraw a amt)                                            
  (account (- (account-balance a) amt)))                            
                                                                    
(define (deposit a amt)                                             
  (account (+ (account-balance a) amt)))                            
```

The contracts in section 2 provide typical type-like guarantees for
`create` and `balance`. For `withdraw` and `deposit`, however, the
contracts check and guarantee the more complicated constraints on
`balance` and `deposit`.  The contract on the second argument to
`withdraw` uses `(balance acc)` to check whether the supplied withdrawal
amount is small enough, where `acc` is the name given within `->i` to
the function’s first argument. The contract on the result of `withdraw`
uses both `acc` and `amt` to guarantee that no more than that requested
amount was withdrawn. The contract on `deposit` similarly uses `acc` and
`amount` in the result contract to guarantee that at least as much money
as provided was deposited into the account.

As written above, when a contract check fails, the error message is not
great. The following revision uses `flat-named-contract` within a helper
function `mk-account-contract` to provide better error messages.

```racket
#lang racket                                                               
                                                                           
; section 1: the contract definitions                                      
(struct account (balance))                                                 
(define amount/c natural-number/c)                                         
                                                                           
(define msg> "account a with balance larger than ~a expected")             
(define msg< "account a with balance less than ~a expected")               
                                                                           
(define (mk-account-contract acc amt op msg)                               
  (define balance0 (balance acc))                                          
  (define (ctr a)                                                          
    (and (account? a) (op balance0 (balance a))))                          
  (flat-named-contract (format msg balance0) ctr))                         
                                                                           
; section 2: the exports                                                   
(provide                                                                   
 (contract-out                                                             
  [create   (amount/c . -> . account?)]                                    
  [balance  (account? . -> . amount/c)]                                    
  [withdraw (->i ([acc account?]                                           
                  [amt (acc) (and/c amount/c (<=/c (balance acc)))])       
                 [result (acc amt) (mk-account-contract acc amt >= msg>)])]
  [deposit  (->i ([acc account?]                                           
                  [amt amount/c])                                          
                 [result (acc amt)                                         
                         (mk-account-contract acc amt <= msg<)])]))        
                                                                           
; section 3: the function definitions                                      
(define balance account-balance)                                           
                                                                           
(define (create amt) (account amt))                                        
                                                                           
(define (withdraw a amt)                                                   
  (account (- (account-balance a) amt)))                                   
                                                                           
(define (deposit a amt)                                                    
  (account (+ (account-balance a) amt)))                                   
```

#### 7.3.7. Checking State Changes

The `->i` contract combinator can also ensure that a function only
modifies state according to certain constraints. For example, consider
this contract \(it is a slightly simplified version from the function
`preferences:add-panel` in the framework\):

```racket
(->i ([parent (is-a?/c area-container-window<%>)])     
      [_ (parent)                                      
       (let ([old-children (send parent get-children)])
         (λ (child)                                    
           (andmap eq?                                 
                   (append old-children (list child))  
                   (send parent get-children))))])     
```

It says that the function accepts a single argument, named `parent`, and
that `parent` must be an object matching the interface
`area-container-window<%>`.

The range contract ensures that the function only modifies the children
of `parent` by adding a new child to the front of the list. It
accomplishes this by using the `_` instead of a normal identifier, which
tells the contract library that the range contract does not depend on
the values of any of the results, and thus the contract library
evaluates the expression following the `_` when the function is called,
instead of when it returns. Therefore the call to the `get-children`
method happens before the function under the contract is called. When
the function under contract returns, its result is passed in as `child`,
and the contract ensures that the children after the function return are
the same as the children before the function called, but with one more
child, at the front of the list.

To see the difference in a toy example that focuses on this point,
consider this program

```racket
#lang racket                                           
(define x '())                                         
(define (get-x) x)                                     
(define (f) (set! x (cons 'f x)))                      
(provide                                               
 (contract-out                                         
  [f (->i () [_ (begin (set! x (cons 'ctc x)) any/c)])]
  [get-x (-> (listof symbol?))]))                      
```

If you were to require this module, call `f`, then the result of `get-x`
would be `'(f ctc)`. In contrast, if the contract for `f` were

`(->i` `()` `[res` `(begin` `(set!` `x` `(cons` `'ctc` `x))` `any/c)])`

\(only changing the underscore to `res`\), then the result of `get-x`
would be `'(ctc f)`.

#### 7.3.8. Multiple Result Values

The function `split` consumes a list of `char`s   and delivers the
string that occurs before the first occurrence of   `#\newline` \(if
any\) and the rest of the list:

```racket
(define (split l)                                        
  (define (split l w)                                    
    (cond                                                
      [(null? l) (values (list->string (reverse w)) '())]
      [(char=? #\newline (car l))                        
       (values (list->string (reverse w)) (cdr l))]      
      [else (split (cdr l) (cons (car l) w))]))          
  (split l '()))                                         
```

It is a typical multiple-value function, returning two values by
traversing a single list.

The contract for such a function can use the ordinary function arrow
`->`, since `->` treats `values` specially when it appears as the last
result:

```racket
(provide (contract-out                                  
          [split (-> (listof char?)                     
                     (values string? (listof char?)))]))
```

The contract for such a function can also be written using `->*`:

```racket
(provide (contract-out                                   
          [split (->* ((listof char?))                   
                      ()                                 
                      (values string? (listof char?)))]))
```

As before, the contract for the argument with `->*` is wrapped in an
extra pair of parentheses \(and must always be wrapped like  that\) and
the empty pair of parentheses indicates that  there are no optional
arguments. The contracts for the  results are inside `values`: a string
and a list of  characters.

Now, suppose that we also want to ensure that the first result of
`split` is a prefix of the given word in list format. In that  case, we
need to use the `->i` contract combinator:

```racket
(define (substring-of? s)                                       
  (flat-named-contract                                          
    (format "substring of ~s" s)                                
    (lambda (s2)                                                
      (and (string? s2)                                         
           (<= (string-length s2) (string-length s))            
           (equal? (substring s 0 (string-length s2)) s2)))))   
                                                                
(provide                                                        
 (contract-out                                                  
  [split (->i ([fl (listof char?)])                             
              (values [s (fl) (substring-of? (list->string fl))]
                      [c (listof char?)]))]))                   
```

Like `->*`, the `->i` combinator uses a function over the  argument to
create the range contracts. Yes, it doesn’t just return one  contract
but as many as the function produces values: one contract per  value.
In this case, the second contract is the same as before, ensuring  that
the second result is a list of `char`s. In contrast, the  first contract
strengthens the old one so that the result is a prefix of  the given
word.

This contract is expensive to check, of course. Here is a slightly
cheaper version:

```racket
(provide                                                 
 (contract-out                                           
  [split (->i ([fl (listof char?)])                      
              (values [s (fl) (string-len/c (length fl))]
                      [c (listof char?)]))]))            
```

#### 7.3.9. Fixed but Statically Unknown Arities

Imagine yourself writing a contract for a function that accepts some
other function and a list of numbers that eventually applies the former
to the latter. Unless the arity of the given function matches the length
of the given list, your procedure is in trouble.

Consider this `n-step` function:

```racket
; (number ... -> (union #f number?)) (listof number) -> void
(define (n-step proc inits)                                 
  (let ([inc (apply proc inits)])                           
    (when inc                                               
      (n-step proc (map (λ (x) (+ x inc)) inits)))))        
```

The argument of `n-step` is `proc`, a function `proc` whose results are
either numbers or false, and a list. It then applies `proc` to the list
`inits`. As long as `proc` returns a number, `n-step` treats that number
as an increment for each of the numbers in `inits` and recurs. When
`proc` returns `false`, the loop stops.

Here are two uses:

```racket
; nat -> nat                  
(define (f x)                 
  (printf "~s\n" x)           
  (if (= x 0) #f -1))         
(n-step f '(2))               
                              
; nat nat -> nat              
(define (g x y)               
  (define z (+ x y))          
  (printf "~s\n" (list x y z))
  (if (= z 0) #f -1))         
                              
(n-step g '(1 1))             
```

A contract for `n-step` must specify two aspects of `proc`’s behavior:
its arity must include the number of elements in `inits`, and it must
return either a number or `#f`. The latter is easy, the former is
difficult. At first glance, this appears to suggest a contract that
assigns a _variable-arity_ to `proc`:

```racket
(->* ()                     
     #:rest (listof any/c)  
     (or/c number? false/c))
```

This contract, however, says that the function must accept _any_ number
of arguments, not a _specific_ but _undetermined_ number. Thus, applying
`n-step` to `(lambda (x) x)` and `(list 1)` breaks the contract because
the given function accepts only one argument.

The correct contract uses the `unconstrained-domain->`  combinator,
which specifies only the range of a function, not its  domain. It is
then possible to combine this contract with an arity test to  specify
the correct contract for `n-step`:

```racket
(provide                                          
 (contract-out                                    
  [n-step                                         
   (->i ([proc (inits)                            
          (and/c (unconstrained-domain->          
                  (or/c false/c number?))         
                 (λ (f) (procedure-arity-includes?
                         f                        
                         (length inits))))]       
         [inits (listof number?)])                
        ()                                        
        any)]))                                   
```

### 7.4. Contracts: A Thorough Example

This section develops several different flavors of contracts for one and
the same example: Racket’s `argmax` function. According to  its Racket
documentation, the function  consumes a procedure `proc` and  a
non-empty list of values, `lst`. It

returns the _first_ element in the list `lst` that maximizes the result
of `proc`.

The emphasis on _first_ is ours.

Examples:

```racket
> (argmax add1 (list 1 2 3))                      
3                                                 
> (argmax sqrt (list 0.4 0.9 0.16))               
0.9                                               
> (argmax second '((a 2) (b 3) (c 4) (d 1) (e 4)))
'(c 4)                                            
```

Here is the simplest possible contract for this function:

`version 1`
```racket
#lang racket                                                
                                                            
(define (argmax f lov) ...)                                 
                                                            
(provide                                                    
 (contract-out                                              
  [argmax (-> (-> any/c real?) (and/c pair? list?) any/c)]))
```

This contract captures two essential conditions of the informal
description of `argmax`:

* the given function must produce numbers that are comparable according
  to `<`. In particular, the contract `(-> any/c number?)` would not do,
  because `number?` also recognizes complex numbers in Racket.

* the given list must contain at least one item.

When combined with the name, the contract explains the behavior of
`argmax` at the same level as an ML function type in a  module signature
\(except for the non-empty list aspect\).

Contracts may communicate significantly more than a type signature,
however. Take a look at this second contract for `argmax`:

`version 2`
```racket
#lang racket                                                
                                                            
(define (argmax f lov) ...)                                 
                                                            
(provide                                                    
 (contract-out                                              
  [argmax                                                   
    (->i ([f (-> any/c real?)] [lov (and/c pair? list?)]) ()
         (r (f lov)                                         
            (lambda (r)                                     
              (define f@r (f r))                            
              (for/and ([v lov]) (>= f@r (f v))))))]))      
```

It is a _dependent_ contract that names the two arguments and uses  the
names to impose a predicate on the result. This predicate computes  `(f
r)` – where `r` is the result of `argmax` – and  then validates that
this value is greater than or equal to all values  of `f` on the items
of `lov`.

Is it possible that `argmax` could cheat by returning a random value
that accidentally maximizes `f` over all elements of `lov`?  With a
contract, it is possible to rule out this possibility:

`version 2 rev. a`
```racket
#lang racket                                                
                                                            
(define (argmax f lov) ...)                                 
                                                            
(provide                                                    
 (contract-out                                              
  [argmax                                                   
    (->i ([f (-> any/c real?)] [lov (and/c pair? list?)]) ()
         (r (f lov)                                         
            (lambda (r)                                     
              (define f@r (f r))                            
              (and (memq r lov)                             
                   (for/and ([v lov]) (>= f@r (f v)))))))]))
```

The `memq` function ensures that `r` is _intensionally equal_  That is,
"pointer equality" for those who prefer to think at the hardware level.
to one of the members of `lov`. Of course, a  moment’s worth of
reflection shows that it is impossible to make up such a  value.
Functions are opaque values in Racket and without applying a  function,
it is impossible to determine whether some random input value  produces
an output value or triggers some exception. So we ignore this
possibility from here on.

Version 2 formulates the overall sentiment of `argmax`’s  documentation,
but it fails to bring across that the result is the  _first_ element of
the given list that maximizes the given function  `f`. Here is a version
that communicates this second aspect of  the informal documentation:

`version 3`
```racket
#lang racket                                                         
                                                                     
(define (argmax f lov) ...)                                          
                                                                     
(provide                                                             
 (contract-out                                                       
  [argmax                                                            
    (->i ([f (-> any/c real?)] [lov (and/c pair? list?)]) ()         
         (r (f lov)                                                  
            (lambda (r)                                              
              (define f@r (f r))                                     
              (and (for/and ([v lov]) (>= f@r (f v)))                
                   (eq? (first (memf (lambda (v) (= (f v) f@r)) lov))
                        r)))))]))                                    
```

That is, the `memf` function determines the first element of  `lov`
whose value under `f` is equal to `r`’s value  under `f`. If this
element is intensionally equal to `r`,  the result of `argmax` is
correct.

This second refinement step introduces two problems. First, both
conditions  recompute the values of `f` for all elements of `lov`.
Second,  the contract is now quite difficult to read. Contracts should
have a concise  formulation that a client can comprehend with a simple
scan. Let us  eliminate the readability problem with two auxiliary
functions that have  reasonably meaningful names:

`version 3 rev. a`
```racket
#lang racket                                                
                                                            
(define (argmax f lov) ...)                                 
                                                            
(provide                                                    
 (contract-out                                              
  [argmax                                                   
    (->i ([f (-> any/c real?)] [lov (and/c pair? list?)]) ()
         (r (f lov)                                         
            (lambda (r)                                     
              (define f@r (f r))                            
              (and (is-first-max? r f@r f lov)              
                   (dominates-all f@r f lov)))))]))         
                                                            
; where                                                     
                                                            
; f@r is greater or equal to all (f v) for v in lov         
(define (dominates-all f@r f lov)                           
  (for/and ([v lov]) (>= f@r (f v))))                       
                                                            
; r is eq? to the first element v of lov for which (pred? v)
(define (is-first-max? r f@r f lov)                         
  (eq? (first (memf (lambda (v) (= (f v) f@r)) lov)) r))    
```

The names of the two predicates express their functionality and, in
principle, render it unnecessary to read their definitions.

This step leaves us with the problem of the newly introduced
inefficiency.  To avoid the recomputation of `(f v)` for all `v` on
`lov`, we change the contract so that it computes these values and
reuses them as needed:

`version 3 rev. b`
```racket
#lang racket                                                        
                                                                    
(define (argmax f lov) ...)                                         
                                                                    
(provide                                                            
 (contract-out                                                      
  [argmax                                                           
    (->i ([f (-> any/c real?)] [lov (and/c pair? list?)]) ()        
         (r (f lov)                                                 
            (lambda (r)                                             
              (define f@r (f r))                                    
              (define flov (map f lov))                             
              (and (is-first-max? r f@r (map list lov flov))        
                   (dominates-all f@r flov)))))]))                  
                                                                    
; where                                                             
                                                                    
; f@r is greater or equal to all f@v in flov                        
(define (dominates-all f@r flov)                                    
  (for/and ([f@v flov]) (>= f@r f@v)))                              
                                                                    
; r is (first x) for the first x in lov+flov s.t. (= (second x) f@r)
(define (is-first-max? r f@r lov+flov)                              
  (define fst (first lov+flov))                                     
  (if (= (second fst) f@r)                                          
      (eq? (first fst) r)                                           
      (is-first-max? r f@r (rest lov+flov))))                       
```

Now the predicate on the result once again computes all values of `f`
for elements of `lov` once.

> The word "eager" comes from the literature on the linguistics of
> contracts.

Version 3 may still be too eager when it comes to calling `f`. While
Racket’s `argmax` always calls `f` no matter how many items  `lov`
contains, let us imagine for illustrative purposes that our  own
implementation first checks whether the list is a singleton.  If so,
the first element would be the only element of `lov` and in that  case
there would be no need to compute `(f r)`. The `argmax` of Racket
implicitly argues that it not only promises the first value that
maximizes `f` over `lov` but also that `f` produces/produced a value for
the result.  As a matter of fact, since `f` may diverge or raise an
exception  for some inputs, `argmax` should avoid calling `f` when
possible.

The following contract demonstrates how a higher-order dependent
contract  needs to be adjusted so as to avoid being over-eager:

`version 4`
```racket
#lang racket                                                        
                                                                    
(define (argmax f lov)                                              
  (if (empty? (rest lov))                                           
      (first lov)                                                   
      ...))                                                         
                                                                    
(provide                                                            
 (contract-out                                                      
  [argmax                                                           
    (->i ([f (-> any/c real?)] [lov (and/c pair? list?)]) ()        
         (r (f lov)                                                 
            (lambda (r)                                             
              (cond                                                 
                [(empty? (rest lov)) (eq? (first lov) r)]           
                [else                                               
                 (define f@r (f r))                                 
                 (define flov (map f lov))                          
                 (and (is-first-max? r f@r (map list lov flov))     
                      (dominates-all f@r flov))]))))]))             
                                                                    
; where                                                             
                                                                    
; f@r is greater or equal to all f@v in flov                        
(define (dominates-all f@r lov) ...)                                
                                                                    
; r is (first x) for the first x in lov+flov s.t. (= (second x) f@r)
(define (is-first-max? r f@r lov+flov) ...)                         
```

Note that such considerations don’t apply to the world of first-order
contracts. Only a higher-order \(or lazy\) language forces the
programmer to  express contracts with such precision.

The problem of diverging or exception-raising functions should alert the
reader to the even more general problem of functions with side-effects.
If  the given function `f` has visible effects – say it logs its calls
to a file – then the clients of `argmax` will be able to observe  two
sets of logs for each call to `argmax`. To be precise, if the  list of
values contains more than one element, the log will contain two  calls
of `f` per value on `lov`. If `f` is expensive  to compute, doubling the
calls imposes a high cost.

To avoid this cost and to signal problems with overly eager contracts, a
contract system could record the i/o of contracted function arguments
and  use these hashtables in the dependency specification. This is a
topic of  on-going research in PLT. Stay tuned.

### 7.5. Contracts on Structures

Modules deal with structures in two ways. First they export `struct`
definitions, i.e., the ability to create structs of a certain kind, to
access their fields, to modify them, and to distinguish structs of this
kind against every other kind of value in the world. Second, on occasion
a module exports a specific struct and wishes to promise that its fields
contain values of a certain kind. This section explains how to protect
structs with contracts for both uses.

#### 7.5.1. Guarantees for a Specific Value

If your module defines a variable to be a structure, then you can
specify the structure’s shape using `struct/c`:

```racket
#lang racket                                    
(require lang/posn)                             
                                                
(define origin (make-posn 0 0))                 
                                                
(provide (contract-out                          
          [origin (struct/c posn zero? zero?)]))
```

In this example, the module imports a library for representing
positions, which exports a `posn` structure. One of the `posn`s it
creates and exports stands for the origin, i.e., `(0,0)`, of the grid.

> See also `vector/c` and similar contract combinators for \(flat\)
> compound data.

#### 7.5.2. Guarantees for All Values

The book _[How to Design Programs](http://www.htdp.org)_ teaches that
`posn`s should contain only numbers in their two fields. With contracts
we would enforce this informal data definition as follows:

```racket
#lang racket                                     
(struct posn (x y))                              
                                                 
(provide (contract-out                           
          [struct posn ((x number?) (y number?))]
          [p-okay posn?]                         
          [p-sick posn?]))                       
                                                 
(define p-okay (posn 10 20))                     
(define p-sick (posn 'a 'b))                     
```

This module exports the entire structure definition: `posn`, `posn?`,
`posn-x`, `posn-y`, `set-posn-x!`, and `set-posn-y!`. Each function
enforces or promises that the two fields of a `posn` structure are
numbers — when the values flow across the module boundary.  Thus, if a
client calls `posn` on `10` and `'a`, the contract system signals a
contract violation.

The creation of `p-sick` inside of the `posn` module, however, does not
violate the contracts. The function `posn` is used internally, so `'a`
and `'b` don’t cross the module boundary. Similarly, when `p-sick`
crosses the boundary of `posn`, the contract promises a `posn?` and
nothing else. In particular, this check does _not_ require that the
fields of `p-sick` are numbers.

The association of contract checking with module boundaries implies that
`p-okay` and `p-sick` look alike from a client’s perspective until the
client extracts the pieces:

```racket
#lang racket           
(require lang/posn)    
                       
... (posn-x p-sick) ...
```

Using `posn-x` is the only way the client can find out what a `posn`
contains in the `x` field. The application of `posn-x` sends `p-sick`
back into the `posn` module and the result value – `'a` here – back to
the client, again across the module boundary. At this very point, the
contract system discovers that a promise is broken. Specifically,
`posn-x` doesn’t return a number but a symbol and is therefore blamed.

This specific example shows that the explanation for a contract
violation doesn’t always pinpoint the source of the error. The good news
is that the error is located in the `posn` module. The bad news is that
the explanation is misleading. Although it is true that `posn-x`
produced a symbol instead of a number, it is the fault of the programmer
who created a `posn` from symbols, i.e., the programmer who added

`(define` `p-sick` `(posn` `'a` `'b))`

to the module. So, when you are looking for bugs based on contract
violations, keep this example in mind.

If we want to fix the contract for `p-sick` so that the error is caught
when `sick` is exported, a single change suffices:

```racket
(provide                                    
 (contract-out                              
  ...                                       
  [p-sick (struct/c posn number? number?)]))
```

That is, instead of exporting `p-sick` as a plain `posn?`, we use a
`struct/c` contract to enforce constraints on its components.

#### 7.5.3. Checking Properties of Data Structures

Contracts written using `struct/c` immediately check the fields of the
data structure, but sometimes this can have disastrous effects on the
performance of a program that does not, itself, inspect the entire data
structure.

As an example, consider the binary search tree search algorithm. A
binary search tree is like a binary tree, except that the numbers are
organized in the tree to make searching the tree fast. In particular,
for each interior node in the tree, all of the numbers in the left
subtree are smaller than the number in the node, and all of the numbers
in the right subtree are larger than the number in the node.

We can implement a search function `in?` that takes advantage of the
structure of the binary search tree.

```racket
#lang racket                                                 
                                                             
(struct node (val left right))                               
                                                             
; determines if `n' is in the binary search tree `b',        
; exploiting the binary search tree invariant                
(define (in? n b)                                            
  (cond                                                      
    [(null? b) #f]                                           
    [else (cond                                              
            [(= n (node-val b))                              
             #t]                                             
            [(< n (node-val b))                              
             (in? n (node-left b))]                          
            [(> n (node-val b))                              
             (in? n (node-right b))])]))                     
                                                             
; a predicate that identifies binary search trees            
(define (bst-between? b low high)                            
  (or (null? b)                                              
      (and (<= low (node-val b) high)                        
           (bst-between? (node-left b) low (node-val b))     
           (bst-between? (node-right b) (node-val b) high))))
                                                             
(define (bst? b) (bst-between? b -inf.0 +inf.0))             
                                                             
(provide (struct-out node))                                  
(provide (contract-out                                       
          [bst? (any/c . -> . boolean?)]                     
          [in? (number? bst? . -> . boolean?)]))             
```

In a full binary search tree, this means that the `in?` function only
has to explore a logarithmic number of nodes.

The contract on `in?` guarantees that its input is a binary search tree.
But a little careful thought reveals that this contract defeats the
purpose of the binary search tree algorithm. In particular, consider the
inner `cond` in the `in?` function. This is where the `in?` function
gets its speed: it avoids searching an entire subtree at each recursive
call. Now compare that to the `bst-between?` function. In the case that
it returns `#t`, it traverses the entire tree, meaning that the speedup
of `in?` is lost.

In order to fix that, we can employ a new strategy for checking the
binary search tree contract. In particular, if we only checked the
contract on the nodes that `in?` looks at, we can still guarantee that
the tree is at least partially well-formed, but without changing the
complexity.

To do that, we need to use `struct/dc` to define `bst-between?`. Like
`struct/c`, `struct/dc` defines a contract for a structure. Unlike
`struct/c`, it allows fields to be marked as lazy, so that the contracts
are only checked when the matching selector is called. Also, it does not
allow mutable fields to be marked as lazy.

The `struct/dc` form accepts a contract for each field of the struct and
returns a contract on the struct. More interestingly, `struct/dc` allows
us to write dependent contracts, i.e., contracts where some of the
contracts on the fields depend on the values of other fields. We can use
this to define the binary search tree contract:

```racket
#lang racket                                                            
                                                                        
(struct node (val left right))                                          
                                                                        
; determines if `n' is in the binary search tree `b'                    
(define (in? n b) ... as before ...)                                    
                                                                        
; bst-between : number number -> contract                               
; builds a contract for binary search trees                             
; whose values are between low and high                                 
(define (bst-between/c low high)                                        
  (or/c null?                                                           
        (struct/dc node [val (between/c low high)]                      
                        [left (val) #:lazy (bst-between/c low val)]     
                        [right (val) #:lazy (bst-between/c val high)])))
                                                                        
(define bst/c (bst-between/c -inf.0 +inf.0))                            
                                                                        
(provide (struct-out node))                                             
(provide (contract-out                                                  
          [bst/c contract?]                                             
          [in? (number? bst/c . -> . boolean?)]))                       
```

In general, each use of `struct/dc` must name the fields and then
specify contracts for each field. In the above, the `val` field is a
contract that accepts values between `low` and `high`. The `left` and
`right` fields are dependent on the value of the `val` field, indicated
by their second sub-expressions. They are also marked with the `#:lazy`
keyword to indicate that they should be checked only when the
appropriate accessor is called on the struct instance. Their contracts
are built by recursive calls to the `bst-between/c` function. Taken
together, this contract ensures the same thing that the `bst-between?`
function checked in the original example, but here the checking only
happens as `in?` explores the tree.

Although this contract improves the performance of `in?`, restoring it
to the logarithmic behavior that the contract-less version had, it is
still imposes a fairly large constant overhead. So, the contract library
also provides `define-opt/c` that brings down that constant factor by
optimizing its body. Its shape is just like the `define` above. It
expects its body to be a contract and then optimizes that contract.

```racket
(define-opt/c (bst-between/c low high)                                  
  (or/c null?                                                           
        (struct/dc node [val (between/c low high)]                      
                        [left (val) #:lazy (bst-between/c low val)]     
                        [right (val) #:lazy (bst-between/c val high)])))
```

### 7.6. Abstract Contracts using `#:exists` and `#:∃`

The contract system provides existential contracts that can protect
abstractions, ensuring that clients of your module cannot depend on the
precise representation choices you make for your data structures.

> You can type `#:exists` instead of `#:∃` if you cannot easily type
> unicode characters; in DrRacket, typing `\exists` followed by either
> alt-\ or control-\ \(depending on your platform\) will produce `∃`.

The `contract-out` form allows you to write

`#:∃` `name-of-a-new-contract`

as one of its clauses. This declaration introduces the variable
`name-of-a-new-contract`, binding it to a new contract that hides
information about the values it protects.

As an example, consider this \(simple\) implementation of a queue
datastructure:

```racket
#lang racket                                             
(define empty '())                                       
(define (enq top queue) (append queue (list top)))       
(define (next queue) (car queue))                        
(define (deq queue) (cdr queue))                         
(define (empty? queue) (null? queue))                    
                                                         
(provide                                                 
 (contract-out                                           
  [empty (listof integer?)]                              
  [enq (-> integer? (listof integer?) (listof integer?))]
  [next (-> (listof integer?) integer?)]                 
  [deq (-> (listof integer?) (listof integer?))]         
  [empty? (-> (listof integer?) boolean?)]))             
```

This code implements a queue purely in terms of lists, meaning that
clients of this data structure might use `car` and `cdr` directly on the
data structure \(perhaps accidentally\) and thus any change in the
representation \(say to a more efficient representation that supports
amortized constant time enqueue and dequeue operations\) might break
client code.

To ensure that the queue representation is abstract, we can use `#:∃` in
the `contract-out` expression, like this:

```racket
(provide                         
 (contract-out                   
  #:∃ queue                      
  [empty queue]                  
  [enq (-> integer? queue queue)]
  [next (-> queue integer?)]     
  [deq (-> queue queue)]         
  [empty? (-> queue boolean?)])) 
```

Now, if clients of the data structure try to use `car` and `cdr`, they
receive an error, rather than mucking about with the internals of the
queues.

See also Exists Contracts and Predicates.

### 7.7. Additional Examples

This section illustrates the current state of Racket’s contract
implementation with a series of examples from _Design by Contract, by
Example_ \[Mitchell02\].

Mitchell and McKim’s principles for design by contract DbC are derived
from the 1970s style algebraic specifications. The overall goal of DbC
is   to specify the constructors of an algebra in terms of its
observers. While we reformulate Mitchell and McKim’s terminology and
we use a mostly applicative approach, we   retain their terminology of
“classes” and “objects”:

* **Separate queries from commands.**

  A _query_ returns a result but does not change the observable
  properties of an object. A _command_ changes the visible properties of
  an object, but does not return a result. In applicative implementation
  a command typically returns an new object of the same class.

* **Separate basic queries from derived queries.**

  A _derived query_ returns a result that is computable in terms of
  basic queries.

* **For each derived query, write a post-condition contract that
  specifies the result in terms of the basic queries.**

* **For each command, write a post-condition contract that specifies the
  changes to the observable properties in terms of the basic queries.**

* **For each query and command, decide on a suitable pre-condition
  contract.**

Each of the following sections corresponds to a chapter in  Mitchell and
McKim’s book \(but not all chapters show up  here\). We recommend that
you read the contracts first \(near  the end of the first modules\),
then the implementation \(in  the first modules\), and then the test
module \(at the end of  each section\).

Mitchell and McKim use Eiffel as the underlying programming language and
employ a conventional imperative programming style. Our long-term goal
is  to transliterate their examples into applicative Racket,
structure-oriented imperative Racket, and Racket’s class system.

Note: To mimic Mitchell and McKim’s informal notion of parametericity
\(parametric polymorphism\), we use first-class contracts. At several
places, this use of first-class contracts improves on Mitchell and
McKim’s  design \(see comments in interfaces\).

#### 7.7.1. A Customer-Manager Component

This first module contains some struct definitions in a separate module
in order to better track bugs.

```racket
#lang racket                                              
; data definitions                                        
                                                          
(define id? symbol?)                                      
(define id-equal? eq?)                                    
(define-struct basic-customer (id name address) #:mutable)
                                                          
; interface                                               
(provide                                                  
 (contract-out                                            
  [id?                   (-> any/c boolean?)]             
  [id-equal?             (-> id? id? boolean?)]           
  [struct basic-customer ((id id?)                        
                          (name string?)                  
                          (address string?))]))           
; end of interface                                        
```

This module contains the program that uses the above.

```racket
#lang racket                                                
                                                            
(require "1.rkt") ; the module just above                   
                                                            
; implementation                                            
; [listof (list basic-customer? secret-info)]               
(define all '())                                            
                                                            
(define (find c)                                            
  (define (has-c-as-key p)                                  
    (id-equal? (basic-customer-id (car p)) c))              
  (define x (filter has-c-as-key all))                      
  (if (pair? x) (car x) x))                                 
                                                            
(define (active? c)                                         
  (pair? (find c)))                                         
                                                            
(define not-active? (compose not active? basic-customer-id))
                                                            
(define count 0)                                            
(define (get-count) count)                                  
                                                            
(define (add c)                                             
  (set! all (cons (list c 'secret) all))                    
  (set! count (+ count 1)))                                 
                                                            
(define (name id)                                           
  (define bc-with-id (find id))                             
  (basic-customer-name (car bc-with-id)))                   
                                                            
(define (set-name id name)                                  
  (define bc-with-id (find id))                             
  (set-basic-customer-name! (car bc-with-id) name))         
                                                            
(define c0 0)                                               
; end of implementation                                     
                                                            
(provide                                                    
 (contract-out                                              
  ; how many customers are in the db?                       
  [get-count (-> natural-number/c)]                         
  ; is the customer with this id active?                    
  [active?   (-> id? boolean?)]                             
  ; what is the name of the customer with this id?          
  [name      (-> (and/c id? active?) string?)]              
  ; change the name of the customer with this id            
  [set-name  (->i ([id id?] [nn string?])                   
                  [result any/c] ; result contract          
                  #:post (id nn) (string=? (name id) nn))]  
                                                            
  [add       (->i ([bc (and/c basic-customer? not-active?)])
                  ; A pre-post condition contract must use  
                  ; a side-effect to express this contract  
                  ; via post-conditions                     
                  #:pre () (set! c0 count)                  
                  [result any/c] ; result contract          
                  #:post () (> count c0))]))                
```

The tests:

```racket
#lang racket                                                  
(require rackunit rackunit/text-ui "1.rkt" "1b.rkt")          
                                                              
(add (make-basic-customer 'mf "matthias" "brookstone"))       
(add (make-basic-customer 'rf "robby" "beverly hills park"))  
(add (make-basic-customer 'fl "matthew" "pepper clouds town"))
(add (make-basic-customer 'sk "shriram" "i city"))            
                                                              
(run-tests                                                    
 (test-suite                                                  
  "manager"                                                   
  (test-equal? "id lookup" "matthias" (name 'mf))             
  (test-equal? "count" 4 (get-count))                         
  (test-true "active?" (active? 'mf))                         
  (test-false "active? 2" (active? 'kk))                      
  (test-true "set name" (void? (set-name 'mf "matt")))))      
```

#### 7.7.2. A Parameteric \(Simple\) Stack

```racket
#lang racket                                                      
                                                                  
; a contract utility                                              
(define (eq/c x) (lambda (y) (eq? x y)))                          
                                                                  
(define-struct stack (list p? eq))                                
                                                                  
(define (initialize p? eq) (make-stack '() p? eq))                
(define (push s x)                                                
  (make-stack (cons x (stack-list s)) (stack-p? s) (stack-eq s))) 
(define (item-at s i) (list-ref (reverse (stack-list s)) (- i 1)))
(define (count s) (length  (stack-list s)))                       
(define (is-empty? s) (null? (stack-list s)))                     
(define not-empty? (compose not is-empty?))                       
(define (pop s) (make-stack (cdr (stack-list s))                  
                            (stack-p? s)                          
                            (stack-eq s)))                        
(define (top s) (car (stack-list s)))                             
                                                                  
(provide                                                          
 (contract-out                                                    
  ; predicate                                                     
  [stack?     (-> any/c boolean?)]                                
                                                                  
  ; primitive queries                                             
  ; how many items are on the stack?                              
  [count      (-> stack? natural-number/c)]                       
                                                                  
  ; which item is at the given position?                          
  [item-at                                                        
   (->d ([s stack?] [i (and/c positive? (<=/c (count s)))])       
        ()                                                        
        [result (stack-p? s)])]                                   
                                                                  
  ; derived queries                                               
  ; is the stack empty?                                           
  [is-empty?                                                      
   (->d ([s stack?])                                              
        ()                                                        
        [result (eq/c (= (count s) 0))])]                         
                                                                  
  ; which item is at the top of the stack                         
  [top                                                            
   (->d ([s (and/c stack? not-empty?)])                           
        ()                                                        
        [t (stack-p? s)] ; a stack item, t is its name            
        #:post-cond                                               
        ([stack-eq s] t (item-at s (count s))))]                  
                                                                  
  ; creation                                                      
  [initialize                                                     
   (->d ([p contract?] [s (p p . -> . boolean?)])                 
        ()                                                        
        ; Mitchell and McKim use (= (count s) 0) here to express  
        ; the post-condition in terms of a primitive query        
        [result (and/c stack? is-empty?)])]                       
                                                                  
  ; commands                                                      
  ; add an item to the top of the stack                           
  [push                                                           
   (->d ([s stack?] [x (stack-p? s)])                             
        ()                                                        
        [sn stack?] ; result kind                                 
        #:post-cond                                               
        (and (= (+ (count s) 1) (count sn))                       
             ([stack-eq s] x (top sn))))]                         
                                                                  
  ; remove the item at the top of the stack                       
  [pop                                                            
   (->d ([s (and/c stack? not-empty?)])                           
        ()                                                        
        [sn stack?] ; result kind                                 
        #:post-cond                                               
        (= (- (count s) 1) (count sn)))]))                        
```

The tests:

```racket
#lang racket                                           
(require rackunit rackunit/text-ui "2.rkt")            
                                                       
(define s0 (initialize (flat-contract integer?) =))    
(define s2 (push (push s0 2) 1))                       
                                                       
(run-tests                                             
 (test-suite                                           
  "stack"                                              
  (test-true                                           
   "empty"                                             
   (is-empty? (initialize (flat-contract integer?) =)))
  (test-true "push" (stack? s2))                       
  (test-true                                           
   "push exn"                                          
   (with-handlers ([exn:fail:contract? (lambda _ #t)]) 
     (push (initialize (flat-contract integer?)) 'a)   
     #f))                                              
  (test-true "pop" (stack? (pop s2)))                  
  (test-equal? "top" (top s2) 1)                       
  (test-equal? "toppop" (top (pop s2)) 2)))            
```

#### 7.7.3. A Dictionary

```racket
#lang racket                                                            
                                                                        
; a shorthand for use below                                             
(define-syntax ⇒                                                        
  (syntax-rules ()                                                      
    [(⇒ antecedent consequent) (if antecedent consequent #t)]))         
                                                                        
; implementation                                                        
(define-struct dictionary (l value? eq?))                               
; the keys should probably be another parameter (exercise)              
                                                                        
(define (initialize p eq) (make-dictionary '() p eq))                   
(define (put d k v)                                                     
  (make-dictionary (cons (cons k v) (dictionary-l d))                   
                   (dictionary-value? d)                                
                   (dictionary-eq? d)))                                 
(define (rem d k)                                                       
  (make-dictionary                                                      
   (let loop ([l (dictionary-l d)])                                     
     (cond                                                              
       [(null? l) l]                                                    
       [(eq? (caar l) k) (loop (cdr l))]                                
       [else (cons (car l) (loop (cdr l)))]))                           
   (dictionary-value? d)                                                
   (dictionary-eq? d)))                                                 
(define (count d) (length (dictionary-l d)))                            
(define (value-for d k) (cdr (assq k (dictionary-l d))))                
(define (has? d k) (pair? (assq k (dictionary-l d))))                   
(define (not-has? d) (lambda (k) (not (has? d k))))                     
; end of implementation                                                 
                                                                        
; interface                                                             
(provide                                                                
 (contract-out                                                          
  ; predicates                                                          
  [dictionary? (-> any/c boolean?)]                                     
  ; basic queries                                                       
  ; how many items are in the dictionary?                               
  [count       (-> dictionary? natural-number/c)]                       
  ; does the dictionary define key k?                                   
  [has?        (->d ([d dictionary?] [k symbol?])                       
                    ()                                                  
                    [result boolean?]                                   
                    #:post-cond                                         
                    ((zero? (count d)) . ⇒ . (not result)))]            
  ; what is the value of key k in this dictionary?                      
  [value-for   (->d ([d dictionary?]                                    
                     [k (and/c symbol? (lambda (k) (has? d k)))])       
                    ()                                                  
                    [result (dictionary-value? d)])]                    
  ; initialization                                                      
  ; post condition: for all k in symbol, (has? d k) is false.           
  [initialize  (->d ([p contract?] [eq (p p . -> . boolean?)])          
                    ()                                                  
                    [result (and/c dictionary? (compose zero? count))])]
  ; commands                                                            
  ; Mitchell and McKim say that put shouldn't consume Void (null ptr)   
  ; for v. We allow the client to specify a contract for all values     
  ; via initialize. We could do the same via a key? parameter           
  ; (exercise). add key k with value v to this dictionary               
  [put         (->d ([d dictionary?]                                    
                     [k (and/c symbol? (not-has? d))]                   
                     [v (dictionary-value? d)])                         
                    ()                                                  
                    [result dictionary?]                                
                    #:post-cond                                         
                    (and (has? result k)                                
                         (= (count d) (- (count result) 1))             
                         ([dictionary-eq? d] (value-for result k) v)))] 
  ; remove key k from this dictionary                                   
  [rem         (->d ([d dictionary?]                                    
                     [k (and/c symbol? (lambda (k) (has? d k)))])       
                    ()                                                  
                    [result (and/c dictionary? not-has?)]               
                    #:post-cond                                         
                    (= (count d) (+ (count result) 1)))]))              
; end of interface                                                      
```

The tests:

```racket
#lang racket                                                             
(require rackunit rackunit/text-ui "3.rkt")                              
                                                                         
(define d0 (initialize (flat-contract integer?) =))                      
(define d (put (put (put d0 'a 2) 'b 2) 'c 1))                           
                                                                         
(run-tests                                                               
 (test-suite                                                             
  "dictionaries"                                                         
  (test-equal? "value for" 2 (value-for d 'b))                           
  (test-false "has?" (has? (rem d 'b) 'b))                               
  (test-equal? "count" 3 (count d))                                      
  (test-case "contract check for put: symbol?"                           
             (define d0 (initialize (flat-contract integer?) =))         
             (check-exn exn:fail:contract? (lambda () (put d0 "a" 2))))))
```

#### 7.7.4. A Queue

```racket
#lang racket                                                              
                                                                          
; Note: this queue doesn't implement the capacity restriction             
; of Mitchell and McKim's queue but this is easy to add.                  
                                                                          
; a contract utility                                                      
(define (all-but-last l) (reverse (cdr (reverse l))))                     
(define (eq/c x) (lambda (y) (eq? x y)))                                  
                                                                          
; implementation                                                          
(define-struct queue (list p? eq))                                        
                                                                          
(define (initialize p? eq) (make-queue '() p? eq))                        
(define items queue-list)                                                 
(define (put q x)                                                         
  (make-queue (append (queue-list q) (list x))                            
              (queue-p? q)                                                
              (queue-eq q)))                                              
(define (count s) (length  (queue-list s)))                               
(define (is-empty? s) (null? (queue-list s)))                             
(define not-empty? (compose not is-empty?))                               
(define (rem s)                                                           
  (make-queue (cdr (queue-list s))                                        
              (queue-p? s)                                                
              (queue-eq s)))                                              
(define (head s) (car (queue-list s)))                                    
                                                                          
; interface                                                               
(provide                                                                  
 (contract-out                                                            
  ; predicate                                                             
  [queue?     (-> any/c boolean?)]                                        
                                                                          
  ; primitive queries                                                     
  ; Imagine providing this 'query' for the interface of the module        
  ; only. Then in Racket there is no reason to have count or is-empty?    
  ; around (other than providing it to clients). After all items is       
  ; exactly as cheap as count.                                            
  [items      (->d ([q queue?]) () [result (listof (queue-p? q))])]       
                                                                          
  ; derived queries                                                       
  [count      (->d ([q queue?])                                           
                   ; We could express this second part of the post        
                   ; condition even if count were a module "attribute"    
                   ; in the language of Eiffel; indeed it would use the   
                   ; exact same syntax (minus the arrow and domain).      
                   ()                                                     
                   [result (and/c natural-number/c                        
                                  (=/c (length (items q))))])]            
                                                                          
  [is-empty?  (->d ([q queue?])                                           
                   ()                                                     
                   [result (and/c boolean?                                
                                  (eq/c (null? (items q))))])]            
                                                                          
  [head       (->d ([q (and/c queue? (compose not is-empty?))])           
                   ()                                                     
                   [result (and/c (queue-p? q)                            
                                  (eq/c (car (items q))))])]              
  ; creation                                                              
  [initialize (-> contract?                                               
                  (contract? contract? . -> . boolean?)                   
                  (and/c queue? (compose null? items)))]                  
                                                                          
  ; commands                                                              
  [put        (->d ([oldq queue?] [i (queue-p? oldq)])                    
                   ()                                                     
                   [result                                                
                    (and/c                                                
                     queue?                                               
                     (lambda (q)                                          
                       (define old-items (items oldq))                    
                       (equal? (items q) (append old-items (list i)))))])]
                                                                          
  [rem        (->d ([oldq (and/c queue? (compose not is-empty?))])        
                   ()                                                     
                   [result                                                
                    (and/c queue?                                         
                           (lambda (q)                                    
                             (equal? (cdr (items oldq)) (items q))))])])) 
; end of interface                                                        
```

The tests:

```racket
#lang racket                                                      
(require rackunit rackunit/text-ui "5.rkt")                       
                                                                  
(define s (put (put (initialize (flat-contract integer?) =) 2) 1))
                                                                  
(run-tests                                                        
 (test-suite                                                      
  "queue"                                                         
  (test-true                                                      
   "empty"                                                        
   (is-empty? (initialize (flat-contract integer?) =)))           
  (test-true "put" (queue? s))                                    
  (test-equal? "count" 2 (count s))                               
  (test-true "put exn"                                            
             (with-handlers ([exn:fail:contract? (lambda _ #t)])  
               (put (initialize (flat-contract integer?)) 'a)     
               #f))                                               
  (test-true "remove" (queue? (rem s)))                           
  (test-equal? "head" 2 (head s))))                               
```

### 7.8. Building New Contracts

Contracts are represented internally as functions that accept
information about the contract \(who is to blame, source locations,
etc.\) and produce projections \(in the spirit of Dana Scott\) that
enforce the contract.

In a general sense, a projection is a function that accepts an arbitrary
value, and returns a value that satisfies the corresponding contract.
For example, a projection that accepts only integers corresponds to the
contract `(flat-contract integer?)`, and can be written like this:

```racket
(define int-proj                      
  (λ (x)                              
    (if (integer? x)                  
        x                             
        (signal-contract-violation))))
```

As a second example, a projection that accepts unary functions on
integers looks like this:

```racket
(define int->int-proj                        
  (λ (f)                                     
    (if (and (procedure? f)                  
             (procedure-arity-includes? f 1))
        (λ (x) (int-proj (f (int-proj x))))  
        (signal-contract-violation))))       
```

Although these projections have the right error behavior, they are not
quite ready for use as contracts, because they do not accommodate blame
and do not provide good error messages. In order to accommodate these,
contracts do not just use simple projections, but use functions that
accept a blame object encapsulating the names of two parties that are
the candidates for blame, as well as a record of the source location
where the contract was established and the name of the contract. They
can then, in turn, pass that information to `raise-blame-error` to
signal a good error message.

Here is the first of those two projections, rewritten for use in the
contract system:

```racket
(define (int-proj blame)                     
  (λ (x)                                     
    (if (integer? x)                         
        x                                    
        (raise-blame-error                   
         blame                               
         x                                   
         '(expected: "<integer>" given: "~e")
         x))))                               
```

The new argument specifies who is to be blamed for positive and negative
contract violations.

Contracts, in this system, are always established between two parties.
One party, called the server, provides some value according to the
contract, and the other, the client, consumes the value, also according
to the contract. The server is called the positive position and the
client the negative position. So, in the case of just the integer
contract, the only thing that can go wrong is that the value provided is
not an integer. Thus, only the positive party \(the server\) can ever
accrue blame.  The `raise-blame-error` function always blames the
positive party.

Compare that to the projection for our function contract:

```racket
(define (int->int-proj blame)                                 
  (define dom (int-proj (blame-swap blame)))                  
  (define rng (int-proj blame))                               
  (λ (f)                                                      
    (if (and (procedure? f)                                   
             (procedure-arity-includes? f 1))                 
        (λ (x) (rng (f (dom x))))                             
        (raise-blame-error                                    
         blame                                                
         f                                                    
         '(expected "a procedure of one argument" given: "~e")
         f))))                                                
```

In this case, the only explicit blame covers the situation where either
a non-procedure is supplied to the contract or the procedure does not
accept one argument. As with the integer projection, the blame here also
lies with the producer of the value, which is why `raise-blame-error` is
passed `blame` unchanged.

The checking for the domain and range are delegated to the `int-proj`
function, which is supplied its arguments in the first two lines of the
`int->int-proj` function. The trick here is that, even though the
`int->int-proj` function always blames what it sees as positive, we can
swap the blame parties by calling `blame-swap` on the given blame
object, replacing the positive party with the negative party and vice
versa.

This technique is not merely a cheap trick to get the example to work,
however. The reversal of the positive and the negative is a natural
consequence of the way functions behave. That is, imagine the flow of
values in a program between two modules. First, one module \(the
server\) defines a function, and then that module is required by another
\(the client\). So far, the function itself has to go from the original,
providing module to the requiring module. Now, imagine that the
requiring module invokes the function, supplying it an argument. At this
point, the flow of values reverses. The argument is traveling back from
the requiring module to the providing module! The client is “serving”
the argument to the server, and the server is receiving that value as a
client. And finally, when the function produces a result, that result
flows back in the original direction from server to client. Accordingly,
the contract on the domain reverses the positive and the negative blame
parties, just like the flow of values reverses.

We can use this insight to generalize the function contracts and build a
function that accepts any two contracts and returns a contract for
functions between them.

This projection also goes further and uses `blame-add-context` to
improve the error messages when a contract violation is detected.

```racket
(define (make-simple-function-contract dom-proj range-proj)     
  (λ (blame)                                                    
    (define dom (dom-proj (blame-add-context blame              
                                             "the argument of"  
                                             #:swap? #t)))      
    (define rng (range-proj (blame-add-context blame            
                                               "the range of")))
    (λ (f)                                                      
      (if (and (procedure? f)                                   
               (procedure-arity-includes? f 1))                 
          (λ (x) (rng (f (dom x))))                             
          (raise-blame-error                                    
           blame                                                
           f                                                    
           '(expected "a procedure of one argument" given: "~e")
           f)))))                                               
```

While these projections are supported by the contract library and can be
used to build new contracts, the contract library also supports a
different API for projections that can be more efficient. Specifically,
a late neg projection accepts a blame object without the negative blame
information and then returns a function that accepts both the value to
be contracted and the name of the negative party, in that order. The
returned function then in turn returns the value with the contract.
Rewriting `int->int-proj` to use this API looks like this:

```racket
(define (int->int-proj blame)                                 
  (define dom-blame (blame-add-context blame                  
                                       "the argument of"      
                                       #:swap? #t))           
  (define rng-blame (blame-add-context blame "the range of")) 
  (define (check-int v to-blame neg-party)                    
    (unless (integer? v)                                      
      (raise-blame-error                                      
       to-blame #:missing-party neg-party                     
       v                                                      
       '(expected "an integer" given: "~e")                   
       v)))                                                   
  (λ (f neg-party)                                            
    (if (and (procedure? f)                                   
             (procedure-arity-includes? f 1))                 
        (λ (x)                                                
          (check-int x dom-blame neg-party)                   
          (define ans (f x))                                  
          (check-int ans rng-blame neg-party)                 
          ans)                                                
        (raise-blame-error                                    
         blame #:missing-party neg-party                      
         f                                                    
         '(expected "a procedure of one argument" given: "~e")
         f))))                                                
```

The advantage of this style of contract is that the `blame` argument can
be supplied on the server side of the contract boundary and the result
can be used for each different client. With the simpler situation, a new
blame object has to be created for each client.

One final problem remains before this contract can be used with the rest
of the contract system. In the function above, the contract is
implemented by creating a wrapper function for `f`, but this wrapper
function does not cooperate with `equal?`, nor does it let the runtime
system know that there is a relationship between the result function and
`f`, the input function.

To remedy these two problems, we should use chaperones instead of just
using `λ` to create the wrapper function. Here is the `int->int-proj`
function rewritten to use a chaperone:

```racket
(define (int->int-proj blame)                                 
  (define dom-blame (blame-add-context blame                  
                                       "the argument of"      
                                       #:swap? #t))           
  (define rng-blame (blame-add-context blame "the range of")) 
  (define (check-int v to-blame neg-party)                    
    (unless (integer? v)                                      
      (raise-blame-error                                      
       to-blame #:missing-party neg-party                     
       v                                                      
       '(expected "an integer" given: "~e")                   
       v)))                                                   
  (λ (f neg-party)                                            
    (if (and (procedure? f)                                   
             (procedure-arity-includes? f 1))                 
        (chaperone-procedure                                  
         f                                                    
         (λ (x)                                               
           (check-int x dom-blame neg-party)                  
           (values (λ (ans)                                   
                     (check-int ans rng-blame neg-party)      
                     ans)                                     
                   x)))                                       
        (raise-blame-error                                    
         blame #:missing-party neg-party                      
         f                                                    
         '(expected "a procedure of one argument" given: "~e")
         f))))                                                
```

Projections like the ones described above, but suited to other, new
kinds of value you might make, can be used with the contract library
primitives. Specifically, we can use `make-chaperone-contract` to build
it:

```racket
(define int->int-contract               
  (make-contract                        
   #:name 'int->int                     
   #:late-neg-projection int->int-proj))
```

and then combine it with a value and get some contract checking.

```racket                            
(define/contract (f x)               
  int->int-contract                  
  "not an int")                      
```                                  
                                     
```racket                            
> (f #f)                             
f: contract violation;               
 expected an integer                 
  given: #f                          
  in: the argument of                
      int->int                       
  contract from: (function f)        
  blaming: top-level                 
   (assuming the contract is correct)
  at: eval:5.0                       
> (f 1)                              
f: broke its own contract;           
 promised an integer                 
  produced: "not an int"             
  in: the range of                   
      int->int                       
  contract from: (function f)        
  blaming: (function f)              
   (assuming the contract is correct)
  at: eval:5.0                       
```                                  

#### 7.8.1. Contract Struct Properties

The `make-chaperone-contract` function is okay for one-off contracts,
but often you want to make many different contracts that differ only in
some pieces. The best way to do that is to use a `struct` with either
`prop:contract`, `prop:chaperone-contract`, or `prop:flat-contract`.

For example, lets say we wanted to make a simple form of the `->`
contract that accepts one contract for the range and one for the domain.
We should define a struct with two fields and use
`build-chaperone-contract-property` to construct the chaperone contract
property we need.

```racket
(struct simple-arrow (dom rng)                 
  #:property prop:chaperone-contract           
  (build-chaperone-contract-property           
   #:name                                      
   (λ (arr) (simple-arrow-name arr))           
   #:late-neg-projection                       
   (λ (arr) (simple-arrow-late-neg-proj arr))))
```

To do the automatic coercion of values like `integer?` and `#f` into
contracts, we need to call `coerce-chaperone-contract` \(note that this
rejects impersonator contracts and does not insist on flat contracts; to
do either of those things, call `coerce-contract` or
`coerce-flat-contract` instead\).

```racket
(define (simple-arrow-contract dom rng)                       
  (simple-arrow (coerce-contract 'simple-arrow-contract dom)  
                (coerce-contract 'simple-arrow-contract rng)))
```

To define `simple-arrow-name` is straight-forward; it needs to return an
s-expression representing the contract:

```racket
(define (simple-arrow-name arr)                 
  `(-> ,(contract-name (simple-arrow-dom arr))  
       ,(contract-name (simple-arrow-rng arr))))
```

And we can define the projection using a generalization of the
projection we defined earlier, this time using chaperones:

```racket
(define (simple-arrow-late-neg-proj arr)                                 
  (define dom-ctc (get/build-late-neg-projection (simple-arrow-dom arr)))
  (define rng-ctc (get/build-late-neg-projection (simple-arrow-rng arr)))
  (λ (blame)                                                             
    (define dom+blame (dom-ctc (blame-add-context blame                  
                                                  "the argument of"      
                                                  #:swap? #t)))          
    (define rng+blame (rng-ctc (blame-add-context blame "the range       
of")))                                                                   
    (λ (f neg-party)                                                     
      (if (and (procedure? f)                                            
               (procedure-arity-includes? f 1))                          
          (chaperone-procedure                                           
           f                                                             
           (λ (arg)                                                      
             (values                                                     
              (λ (result) (rng+blame result neg-party))                  
              (dom+blame arg neg-party))))                               
          (raise-blame-error                                             
           blame #:missing-party neg-party                               
           f                                                             
           '(expected "a procedure of one argument" given: "~e")         
           f)))))                                                        
```

```racket                                  
(define/contract (f x)                     
  (simple-arrow-contract integer? boolean?)
  "not a boolean")                         
```                                        
                                           
```racket                                  
> (f #f)                                   
f: contract violation                      
  expected: integer?                       
  given: #f                                
  in: the argument of                      
      (-> integer? boolean?)               
  contract from: (function f)              
  blaming: top-level                       
   (assuming the contract is correct)      
  at: eval:12.0                            
> (f 1)                                    
f: broke its own contract                  
  promised: boolean?                       
  produced: "not a boolean"                
  in: the range of                         
      (-> integer? boolean?)               
  contract from: (function f)              
  blaming: (function f)                    
   (assuming the contract is correct)      
  at: eval:12.0                            
```                                        

#### 7.8.2. With all the Bells and Whistles

There are a number of optional pieces to a contract that
`simple-arrow-contract` did not add. In this section, we walk through
all of them to show examples of how they can be implemented.

The first is a first-order check. This is used by `or/c` in order to
determine which of the higher-order argument contracts to use when it
sees a value. Here’s the function for our simple arrow contract.

```racket
(define (simple-arrow-first-order ctc)          
  (λ (v) (and (procedure? v)                    
              (procedure-arity-includes? v 1))))
```

It accepts a value and returns `#f` if the value is guaranteed not to
satisfy the contract, and `#t` if, as far as we can tell, the value
satisfies the contract, just be inspecting first-order properties of the
value.

The next is random generation. Random generation in the contract library
consists of two pieces: the ability to randomly generate values
satisfying the contract and the ability to exercise values that match
the contract that are given, in the hopes of finding bugs in them \(and
also to try to get them to produce interesting values to be used
elsewhere during generation\).

To exercise contracts, we need to implement a function that is given a
`arrow-contract` struct and some fuel. It should return two values: a
function that accepts values of the contract and exercises them, plus a
list of values that the exercising process will always produce. In the
case of our simple contract, we know that we can always produce values
of the range, as long as we can generate values of the domain \(since we
can just call the function\). So, here’s a function that matches the
`exercise` argument of `build-chaperone-contract-property`’s contract:

```racket
(define (simple-arrow-contract-exercise arr)                        
  (define env (contract-random-generate-get-current-environment))   
  (λ (fuel)                                                         
    (define dom-generate                                            
      (contract-random-generate/choose (simple-arrow-dom arr) fuel))
    (cond                                                           
      [dom-generate                                                 
       (values                                                      
        (λ (f) (contract-random-generate-stash                      
                env                                                 
                (simple-arrow-rng arr)                              
                (f (dom-generate))))                                
        (list (simple-arrow-rng arr)))]                             
      [else                                                         
       (values void '())])))                                        
```

If the domain contract can be generated, then we know we can do some
good via exercising. In that case, we return a procedure that calls `f`
\(the function matching the contract\) with something that we generated
from the domain, and we stash the result value in the environment too.
We also return `(simple-arrow-rng arr)` to indicate that exercising will
always produce something of that contract.

If we cannot, then we simply return a function that does no exercising
\(`void`\) and the empty list \(indicating that we won’t generate any
values\).

Then, to generate values matching the contract, we define a function
that when given the contract and some fuel, makes up a random function.
To help make it a more effective testing function, we can exercise any
arguments it receives, and also stash them into the generation
environment, but only if we can generate values of the range contract.

```racket
(define (simple-arrow-contract-generate arr)                              
  (λ (fuel)                                                               
    (define env (contract-random-generate-get-current-environment))       
    (define rng-generate                                                  
      (contract-random-generate/choose (simple-arrow-rng arr) fuel))      
    (cond                                                                 
      [rng-generate                                                       
       (λ ()                                                              
         (λ (arg)                                                         
           (contract-random-generate-stash env (simple-arrow-dom arr) arg)
           (rng-generate)))]                                              
      [else                                                               
       #f])))                                                             
```

When the random generation pulls something out of the environment, it
needs to be able to tell if a value that has been passed to
`contract-random-generate-stash` is a candidate for the contract it is
trying to generate. Of course, it the contract passed to
`contract-random-generate-stash` is an exact match, then it can use it.
But it can also use the value if the contract is stronger \(in the sense
that it accepts fewer values\).

To provide that functionality, we implement this function:

```racket
(define (simple-arrow-first-stronger? this that)     
  (and (simple-arrow? that)                          
       (contract-stronger? (simple-arrow-dom that)   
                           (simple-arrow-dom this))  
       (contract-stronger? (simple-arrow-rng this)   
                           (simple-arrow-rng that))))
```

This function accepts `this` and `that`, two contracts. It is guaranteed
that `this` will be one of our simple arrow contracts, since we’re
supplying this function together with the simple arrow implementation.
But the `that` argument might be any contract. This function checks to
see if `that` is also a simple arrow contract and, if so compares the
domain and range. Of course, there are other contracts that we could
also check for \(e.g., contracts built using `->` or `->*`\), but we do
not need to. The stronger function is allowed to return `#f` if it
doesn’t know the answer but if it returns `#t`, then the contract really
must be stronger.

Now that we have all of the pieces implemented, we need to pass them to
`build-chaperone-contract-property` so the contract system starts using
them:

```racket
(struct simple-arrow (dom rng)                                    
  #:property prop:custom-write contract-custom-write-property-proc
  #:property prop:chaperone-contract                              
  (build-chaperone-contract-property                              
   #:name                                                         
   (λ (arr) (simple-arrow-name arr))                              
   #:late-neg-projection                                          
   (λ (arr) (simple-arrow-late-neg-proj arr))                     
   #:first-order simple-arrow-first-order                         
   #:stronger simple-arrow-first-stronger?                        
   #:generate simple-arrow-contract-generate                      
   #:exercise simple-arrow-contract-exercise))                    
(define (simple-arrow-contract dom rng)                           
  (simple-arrow (coerce-contract 'simple-arrow-contract dom)      
                (coerce-contract 'simple-arrow-contract rng)))    
```

We also add a `prop:custom-write` property so that the contracts print
properly, e.g.:

```racket
> (simple-arrow-contract integer? integer?)
(-> integer? integer?)                     
```

\(We use `prop:custom-write` because the contract library can not depend
on

`#lang` `racket/generic`

but yet still wants to provide some help to make it easy to use the
right printer.\)

Now that that’s done, we can use the new functionality. Here’s a random
function, generated by the contract library, using our
`simple-arrow-contract-generate` function:

```racket                                     
(define a-random-function                     
  (contract-random-generate                   
   (simple-arrow-contract integer? integer?)))
```                                           
                                              
```racket                                     
> (a-random-function 0)                       
-133.0                                        
> (a-random-function 1)                       
-70.0                                         
```                                           

Here’s how the contract system can now automatically find bugs in
functions that consume simple arrow contracts:

```racket                                           
(define/contract (misbehaved-f f)                   
  (-> (simple-arrow-contract integer? boolean?) any)
  (f "not an integer"))                             
```                                                 
                                                    
```racket                                           
> (contract-exercise misbehaved-f)                  
misbehaved-f: broke its own contract                
  promised: integer?                                
  produced: "not an integer"                        
  in: the argument of                               
      the 1st argument of                           
      (-> (-> integer? boolean?) any)               
  contract from: (function misbehaved-f)            
  blaming: (function misbehaved-f)                  
   (assuming the contract is correct)               
  at: eval:25.0                                     
```                                                 

And if we hadn’t implemented `simple-arrow-first-order`, then `or/c`
would not be able to tell which branch of the `or/c` to use in this
program:

```racket                                    
(define/contract (maybe-accepts-a-function f)
  (or/c (simple-arrow-contract real? real?)  
        (-> real? real? real?)               
        real?)                               
  (if (procedure? f)                         
      (if (procedure-arity-includes f 1)     
          (f 1132)                           
          (f 11 2))                          
      f))                                    
```                                          
                                             
```racket                                    
> (maybe-accepts-a-function sqrt)            
maybe-accepts-a-function: contract violation 
  expected: real?                            
  given: #<procedure:sqrt>                   
  in: the argument of                        
      a part of the or/c of                  
      (or/c                                  
       (-> real? real?)                      
       (-> real? real? real?)                
       real?)                                
  contract from:                             
      (function maybe-accepts-a-function)    
  blaming: top-level                         
   (assuming the contract is correct)        
  at: eval:27.0                              
> (maybe-accepts-a-function 123)             
123                                          
```                                          

### 7.9. Gotchas

#### 7.9.1. Contracts and `eq?`

As a general rule, adding a contract to a program should either leave
the behavior of the program unchanged, or should signal a contract
violation. And this is almost true for Racket contracts, with one
exception: `eq?`.

The `eq?` procedure is designed to be fast and does not provide much in
the way of guarantees, except that if it returns true, it means that the
two values behave identically in all respects. Internally, this is
implemented as pointer equality at a low-level so it exposes information
about how Racket is implemented \(and how contracts are implemented\).

Contracts interact poorly with `eq?` because function contract checking
is implemented internally as wrapper functions. For example, consider
this module:

```racket
#lang racket                                              
                                                          
(define (make-adder x)                                    
  (if (= 1 x)                                             
      add1                                                
      (lambda (y) (+ x y))))                              
(provide (contract-out                                    
          [make-adder (-> number? (-> number? number?))]))
```

It exports the `make-adder` function that is the usual curried addition
function, except that it returns Racket’s `add1` when its input is `1`.

You might expect that

```racket
(eq? (make-adder 1) 
     (make-adder 1))
```

would return `#t`, but it does not. If the contract were changed to
`any/c` \(or even `(-> number? any/c)`\), then the `eq?` call would
return `#t`.

Moral: Do not use `eq?` on values that have contracts.

#### 7.9.2. Contract boundaries and `define/contract`

The contract boundaries established by `define/contract`, which creates
a nested contract boundary, are sometimes unintuitive. This is
especially true when multiple functions or other values with contracts
interact. For example, consider these two interacting functions:

```racket
> (define/contract (f x)             
    (-> integer? integer?)           
    x)                               
> (define/contract (g)               
    (-> string?)                     
    (f "not an integer"))            
> (g)                                
f: contract violation                
  expected: integer?                 
  given: "not an integer"            
  in: the 1st argument of            
      (-> integer? integer?)         
  contract from: (function f)        
  blaming: top-level                 
   (assuming the contract is correct)
  at: eval:2.0                       
```

One might expect that the function `g` will be blamed for breaking the
terms of its contract with `f`. Blaming `g` would be right if `f` and
`g` were directly establishing contracts with each other. They aren’t,
however. Instead, the access between `f` and `g` is mediated through the
top-level of the enclosing module.

More precisely, `f` and the top-level of the module have the `(->
integer? integer?)` contract mediating their interaction; `g` and the
top-level have `(-> string?)` mediating their interaction, but there is
no contract directly between `f` and `g`. This means that the reference
to `f` in the body of `g` is really the top-level of the module’s
responsibility, not `g`’s. In other words, the function `f` has been
given to `g` with no contract between `g` and the top-level and thus the
top-level is blamed.

If we wanted to add a contract between `g` and the top-level, we can use
`define/contract`’s `#:freevar` declaration and see the expected blame:

```racket
> (define/contract (f x)              
    (-> integer? integer?)            
    x)                                
> (define/contract (g)                
    (-> string?)                      
    #:freevar f (-> integer? integer?)
    (f "not an integer"))             
> (g)                                 
f: contract violation                 
  expected: integer?                  
  given: "not an integer"             
  in: the 1st argument of             
      (-> integer? integer?)          
  contract from: top-level            
  blaming: (function g)               
   (assuming the contract is correct) 
  at: eval:6.0                        
```

Moral: if two values with contracts should interact,        put them in
separate modules with contracts at        the module boundary or use
`#:freevar`.

#### 7.9.3. Exists Contracts and Predicates

Much like the `eq?` example above, `#:∃` contracts can change the
behavior of a program.

Specifically, the `null?` predicate \(and many other predicates\) return
`#f` for `#:∃` contracts, and changing one of those contracts to `any/c`
means that `null?` might now return `#t` instead, resulting in
arbitrarily different behavior depending on how this boolean might flow
around in the program.

```racket
 #lang racket/exists package: [base](https://pkgs.racket-lang.org/package/base)
```

To work around the above problem, the `racket/exists` library behaves
just like `racket`, but predicates signal errors when given `#:∃`
contracts.

Moral: Do not use predicates on `#:∃` contracts, but if you’re not sure,
use `racket/exists` to be safe.

#### 7.9.4. Defining Recursive Contracts

When defining a self-referential contract, it is natural to use
`define`. For example, one might try to write a contract on streams like
this:

```racket
> (define stream/c                                   
    (promise/c                                       
     (or/c null?                                     
           (cons/c number? stream/c))))              
stream/c: undefined;                                 
 cannot reference an identifier before its definition
  in module: top-level                               
```

Unfortunately, this does not work because the value of `stream/c` is
needed before it is defined. Put another way, all of the combinators
evaluate their arguments eagerly, even though the values that they
accept do not.

Instead, use

```racket
(define stream/c                                     
  (promise/c                                         
   (or/c                                             
    null?                                            
    (cons/c number? (recursive-contract stream/c)))))
```

The use of `recursive-contract` delays the evaluation of the identifier
`stream/c` until after the contract is first checked, long enough to
ensure that `stream/c` is defined.

See also Checking Properties of Data Structures.

#### 7.9.5. Mixing `set!` and `contract-out`

The contract library assumes that variables exported via `contract-out`
are not assigned to, but does not enforce it. Accordingly, if you try to
`set!` those variables, you may be surprised. Consider the following
example:

```racket
> (module server racket                           
    (define (inc-x!) (set! x (+ x 1)))            
    (define x 0)                                  
    (provide (contract-out [inc-x! (-> void?)]    
                           [x integer?])))        
> (module client racket                           
    (require 'server)                             
                                                  
    (define (print-latest) (printf "x is ~s\n" x))
                                                  
    (print-latest)                                
    (inc-x!)                                      
    (print-latest))                               
> (require 'client)                               
x is 0                                            
x is 0                                            
```

Both calls to `print-latest` print `0`, even though the value of `x` has
been incremented \(and the change is visible inside the module `x`\).

To work around this, export accessor functions, rather than exporting
the variable directly, like this:

```racket
#lang racket                                  
                                              
(define (get-x) x)                            
(define (inc-x!) (set! x (+ x 1)))            
(define x 0)                                  
(provide (contract-out [inc-x! (-> void?)]    
                       [get-x (-> integer?)]))
```

Moral: This is a bug that we will address in a future release.

## 8. Input and Output

> A Racket port corresponds to the Unix notion of a stream \(not to be
> confused with `racket/stream`’s streams\).

A Racket _port_ represents a source or sink of data, such as a file, a
terminal, a TCP connection, or an in-memory string.  Ports provide
sequential access in which data can be read or written a piece of a
time, without requiring the data to be consumed or produced all at once.
More specifically, an _input port_ represents a source from which a
program can read data, and an _output port_ represents a sink to which a
program can write data.

    8.1 Varieties of Ports              
    8.2 Default Ports                   
    8.3 Reading and Writing Racket Data 
    8.4 Datatypes and Serialization     
    8.5 Bytes, Characters, and Encodings
    8.6 I/O Patterns                    

### 8.1. Varieties of Ports

Various functions create various kinds of ports. Here are a few
examples:

* **Files:** The `open-output-file` function opens a   file for writing,
  and `open-input-file` opens a file for   reading.

  Examples:

  ```racket
> (define out (open-output-file "data"))
  > (display "hello" out)                 
  > (close-output-port out)               
  > (define in (open-input-file "data"))  
  > (read-line in)                        
  "hello"                                 
  > (close-input-port in)                 
```

  If a file exists already, then `open-output-file` raises an exception
  by default. Supply an option like `#:exists 'truncate` or `#:exists
  'update` to re-write or update the file:

  Examples:

  ```racket
> (define out (open-output-file "data" #:exists 'truncate))
  > (display "howdy" out)                                    
  > (close-output-port out)                                  
```

  Instead of having to match the open calls with close calls, most
  Racket programmers will use the `call-with-input-file` and
  `call-with-output-file` functions which take a function to call to
  carry out the desired operation. This function gets as its only
  argument the port, which is automatically opened and closed for the
  operation.

  Examples:

  ```racket
> (call-with-output-file "data"                    
                            #:exists 'truncate       
                            (lambda (out)            
                              (display "hello" out)))
  > (call-with-input-file "data"                     
                          (lambda (in)               
                            (read-line in)))         
  "hello"                                            
```

* **Strings:** The `open-output-string` function creates a port that
  accumulates data into a string, and `get-output-string` extracts the
  accumulated string. The `open-input-string` function creates a port to
  read from a string.

  Examples:

  ```racket
> (define p (open-output-string))                    
  > (display "hello" p)                                
  > (get-output-string p)                              
  "hello"                                              
  > (read-line (open-input-string "goodbye\nfarewell"))
  "goodbye"                                            
```

* **TCP Connections:** The `tcp-connect` function creates both an input
  port and an output port for the client side of a TCP communication.
  The `tcp-listen` function creates a server, which accepts connections
  via `tcp-accept`.

  Examples:

  ```racket
> (define server (tcp-listen 12345))                          
  > (define-values (c-in c-out) (tcp-connect "localhost" 12345))
  > (define-values (s-in s-out) (tcp-accept server))            
  > (display "hello\n" c-out)                                   
  > (close-output-port c-out)                                   
  > (read-line s-in)                                            
  "hello"                                                       
  > (read-line s-in)                                            
  #<eof>                                                        
```

* **Process Pipes:** The `subprocess` function runs a new process at the
  OS level and returns ports that correspond to the subprocess’s stdin,
  stdout, and stderr. \(The first three arguments can be certain kinds
  of existing ports to connect directly to the subprocess, instead of
  creating new ports.\)

  Examples:

  ```racket
> (define-values (p stdout stdin stderr)     
      (subprocess #f #f #f "/usr/bin/wc" "-w"))
  > (display "a b c\n" stdin)                  
  > (close-output-port stdin)                  
  > (read-line stdout)                         
  "       3"                                   
  > (close-input-port stdout)                  
  > (close-input-port stderr)                  
```

* **Internal Pipes:** The `make-pipe` function returns two ports that
  are ends of a pipe. This kind of pipe is internal to Racket, and not
  related to OS-level pipes for communicating between different
  processes.

  Examples:

  ```racket
> (define-values (in out) (make-pipe))
  > (display "garbage" out)             
  > (close-output-port out)             
  > (read-line in)                      
  "garbage"                             
```

### 8.2. Default Ports

For most simple I/O functions, the target port is an optional argument,
and the default is the _current input port_ or _current output port_.
Furthermore, error messages are written to the _current error port_,
which is an output port. The `current-input-port`,
`current-output-port`, and `current-error-port` functions return the
corresponding current ports.

Examples:

```racket
> (display "Hi")                                 
Hi                                               
> (display "Hi" (current-output-port)) ; the same
Hi                                               
```

If you start the `racket` program in a terminal, then the current input,
output, and error ports are all connected to the terminal. More
generally, they are connected to the OS-level stdin, stdout, and stderr.
In this guide, the examples show output written to stdout in purple, and
output written to stderr in red italics.

Examples:

```racket
(define (swing-hammer)                   
  (display "Ouch!" (current-error-port)))
                                         
> (swing-hammer)                         
Ouch!                                    
```

The current-port functions are actually parameters, which means that
their values can be set with `parameterize`.

> See Dynamic Binding: `parameterize` for an introduction to parameters.

Example:

```racket
> (let ([s (open-output-string)])         
    (parameterize ([current-error-port s])
      (swing-hammer)                      
      (swing-hammer)                      
      (swing-hammer))                     
    (get-output-string s))                
"Ouch!Ouch!Ouch!"                         
```

### 8.3. Reading and Writing Racket Data

As noted throughout Built-In Datatypes, Racket provides three ways to
print an instance of a built-in value:

* `print`, which prints a value in the same way that is it printed for a
  REPL result; and

* `write`, which prints a value in such a way that `read` on the output
  produces the value back; and

* `display`, which tends to reduce a value to just its character or byte
  content—at least for those datatypes that are primarily about
  characters or bytes, otherwise it falls back to the same output as
  `write`.

Here are some examples using each:

```racket            ```racket            ```racket             
> (print 1/2)        > (write 1/2)        > (display 1/2)       
1/2                  1/2                  1/2                   
> (print #\x)        > (write #\x)        > (display #\x)       
#\x                  #\x                  x                     
> (print "hello")    > (write "hello")    > (display "hello")   
"hello"              "hello"              hello                 
> (print #"goodbye") > (write #"goodbye") > (display #"goodbye")
#"goodbye"           #"goodbye"           goodbye               
> (print '|pea pod|) > (write '|pea pod|) > (display '|pea pod|)
'|pea pod|           |pea pod|            pea pod               
> (print '("i" pod)) > (write '("i" pod)) > (display '("i" pod))
'("i" pod)           ("i" pod)            (i pod)               
> (print write)      > (write write)      > (display write)     
#<procedure:write>   #<procedure:write>   #<procedure:write>    
```                  ```                  ```                   

Overall, `print` corresponds to the expression layer of Racket syntax,
`write` corresponds to the reader layer, and `display` roughly
corresponds to the character layer.

The `printf` function supports simple formatting of data and text. In
the format string supplied to `printf`, `~a` `display`s the next
argument, `~s` `write`s the next argument, and `~v` `print`s the next
argument.

Examples:

```racket
(define (deliver who when what)                        
  (printf "Items ~a for shopper ~s: ~v" who when what))
                                                       
> (deliver '("list") '("John") '("milk"))              
Items (list) for shopper ("John"): '("milk")           
```

After using `write`, as opposed to `display` or `print`, many forms of
data can be read back in using `read`. The same values `print`ed can
also be parsed by `read`, but the result may have extra quote forms,
since a `print`ed form is meant to be read like an expression.

Examples:

```racket
> (define-values (in out) (make-pipe))           
> (write "hello" out)                            
> (read in)                                      
"hello"                                          
> (write '("alphabet" soup) out)                 
> (read in)                                      
'("alphabet" soup)                               
> (write #hash((a . "apple") (b . "banana")) out)
> (read in)                                      
'#hash((a . "apple") (b . "banana"))             
> (print '("alphabet" soup) out)                 
> (read in)                                      
”("alphabet" soup)                               
> (display '("alphabet" soup) out)               
> (read in)                                      
'(alphabet soup)                                 
```

### 8.4. Datatypes and Serialization

Prefab structure types \(see Prefab Structure Types\) automatically
support _serialization_: they can be written to an output stream, and a
copy can be read back in from an input stream:

```racket
> (define-values (in out) (make-pipe))
> (write #s(sprout bean) out)         
> (read in)                           
'#s(sprout bean)                      
```

Other structure types created by `struct`, which offer more abstraction
than prefab structure types, normally `write` either using `#<....>`
notation \(for opaque structure types\) or using `#(....)` vector
notation \(for transparent structure types\). In neither case can the
result be read back in as an instance of the structure type:

```racket
> (struct posn (x y))                 
> (write (posn 1 2))                  
#<posn>                               
> (define-values (in out) (make-pipe))
> (write (posn 1 2) out)              
> (read in)                           
pipe::1: read: bad syntax `#<`        
```

```racket
> (struct posn (x y) #:transparent)   
> (write (posn 1 2))                  
#(struct:posn 1 2)                    
> (define-values (in out) (make-pipe))
> (write (posn 1 2) out)              
> (define v (read in))                
> v                                   
'#(struct:posn 1 2)                   
> (posn? v)                           
#f                                    
> (vector? v)                         
#t                                    
```

The `serializable-struct` form defines a structure type that can be
`serialize`d to a value that can be printed using `write` and restored
via `read`. The `serialize`d result can be `deserialize`d to get back an
instance of the original structure type. The serialization form and
functions are provided by the `racket/serialize` library.

Examples:

```racket
> (require racket/serialize)                             
> (serializable-struct posn (x y) #:transparent)         
> (deserialize (serialize (posn 1 2)))                   
(posn 1 2)                                               
> (write (serialize (posn 1 2)))                         
((3) 1 ((#f . deserialize-info:posn-v0)) 0 () () (0 1 2))
> (define-values (in out) (make-pipe))                   
> (write (serialize (posn 1 2)) out)                     
> (deserialize (read in))                                
(posn 1 2)                                               
```

In addition to the names bound by `struct`, `serializable-struct` binds
an identifier with deserialization information, and it automatically
`provide`s the deserialization identifier from a module context. This
deserialization identifier is accessed reflectively when a value is
deserialized.

### 8.5. Bytes, Characters, and Encodings

Functions like `read-line`, `read`, `display`, and `write` all work in
terms of characters \(which correspond to Unicode scalar values\).
Conceptually, they are implemented in terms of `read-char` and
`write-char`.

More primitively, ports read and write bytes, instead of characters. The
functions `read-byte` and `write-byte` read and write raw bytes. Other
functions, such as `read-bytes-line`, build on top of byte operations
instead of character operations.

In fact, the `read-char` and `write-char` functions are conceptually
implemented in terms of `read-byte` and `write-byte`. When a single
byte’s value is less than 128, then it corresponds to an ASCII
character. Any other byte is treated as part of a UTF-8 sequence, where
UTF-8 is a particular standard way of encoding Unicode scalar values in
bytes \(which has the nice property that ASCII characters are encoded as
themselves\). Thus, a single `read-char` may call `read-byte` multiple
times, and a single `write-char` may generate multiple output bytes.

The `read-char` and `write-char` operations _always_ use a UTF-8
encoding. If you have a text stream that uses a different encoding, or
if you want to generate a text stream in a different encoding, use
`reencode-input-port` or `reencode-output-port`. The
`reencode-input-port` function converts an input stream from an encoding
that you specify into a UTF-8 stream; that way, `read-char` sees UTF-8
encodings, even though the original used a different encoding. Beware,
however, that `read-byte` also sees the re-encoded data, instead of the
original byte stream.

### 8.6. I/O Patterns

If you want to process individual lines of a file, then you can use
`for` with `in-lines`:

```racket
> (define (upcase-all in)                 
    (for ([l (in-lines in)])              
      (display (string-upcase l))         
      (newline)))                         
> (upcase-all (open-input-string          
               (string-append             
                "Hello, World!\n"         
                "Can you hear me, now?")))
HELLO, WORLD!                             
CAN YOU HEAR ME, NOW?                     
```

If you want to determine whether “hello” appears in a file, then you
could search separate lines, but it’s even easier to simply apply a
regular expression \(see Regular Expressions\) to the stream:

```racket
> (define (has-hello? in)                   
    (regexp-match? #rx"hello" in))          
> (has-hello? (open-input-string "hello"))  
#t                                          
> (has-hello? (open-input-string "goodbye"))
#f                                          
```

If you want to copy one port into another, use `copy-port` from
`racket/port`, which efficiently transfers large blocks when lots of
data is available, but also transfers small blocks immediately if that’s
all that is available:

```racket
> (define o (open-output-string))          
> (copy-port (open-input-string "broom") o)
> (get-output-string o)                    
"broom"                                    
```

## 9. Regular Expressions

> This chapter is a modified version of \[Sitaram05\].

A _regexp_ value encapsulates a pattern that is described by a string or
byte string.  The regexp matcher tries to match this pattern against \(a
portion of\) another string or byte string, which we will call the _text
string_, when you call functions like `regexp-match`.  The text string
is treated as raw text, and not as a pattern.

    9.1 Writing Regexp Patterns                   
    9.2 Matching Regexp Patterns                  
    9.3 Basic Assertions                          
    9.4 Characters and Character Classes          
      9.4.1 Some Frequently Used Character Classes
      9.4.2 POSIX character classes               
    9.5 Quantifiers                               
    9.6 Clusters                                  
      9.6.1 Backreferences                        
      9.6.2 Non-capturing Clusters                
      9.6.3 Cloisters                             
    9.7 Alternation                               
    9.8 Backtracking                              
    9.9 Looking Ahead and Behind                  
      9.9.1 Lookahead                             
      9.9.2 Lookbehind                            
    9.10 An Extended Example                      

> +\[missing\] in \[missing\] provides more on regexps.

### 9.1. Writing Regexp Patterns

A string or byte string can be used directly as a regexp pattern, or it
can be prefixed with `#rx` to form a literal regexp value. For example,
`#rx"abc"` is a string-based regexp value, and `#rx#"abc"` is a byte
string-based regexp value. Alternately, a string or byte string can be
prefixed with `#px`, as in `#px"abc"`, for a slightly extended syntax of
patterns within the string.

Most of the characters in a regexp pattern are meant to match
occurrences of themselves in the text string.  Thus, the pattern
`#rx"abc"` matches a string that contains the characters `a`, `b`, and
`c` in succession. Other characters act as _metacharacters_, and some
character sequences act as _metasequences_.  That is, they specify
something other than their literal selves.  For example, in the pattern
`#rx"a.c"`, the characters `a` and `c` stand for themselves, but the
metacharacter `.` can match _any_ character.  Therefore, the pattern
`#rx"a.c"` matches an `a`, any character, and `c` in succession.

> When we want a literal `\` inside a Racket string or regexp literal, we
> must escape it so that it shows up in the string at all. Racket strings
> use `\` as the escape character, so we end up with two `\`s: one
> Racket-string `\` to escape the regexp `\`, which then escapes the `.`.
> Another character that would need escaping inside a Racket string is
> `"`.

If we needed to match the character `.` itself, we can escape it by
precede it with a `\`.  The character sequence `\.` is thus a
metasequence, since it doesn’t match itself but rather just `.`.  So, to
match `a`, `.`, and `c` in succession, we use the regexp pattern
`#rx"a\\.c"`; the double `\` is an artifact of Racket strings, not the
regexp pattern itself.

The `regexp` function takes a string or byte string and produces a
regexp value. Use `regexp` when you construct a pattern to be matched
against multiple strings, since a pattern is compiled to a regexp value
before it can be used in a match. The `pregexp` function is like
`regexp`, but using the extended syntax. Regexp values as literals with
`#rx` or `#px` are compiled once and for all when they are read.

The `regexp-quote` function takes an arbitrary string and returns a
string for a pattern that matches exactly the original string. In
particular, characters in the input string that could serve as regexp
metacharacters are escaped with a backslash, so that they safely match
only themselves.

```racket
> (regexp-quote "cons") 
"cons"                  
> (regexp-quote "list?")
"list\\?"               
```

The `regexp-quote` function is useful when building a composite regexp
from a mix of regexp strings and verbatim strings.

### 9.2. Matching Regexp Patterns

The `regexp-match-positions` function takes a regexp pattern and a text
string, and it returns a match if the regexp matches \(some part of\)
the text string, or `#f` if the regexp did not match the string. A
successful match produces a list of _index pairs_.

Examples:

```racket
> (regexp-match-positions #rx"brain" "bird")             
#f                                                       
> (regexp-match-positions #rx"needle" "hay needle stack")
'((4 . 10))                                              
```

In the second example, the integers `4` and `10` identify the substring
that was matched. The `4` is the starting \(inclusive\) index, and `10`
the ending \(exclusive\) index of the matching substring:

```racket
> (substring "hay needle stack" 4 10)
"needle"                             
```

In this first example, `regexp-match-positions`’s return list contains
only one index pair, and that pair represents the entire substring
matched by the regexp.  When we discuss subpatterns later, we will see
how a single match operation can yield a list of submatches.

The `regexp-match-positions` function takes optional third and fourth
arguments that specify the indices of the text string within which the
matching should take place.

```racket
> (regexp-match-positions                                   
   #rx"needle"                                              
   "his needle stack -- my needle stack -- her needle stack"
   20 39)                                                   
'((23 . 29))                                                
```

Note that the returned indices are still reckoned relative to the full
text string.

The `regexp-match` function is like `regexp-match-positions`, but
instead of returning index pairs, it returns the matching substrings:

```racket
> (regexp-match #rx"brain" "bird")             
#f                                             
> (regexp-match #rx"needle" "hay needle stack")
'("needle")                                    
```

When `regexp-match` is used with byte-string regexp, the result is a
matching byte substring:

```racket
> (regexp-match #rx#"needle" #"hay needle stack")
'(#"needle")                                     
```

> A byte-string regexp can be applied to a string, and a string regexp can
> be applied to a byte string. In both cases, the result is a byte string.
> Internally, all regexp matching is in terms of bytes, and a string
> regexp is expanded to a regexp that matches UTF-8 encodings of
> characters. For maximum efficiency, use byte-string matching instead of
> string, since matching bytes directly avoids UTF-8 encodings.

If you have data that is in a port, there’s no need to first read it
into a string. Functions like `regexp-match` can match on the port
directly:

```racket
> (define-values (i o) (make-pipe))
> (write "hay needle stack" o)     
> (close-output-port o)            
> (regexp-match #rx#"needle" i)    
'(#"needle")                       
```

The `regexp-match?` function is like `regexp-match-positions`, but
simply returns a boolean indicating whether the match succeeded:

```racket
> (regexp-match? #rx"brain" "bird")             
#f                                              
> (regexp-match? #rx"needle" "hay needle stack")
#t                                              
```

The `regexp-split` function takes two arguments, a regexp pattern and a
text string, and it returns a list of substrings of the text string; the
pattern identifies the delimiter separating the substrings.

```racket
> (regexp-split #rx":" "/bin:/usr/bin:/usr/bin/X11:/usr/local/bin")
'("/bin" "/usr/bin" "/usr/bin/X11" "/usr/local/bin")               
> (regexp-split #rx" " "pea soup")                                 
'("pea" "soup")                                                    
```

If the first argument matches empty strings, then the list of all the
single-character substrings is returned.

```racket
> (regexp-split #rx"" "smithereens")                
'("" "s" "m" "i" "t" "h" "e" "r" "e" "e" "n" "s" "")
```

Thus, to identify one-or-more spaces as the delimiter, take care to use
the regexp `#rx" +"`, not `#rx" *"`.

```racket
> (regexp-split #rx" +" "split pea     soup")                 
'("split" "pea" "soup")                                       
> (regexp-split #rx" *" "split pea     soup")                 
'("" "s" "p" "l" "i" "t" "" "p" "e" "a" "" "s" "o" "u" "p" "")
```

The `regexp-replace` function replaces the matched portion of the text
string by another string.  The first argument is the pattern, the second
the text string, and the third is either the string to be inserted or a
procedure to convert matches to the insert string.

```racket
> (regexp-replace #rx"te" "liberte" "ty")       
"liberty"                                       
> (regexp-replace #rx"." "racket" string-upcase)
"Racket"                                        
```

If the pattern doesn’t occur in the text string, the returned string is
identical to the text string.

The `regexp-replace*` function replaces _all_ matches in the text string
by the insert string:

```racket
> (regexp-replace* #rx"te" "liberte egalite fraternite" "ty")
"liberty egality fratyrnity"                                 
> (regexp-replace* #rx"[ds]" "drracket" string-upcase)       
"Drracket"                                                   
```

### 9.3. Basic Assertions

The _assertions_ `^` and `$` identify the beginning and the end of the
text string, respectively.  They ensure that their adjoining regexps
match at one or other end of the text string:

```racket
> (regexp-match-positions #rx"^contact" "first contact")
#f                                                      
```

The regexp above fails to match because `contact` does not occur at the
beginning of the text string. In

```racket
> (regexp-match-positions #rx"laugh$" "laugh laugh laugh laugh")
'((18 . 23))                                                    
```

the regexp matches the _last_ `laugh`.

The metasequence `\b` asserts that a word boundary exists, but this
metasequence works only with `#px` syntax. In

```racket
> (regexp-match-positions #px"yack\\b" "yackety yack")
'((8 . 12))                                           
```

the `yack` in `yackety` doesn’t end at a word boundary so it isn’t
matched.  The second `yack` does and is.

The metasequence `\B` \(also `#px` only\) has the opposite effect to
`\b`; it asserts that a word boundary does not exist. In

```racket
> (regexp-match-positions #px"an\\B" "an analysis")
'((3 . 5))                                         
```

the `an` that doesn’t end in a word boundary is matched.

### 9.4. Characters and Character Classes

Typically, a character in the regexp matches the same character in the
text string.  Sometimes it is necessary or convenient to use a regexp
metasequence to refer to a single character. For example, the
metasequence `\.` matches the period character.

The metacharacter `.` matches _any_ character \(other than newline in
multi-line mode; see Cloisters\):

```racket
> (regexp-match #rx"p.t" "pet")
'("pet")                       
```

The above pattern also matches `pat`, `pit`, `pot`, `put`, and `p8t`,
but not `peat` or `pfffft`.

A _character class_ matches any one character from a set of characters.
A typical format for this is the _bracketed character class_ `[`...`]`,
which matches any one character from the non-empty sequence of
characters enclosed within the brackets.  Thus, `#rx"p[aeiou]t"` matches
`pat`, `pet`, `pit`, `pot`, `put`, and nothing else.

Inside the brackets, a `-` between two characters specifies the Unicode
range between the characters.  For example, `#rx"ta[b-dgn-p]"` matches
`tab`, `tac`, `tad`, `tag`, `tan`, `tao`, and `tap`.

An initial `^` after the left bracket inverts the set specified by the
rest of the contents; i.e., it specifies the set of characters _other
than_ those identified in the brackets. For example, `#rx"do[^g]"`
matches all three-character sequences starting with `do` except `dog`.

Note that the metacharacter `^` inside brackets means something quite
different from what it means outside.  Most other metacharacters \(`.`,
`*`, `+`, `?`, etc.\) cease to be metacharacters when inside brackets,
although you may still escape them for peace of mind. A `-` is a
metacharacter only when it’s inside brackets, and when it is neither the
first nor the last character between the brackets.

Bracketed character classes cannot contain other bracketed character
classes \(although they contain certain other types of character
classes; see below\).  Thus, a `[` inside a bracketed character class
doesn’t have to be a metacharacter; it can stand for itself. For
example, `#rx"[a[b]"` matches `a`, `[`, and `b`.

Furthermore, since empty bracketed character classes are disallowed, a
`]` immediately occurring after the opening left bracket also doesn’t
need to be a metacharacter.  For example, `#rx"[]ab]"` matches `]`, `a`,
and `b`.

#### 9.4.1. Some Frequently Used Character Classes

In `#px` syntax, some standard character classes can be conveniently
represented as metasequences instead of as explicit bracketed
expressions:  `\d` matches a digit \(the same as `[0-9]`\); `\s` matches
an ASCII whitespace character; and `\w` matches a character that could
be part of a “word”.

> Following regexp custom, we identify “word” characters as
> `[A-Za-z0-9_]`, although these are too restrictive for what a Racketeer
> might consider a “word.”

The upper-case versions of these metasequences stand for the inversions
of the corresponding character classes: `\D` matches a non-digit, `\S` a
non-whitespace character, and `\W` a non-“word” character.

Remember to include a double backslash when putting these metasequences
in a Racket string:

```racket
> (regexp-match #px"\\d\\d"                  
   "0 dear, 1 have 2 read catch 22 before 9")
'("22")                                      
```

These character classes can be used inside a bracketed expression. For
example, `#px"[a-z\\d]"` matches a lower-case letter or a digit.

#### 9.4.2. POSIX character classes

A _POSIX character class_ is a special metasequence of the form
`[:`...`:]` that can be used only inside a bracketed expression in `#px`
syntax.  The POSIX classes supported are

* `[:alnum:]` — ASCII letters and digits

* `[:alpha:]` — ASCII letters

* `[:ascii:]` — ASCII characters

* `[:blank:]` — ASCII widthful whitespace: space and tab

* `[:cntrl:]` — “control” characters: ASCII 0 to 32

* `[:digit:]` — ASCII digits, same as `\d`

* `[:graph:]` — ASCII characters that use ink

* `[:lower:]` — ASCII lower-case letters

* `[:print:]` — ASCII ink-users plus widthful whitespace

* `[:space:]` — ASCII whitespace, same as `\s`

* `[:upper:]` — ASCII upper-case letters

* `[:word:]` — ASCII letters and `_`, same as `\w`

* `[:xdigit:]` — ASCII hex digits

For example, the `#px"[[:alpha:]_]"` matches a letter or underscore.

```racket
> (regexp-match #px"[[:alpha:]_]" "--x--")
'("x")                                    
> (regexp-match #px"[[:alpha:]_]" "--_--")
'("_")                                    
> (regexp-match #px"[[:alpha:]_]" "--:--")
#f                                        
```

The POSIX class notation is valid _only_ inside a bracketed expression.
For instance, `[:alpha:]`, when not inside a bracketed expression, will
not be read as the letter class.  Rather, it is \(from previous
principles\) the character class containing the characters `:`, `a`,
`l`, `p`, `h`.

```racket
> (regexp-match #px"[:alpha:]" "--a--")
'("a")                                 
> (regexp-match #px"[:alpha:]" "--x--")
#f                                     
```

### 9.5. Quantifiers

The _quantifiers_ `*`, `+`, and `?` match respectively: zero or more,
one or more, and zero or one instances of the preceding subpattern.

```racket
> (regexp-match-positions #rx"c[ad]*r" "cadaddadddr")
'((0 . 11))                                          
> (regexp-match-positions #rx"c[ad]*r" "cr")         
'((0 . 2))                                           
> (regexp-match-positions #rx"c[ad]+r" "cadaddadddr")
'((0 . 11))                                          
> (regexp-match-positions #rx"c[ad]+r" "cr")         
#f                                                   
> (regexp-match-positions #rx"c[ad]?r" "cadaddadddr")
#f                                                   
> (regexp-match-positions #rx"c[ad]?r" "cr")         
'((0 . 2))                                           
> (regexp-match-positions #rx"c[ad]?r" "car")        
'((0 . 3))                                           
```

In `#px` syntax, you can use braces to specify much finer-tuned
quantification than is possible with `*`, `+`, `?`:

* The quantifier `{`_m_`}` matches _exactly_ _m_ instances of the
  preceding subpattern; _m_ must be a nonnegative integer.

* The quantifier `{`_m_`,`_n_`}` matches at least _m_ and at most _n_
  instances.  `m` and `n` are nonnegative integers with _m_ less or
  equal to _n_.  You may omit either or both numbers, in which case _m_
  defaults to __0__ and _n_ to infinity.

It is evident that `+` and `?` are abbreviations for `{1,}` and `{0,1}`
respectively, and `*` abbreviates `{,}`, which is the same as `{0,}`.

```racket
> (regexp-match #px"[aeiou]{3}" "vacuous") 
'("uou")                                   
> (regexp-match #px"[aeiou]{3}" "evolve")  
#f                                         
> (regexp-match #px"[aeiou]{2,3}" "evolve")
#f                                         
> (regexp-match #px"[aeiou]{2,3}" "zeugma")
'("eu")                                    
```

The quantifiers described so far are all _greedy_: they match the
maximal number of instances that would still lead to an overall match
for the full pattern.

```racket
> (regexp-match #rx"<.*>" "<tag1> <tag2> <tag3>")
'("<tag1> <tag2> <tag3>")                        
```

To make these quantifiers _non-greedy_, append a `?` to them.
Non-greedy quantifiers match the minimal number of instances needed to
ensure an overall match.

```racket
> (regexp-match #rx"<.*?>" "<tag1> <tag2> <tag3>")
'("<tag1>")                                       
```

The non-greedy quantifiers are `*?`, `+?`, `??`, `{`_m_`}?`, and
`{`_m_`,`_n_`}?`, although `{`_m_`}?` is always the same as `{`_m_`}`.
Note that the metacharacter `?` has two different uses, and both uses
are represented in `??`.

### 9.6. Clusters

_Clustering_—enclosure within parens `(`...`)`—identifies the enclosed
_subpattern_ as a single entity.  It causes the matcher to capture the
_submatch_, or the portion of the string matching the subpattern, in
addition to the overall match:

```racket
> (regexp-match #rx"([a-z]+) ([0-9]+), ([0-9]+)" "jan 1, 1970")
'("jan 1, 1970" "jan" "1" "1970")                              
```

Clustering also causes a following quantifier to treat the entire
enclosed subpattern as an entity:

```racket
> (regexp-match #rx"(pu )*" "pu pu platter")
'("pu pu " "pu ")                           
```

The number of submatches returned is always equal to the number of
subpatterns specified in the regexp, even if a particular subpattern
happens to match more than one substring or no substring at all.

```racket
> (regexp-match #rx"([a-z ]+;)*" "lather; rinse; repeat;")
'("lather; rinse; repeat;" " repeat;")                    
```

Here, the `*`-quantified subpattern matches three times, but it is the
last submatch that is returned.

It is also possible for a quantified subpattern to fail to match, even
if the overall pattern matches.  In such cases, the failing submatch is
represented by `#f`

```racket
> (define date-re                             
    ; match ‘month year' or ‘month day, year';
    ; subpattern matches day, if present      
    #rx"([a-z]+) +([0-9]+,)? *([0-9]+)")      
> (regexp-match date-re "jan 1, 1970")        
'("jan 1, 1970" "jan" "1," "1970")            
> (regexp-match date-re "jan 1970")           
'("jan 1970" "jan" #f "1970")                 
```

#### 9.6.1. Backreferences

Submatches can be used in the insert string argument of the procedures
`regexp-replace` and `regexp-replace*`.  The insert string can use
`\`_n_ as a _backreference_ to refer back to the _n_th submatch, which
is the substring that matched the _n_th subpattern.  A `\0` refers to
the entire match, and it can also be specified as `\&`.

```racket
> (regexp-replace #rx"_(.+?)_"                      
    "the _nina_, the _pinta_, and the _santa maria_"
    "*\\1*")                                        
"the *nina*, the _pinta_, and the _santa maria_"    
> (regexp-replace* #rx"_(.+?)_"                     
    "the _nina_, the _pinta_, and the _santa maria_"
    "*\\1*")                                        
"the *nina*, the *pinta*, and the *santa maria*"    
> (regexp-replace #px"(\\S+) (\\S+) (\\S+)"         
    "eat to live"                                   
    "\\3 \\2 \\1")                                  
"live to eat"                                       
```

Use `\\` in the insert string to specify a literal backslash. Also, `\$`
stands for an empty string, and is useful for separating a backreference
`\`_n_ from an immediately following number.

Backreferences can also be used within a `#px` pattern to refer back to
an already matched subpattern in the pattern. `\`_n_ stands for an exact
repeat of the _n_th submatch. Note that `\0`, which is useful in an
insert string, makes no sense within the regexp pattern, because the
entire regexp has not matched yet so you cannot refer back to it.}

```racket
> (regexp-match #px"([a-z]+) and \\1"   
                "billions and billions")
'("billions and billions" "billions")   
```

Note that the backreference is not simply a repeat of the previous
subpattern.  Rather it is a repeat of the particular substring already
matched by the subpattern.

In the above example, the backreference can only match `billions`.  It
will not match `millions`, even though the subpattern it harks back
to—`([a-z]+)`—would have had no problem doing so:

```racket
> (regexp-match #px"([a-z]+) and \\1"   
                "billions and millions")
#f                                      
```

The following example marks all immediately repeating patterns in a
number string:

```racket
> (regexp-replace* #px"(\\d+)\\1"     
    "123340983242432420980980234"     
    "{\\1,\\1}")                      
"12{3,3}40983{24,24}3242{098,098}0234"
```

The following example corrects doubled words:

```racket
> (regexp-replace* #px"\\b(\\S+) \\1\\b"                          
    (string-append "now is the the time for all good men to "     
                   "to come to the aid of of the party")          
    "\\1")                                                        
"now is the time for all good men to come to the aid of the party"
```

#### 9.6.2. Non-capturing Clusters

It is often required to specify a cluster \(typically for
quantification\) but without triggering the capture of submatch
information.  Such clusters are called _non-capturing_.  To create a
non-capturing cluster, use `(?:` instead of `(` as the cluster opener.

In the following example, a non-capturing cluster eliminates the
“directory” portion of a given Unix pathname, and a capturing cluster
identifies the basename.

> But don’t parse paths with regexps. Use functions like `split-path`,
> instead.

```racket
> (regexp-match #rx"^(?:[a-z]*/)*([a-z]+)$"
                "/usr/local/bin/racket")   
'("/usr/local/bin/racket" "racket")        
```

#### 9.6.3. Cloisters

The location between the `?` and the `:` of a non-capturing cluster is
called a _cloister_. You can put modifiers there that will cause the
enclustered subpattern to be treated specially.  The modifier `i` causes
the subpattern to match case-insensitively:

> The term _cloister_ is a useful, if terminally cute, coinage from the
> abbots of Perl.

```racket
> (regexp-match #rx"(?i:hearth)" "HeartH")
'("HeartH")                               
```

The modifier `m` causes the subpattern to match in _multi-line mode_,
where `.` does not match a newline character, `^` can match just after a
newline, and `$` can match just before a newline.

```racket
> (regexp-match #rx"." "\na\n")                             
'("\n")                                                     
> (regexp-match #rx"(?m:.)" "\na\n")                        
'("a")                                                      
> (regexp-match #rx"^A plan$" "A man\nA plan\nA canal")     
#f                                                          
> (regexp-match #rx"(?m:^A plan$)" "A man\nA plan\nA canal")
'("A plan")                                                 
```

You can put more than one modifier in the cloister:

```racket
> (regexp-match #rx"(?mi:^A Plan$)" "a man\na plan\na canal")
'("a plan")                                                  
```

A minus sign before a modifier inverts its meaning.  Thus, you can use
`-i` in a _subcluster_ to overturn the case-insensitivities caused by an
enclosing cluster.

```racket
> (regexp-match #rx"(?i:the (?-i:TeX)book)"
                "The TeXbook")             
'("The TeXbook")                           
```

The above regexp will allow any casing for `the` and `book`, but it
insists that `TeX` not be differently cased.

### 9.7. Alternation

You can specify a list of _alternate_ subpatterns by separating them by
`|`.  The `|` separates subpatterns in the nearest enclosing cluster
\(or in the entire pattern string if there are no enclosing parens\).

```racket
> (regexp-match #rx"f(ee|i|o|um)" "a small, final fee")          
'("fi" "i")                                                      
> (regexp-replace* #rx"([yi])s(e[sdr]?|ing|ation)"               
                   (string-append                                
                    "analyse an energising organisation"         
                    " pulsing with noisy organisms")             
                   "\\1z\\2")                                    
"analyze an energizing organization pulsing with noisy organisms"
```

Note again that if you wish to use clustering merely to specify a list
of alternate subpatterns but do not want the submatch, use `(?:` instead
of `(`.

```racket
> (regexp-match #rx"f(?:ee|i|o|um)" "fun for all")
'("fo")                                           
```

An important thing to note about alternation is that the leftmost
matching alternate is picked regardless of its length.  Thus, if one of
the alternates is a prefix of a later alternate, the latter may not have
a chance to match.

```racket
> (regexp-match #rx"call|call-with-current-continuation"
                "call-with-current-continuation")       
'("call")                                               
```

To allow the longer alternate to have a shot at matching, place it
before the shorter one:

```racket
> (regexp-match #rx"call-with-current-continuation|call"
                "call-with-current-continuation")       
'("call-with-current-continuation")                     
```

In any case, an overall match for the entire regexp is always preferred
to an overall non-match.  In the following, the longer alternate still
wins, because its preferred shorter prefix fails to yield an overall
match.

```racket
> (regexp-match                                            
   #rx"(?:call|call-with-current-continuation) constrained"
   "call-with-current-continuation constrained")           
'("call-with-current-continuation constrained")            
```

### 9.8. Backtracking

We’ve already seen that greedy quantifiers match the maximal number of
times, but the overriding priority is that the overall match succeed.
Consider

```racket
> (regexp-match #rx"a*a" "aaaa")
'("aaaa")                       
```

The regexp consists of two subregexps: `a*` followed by `a`.  The
subregexp `a*` cannot be allowed to match all four `a`’s in the text
string `aaaa`, even though `*` is a greedy quantifier.  It may match
only the first three, leaving the last one for the second subregexp.
This ensures that the full regexp matches successfully.

The regexp matcher accomplishes this via a process called
_backtracking_.  The matcher tentatively allows the greedy quantifier to
match all four `a`’s, but then when it becomes clear that the overall
match is in jeopardy, it _backtracks_ to a less greedy match of three
`a`’s.  If even this fails, as in the call

```racket
> (regexp-match #rx"a*aa" "aaaa")
'("aaaa")                        
```

the matcher backtracks even further.  Overall failure is conceded only
when all possible backtracking has been tried with no success.

Backtracking is not restricted to greedy quantifiers. Nongreedy
quantifiers match as few instances as possible, and progressively
backtrack to more and more instances in order to attain an overall
match.  There is backtracking in alternation too, as the more rightward
alternates are tried when locally successful leftward ones fail to yield
an overall match.

Sometimes it is efficient to disable backtracking.  For example, we may
wish to commit to a choice, or we know that trying alternatives is
fruitless.  A nonbacktracking regexp is enclosed in `(?>`...`)`.

```racket
> (regexp-match #rx"(?>a+)." "aaaa")
#f                                  
```

In this call, the subregexp `?>a+` greedily matches all four `a`’s, and
is denied the opportunity to backtrack.  So, the overall match is
denied.  The effect of the regexp is therefore to match one or more
`a`’s followed by something that is definitely non-`a`.

### 9.9. Looking Ahead and Behind

You can have assertions in your pattern that look _ahead_ or _behind_ to
ensure that a subpattern does or does not occur. These “look around”
assertions are specified by putting the subpattern checked for in a
cluster whose leading characters are: `?=` \(for positive lookahead\),
`?!` \(negative lookahead\), `?<=` \(positive lookbehind\), `?<!`
\(negative lookbehind\).  Note that the subpattern in the assertion does
not generate a match in the final result; it merely allows or disallows
the rest of the match.

#### 9.9.1. Lookahead

Positive lookahead with `?=` peeks ahead to ensure that its subpattern
_could_ match.

```racket
> (regexp-match-positions #rx"grey(?=hound)"
    "i left my grey socks at the greyhound")
'((28 . 32))                                
```

The regexp `#rx"grey(?=hound)"` matches `grey`, but _only_ if it is
followed by `hound`.  Thus, the first `grey` in the text string is not
matched.

Negative lookahead with `?!` peeks ahead to ensure that its subpattern
_could not_ possibly match.

```racket
> (regexp-match-positions #rx"grey(?!hound)"
    "the gray greyhound ate the grey socks")
'((27 . 31))                                
```

The regexp `#rx"grey(?!hound)"` matches `grey`, but only if it is _not_
followed by `hound`.  Thus the `grey` just before `socks` is matched.

#### 9.9.2. Lookbehind

Positive lookbehind with `?<=` checks that its subpattern _could_ match
immediately to the left of the current position in the text string.

```racket
> (regexp-match-positions #rx"(?<=grey)hound"     
    "the hound in the picture is not a greyhound")
'((38 . 43))                                      
```

The regexp `#rx"(?<=grey)hound"` matches `hound`, but only if it is
preceded by `grey`.

Negative lookbehind with `?<!` checks that its subpattern could not
possibly match immediately to the left.

```racket
> (regexp-match-positions #rx"(?<!grey)hound"     
    "the greyhound in the picture is not a hound")
'((38 . 43))                                      
```

The regexp `#rx"(?<!grey)hound"` matches `hound`, but only if it is
_not_ preceded by `grey`.

Lookaheads and lookbehinds can be convenient when they are not
confusing.

### 9.10. An Extended Example

Here’s an extended example from Friedl’s _Mastering Regular
Expressions_, page 189, that covers many of the features described in
this chapter.  The problem is to fashion a regexp that will match any
and only IP addresses or _dotted quads_: four numbers separated by three
dots, with each number between 0 and 255.

First, we define a subregexp `n0-255` that matches 0 through 255:

```racket
> (define n0-255                    
    (string-append                  
     "(?:"                          
     "\\d|"        ;   0 through   9
     "\\d\\d|"     ;  00 through  99
     "[01]\\d\\d|" ; 000 through 199
     "2[0-4]\\d|"  ; 200 through 249
     "25[0-5]"     ; 250 through 255
     ")"))                          
```

> Note that `n0-255` lists prefixes as preferred alternates, which is
> something we cautioned against in Alternation.  However, since we intend
> to anchor this subregexp explicitly to force an overall match, the order
> of the alternates does not matter.

The first two alternates simply get all single- and double-digit
numbers.  Since 0-padding is allowed, we need to match both 1 and 01.
We need to be careful when getting 3-digit numbers, since numbers above
255 must be excluded.  So we fashion alternates to get 000 through 199,
then 200 through 249, and finally 250 through 255.

An IP-address is a string that consists of four `n0-255`s with three
dots separating them.

```racket
> (define ip-re1                          
    (string-append                        
     "^"        ; nothing before          
     n0-255     ; the first n0-255,       
     "(?:"      ; then the subpattern of  
     "\\."      ; a dot followed by       
     n0-255     ; an n0-255,              
     ")"        ; which is                
     "{3}"      ; repeated exactly 3 times
     "$"))                                
; with nothing following                  
```

Let’s try it out:

```racket
> (regexp-match (pregexp ip-re1) "1.2.3.4")       
'("1.2.3.4")                                      
> (regexp-match (pregexp ip-re1) "55.155.255.265")
#f                                                
```

which is fine, except that we also have

```racket
> (regexp-match (pregexp ip-re1) "0.00.000.00")
'("0.00.000.00")                               
```

All-zero sequences are not valid IP addresses!  Lookahead to the rescue.
Before starting to match `ip-re1`, we look ahead to ensure we don’t have
all zeros.  We could use positive lookahead to ensure there _is_ a digit
other than zero.

```racket
> (define ip-re                                    
    (pregexp                                       
     (string-append                                
       "(?=.*[1-9])" ; ensure there's a non-0 digit
       ip-re1)))                                   
```

Or we could use negative lookahead to ensure that what’s ahead isn’t
composed of _only_ zeros and dots.

```racket
> (define ip-re                                             
    (pregexp                                                
     (string-append                                         
       "(?![0.]*$)" ; not just zeros and dots               
                    ; (note: . is not metachar inside [...])
       ip-re1)))                                            
```

The regexp `ip-re` will match all and only valid IP addresses.

```racket
> (regexp-match ip-re "1.2.3.4")
'("1.2.3.4")                    
> (regexp-match ip-re "0.0.0.0")
#f                              
```

## 10. Exceptions and Control

Racket provides an especially rich set of control operations—not only
operations for raising and catching exceptions, but also operations for
grabbing and restoring portions of a computation.

    10.1 Exceptions        
    10.2 Prompts and Aborts
    10.3 Continuations     

### 10.1. Exceptions

Whenever a run-time error occurs, an _exception_ is raised. Unless the
exception is caught, then it is handled by printing a message associated
with the exception, and then escaping from the computation.

```racket
> (/ 1 0)              
/: division by zero    
> (car 17)             
car: contract violation
  expected: pair?      
  given: 17            
```

To catch an exception, use the `with-handlers` form:

```racket
(with-handlers ([predicate-expr handler-expr] ...)
  body ...+)                                      
```

Each `predicate-expr` in a handler determines a kind of exception that
is caught by the `with-handlers` form, and the value representing the
exception is passed to the handler procedure produced by `handler-expr`.
The result of the `handler-expr` is the result of the `with-handlers`
expression.

For example, a divide-by-zero error raises an instance of the
`exn:fail:contract:divide-by-zero` structure type:

```racket
> (with-handlers ([exn:fail:contract:divide-by-zero?
                   (lambda (exn) +inf.0)])          
    (/ 1 0))                                        
+inf.0                                              
> (with-handlers ([exn:fail:contract:divide-by-zero?
                   (lambda (exn) +inf.0)])          
    (car 17))                                       
car: contract violation                             
  expected: pair?                                   
  given: 17                                         
```

The `error` function is one way to raise your own exception. It packages
an error message and other information into an `exn:fail` structure:

```racket
> (error "crash!")                                    
crash!                                                
> (with-handlers ([exn:fail? (lambda (exn) 'air-bag)])
    (error "crash!"))                                 
'air-bag                                              
```

The `exn:fail:contract:divide-by-zero` and `exn:fail` structure types
are sub-types of the `exn` structure type. Exceptions raised by core
forms and functions always raise an instance of `exn` or one of its
sub-types, but an exception does not have to be represented by a
structure. The `raise` function lets you raise any value as an
exception:

```racket
> (raise 2)                                                     
uncaught exception: 2                                           
> (with-handlers ([(lambda (v) (equal? v 2)) (lambda (v) 'two)])
    (raise 2))                                                  
'two                                                            
> (with-handlers ([(lambda (v) (equal? v 2)) (lambda (v) 'two)])
    (/ 1 0))                                                    
/: division by zero                                             
```

Multiple `predicate-expr`s in a `with-handlers` form let you handle
different kinds of exceptions in different ways. The predicates are
tried in order, and if none of them match, then the exception is
propagated to enclosing contexts.

```racket
> (define (always-fail n)                              
    (with-handlers ([even? (lambda (v) 'even)]         
                    [positive? (lambda (v) 'positive)])
      (raise n)))                                      
> (always-fail 2)                                      
'even                                                  
> (always-fail 3)                                      
'positive                                              
> (always-fail -3)                                     
uncaught exception: -3                                 
> (with-handlers ([negative? (lambda (v) 'negative)])  
   (always-fail -3))                                   
'negative                                              
```

Using `(lambda (v) #t)` as a predicate captures all exceptions, of
course:

```racket
> (with-handlers ([(lambda (v) #t) (lambda (v) 'oops)])
    (car 17))                                          
'oops                                                  
```

Capturing all exceptions is usually a bad idea, however. If the user
types Ctl-C in a terminal window or clicks the Stop button in DrRacket
to interrupt a computation, then normally the `exn:break` exception
should not be caught. To catch only exceptions that represent errors,
use `exn:fail?` as the predicate:

```racket
> (with-handlers ([exn:fail? (lambda (v) 'oops)])   
    (car 17))                                       
'oops                                               
> (with-handlers ([exn:fail? (lambda (v) 'oops)])   
    (break-thread (current-thread)) ; simulate Ctl-C
    (car 17))                                       
user break                                          
```

### 10.2. Prompts and Aborts

When an exception is raised, control escapes out of an arbitrary deep
evaluation context to the point where the exception is caught—or all the
way out if the exception is never caught:

```racket
> (+ 1 (+ 1 (+ 1 (+ 1 (+ 1 (+ 1 (/ 1 0)))))))
/: division by zero                          
```

But if control escapes “all the way out,” why does the REPL keep going
after an error is printed? You might think that it’s because the REPL
wraps every interaction in a `with-handlers` form that catches all
exceptions, but that’s not quite the reason.

The actual reason is that the REPL wraps the interaction with a
_prompt_, which effectively marks the evaluation context with an escape
point. If an exception is not caught, then information about the
exception is printed, and then evaluation _aborts_ to the nearest
enclosing prompt. More precisely, each prompt has a _prompt tag_, and
there is a designated _default prompt tag_ that the uncaught-exception
handler uses to abort.

The `call-with-continuation-prompt` function installs a prompt with a
given prompt tag, and then it evaluates a given thunk under the prompt.
The `default-continuation-prompt-tag` function returns the default
prompt tag. The `abort-current-continuation` function escapes to the
nearest enclosing prompt that has a given prompt tag.

```racket
> (define (escape v)                                   
    (abort-current-continuation                        
     (default-continuation-prompt-tag)                 
     (lambda () v)))                                   
> (+ 1 (+ 1 (+ 1 (+ 1 (+ 1 (+ 1 (escape 0)))))))       
0                                                      
> (+ 1                                                 
     (call-with-continuation-prompt                    
      (lambda ()                                       
        (+ 1 (+ 1 (+ 1 (+ 1 (+ 1 (+ 1 (escape 0))))))))
      (default-continuation-prompt-tag)))              
1                                                      
```

In `escape` above, the value `v` is wrapped in a procedure that is
called after escaping to the enclosing prompt.

Prompts and aborts look very much like exception handling and raising.
Indeed, prompts and aborts are essentially a more primitive form of
exceptions, and `with-handlers` and `raise` are implemented in terms of
prompts and aborts. The power of the more primitive forms is related to
the word “continuation” in the operator names, as we discuss in the next
section.

### 10.3. Continuations

A _continuation_ is a value that encapsulates a piece of an expression’s
evaluation context. The `call-with-composable-continuation` function
captures the _current continuation_ starting outside the current
function call and running up to the nearest enclosing prompt. \(Keep in
mind that each REPL interaction is implicitly wrapped in a prompt.\)

For example, in

`(+` `1` `(+` `1` `(+` `1` `0)))`

at the point where `0` is evaluated, the expression context includes
three nested addition expressions. We can grab that context by changing
`0` to grab the continuation before returning 0:

```racket
> (define saved-k #f)                            
> (define (save-it!)                             
    (call-with-composable-continuation           
     (lambda (k) ; k is the captured continuation
       (set! saved-k k)                          
       0)))                                      
> (+ 1 (+ 1 (+ 1 (save-it!))))                   
3                                                
```

The continuation saved in `save-k` encapsulates the program context `(+
1 (+ 1 (+ 1 ?)))`, where `?` represents a place to plug in a result
value—because that was the expression context when `save-it!` was
called. The continuation is encapsulated so that it behaves like the
function `(lambda (v) (+ 1 (+ 1 (+ 1 v))))`:

```racket
> (saved-k 0)          
3                      
> (saved-k 10)         
13                     
> (saved-k (saved-k 0))
6                      
```

The continuation captured by `call-with-composable-continuation` is
determined dynamically, not syntactically. For example, with

```racket
> (define (sum n)             
    (if (zero? n)             
        (save-it!)            
        (+ n (sum (sub1 n)))))
> (sum 5)                     
15                            
```

the continuation in `saved-k` becomes `(lambda (x) (+ 5 (+ 4 (+ 3 (+ 2
(+ 1 x))))))`:

```racket
> (saved-k 0) 
15            
> (saved-k 10)
25            
```

A more traditional continuation operator in Racket \(or Scheme\) is
`call-with-current-continuation`, which is usually abbreviated
`call/cc`. It is like `call-with-composable-continuation`, but applying
the captured continuation first aborts \(to the current prompt\) before
restoring the saved continuation. In addition, Scheme systems
traditionally support a single prompt at the program start, instead of
allowing new prompts via `call-with-continuation-prompt`. Continuations
as in Racket are sometimes called _delimited continuations_, since a
program can introduce new delimiting prompts, and continuations as
captured by `call-with-composable-continuation` are sometimes called
_composable continuations_, because they do not have a built-in abort.

For an example of how continuations are useful, see \[missing\]. For
specific control operators that have more convenient names than the
primitives described here, see `racket/control`.

## 11. Iterations and Comprehensions

The `for` family of syntactic forms support iteration over _sequences_.
Lists, vectors, strings, byte strings, input ports, and hash tables can
all be used as sequences, and constructors like `in-range` offer even
more kinds of sequences.

Variants of `for` accumulate iteration results in different ways, but
they all have the same syntactic shape. Simplifying for now, the syntax
of `for` is

```racket
(for ([id sequence-expr] ...)
  body ...+)                 
```

A `for` loop iterates through the sequence produced by the
`sequence-expr`. For each element of the sequence, `for` binds the
element to `id`, and then it evaluates the `body`s for side effects.

Examples:

```racket
> (for ([i '(1 2 3)])  
    (display i))       
123                    
> (for ([i "abc"])     
    (printf "~a..." i))
a...b...c...           
> (for ([i 4])         
    (display i))       
0123                   
```

The `for/list` variant of `for` is more Racket-like. It accumulates
`body` results into a list, instead of evaluating `body` only for side
effects. In more technical terms, `for/list` implements a _list
comprehension_.

Examples:

```racket
> (for/list ([i '(1 2 3)])
    (* i i))              
'(1 4 9)                  
> (for/list ([i "abc"])   
    i)                    
'(#\a #\b #\c)            
> (for/list ([i 4])       
    i)                    
'(0 1 2 3)                
```

The full syntax of `for` accommodates multiple sequences to iterate in
parallel, and the `for*` variant nests the iterations instead of running
them in parallel. More variants of `for` and `for*` accumulate `body`
results in different ways.  In all of these variants, predicates that
prune iterations can be included along with bindings.

Before details on the variations of `for`, though, it’s best to see the
kinds of sequence generators that make interesting examples.

### 11.1. Sequence Constructors

The `in-range` function generates a sequence of numbers, given an
optional starting number \(which defaults to `0`\), a number before
which the sequence ends, and an optional step \(which defaults to `1`\).
Using a non-negative integer `k` directly as a sequence is a shorthand
for `(in-range k)`.

Examples:

```racket
> (for ([i 3])                 
    (display i))               
012                            
> (for ([i (in-range 3)])      
    (display i))               
012                            
> (for ([i (in-range 1 4)])    
    (display i))               
123                            
> (for ([i (in-range 1 4 2)])  
    (display i))               
13                             
> (for ([i (in-range 4 1 -1)]) 
    (display i))               
432                            
> (for ([i (in-range 1 4 1/2)])
    (printf " ~a " i))         
 1  3/2  2  5/2  3  7/2        
```

The `in-naturals` function is similar, except that the starting number
must be an exact non-negative integer \(which defaults to `0`\), the
step is always `1`, and there is no upper limit. A `for` loop using just
`in-naturals` will never terminate unless a body expression raises an
exception or otherwise escapes.

Example:

```racket
> (for ([i (in-naturals)]) 
    (if (= i 10)           
        (error "too much!")
        (display i)))      
0123456789                 
too much!                  
```

The `stop-before` and `stop-after` functions construct a new sequence
given a sequence and a predicate. The new sequence is like the given
sequence, but truncated either immediately before or immediately after
the first element for which the predicate returns true.

Example:

```racket
> (for ([i (stop-before "abc def"          
                        char-whitespace?)])
    (display i))                           
abc                                        
```

Sequence constructors like `in-list`, `in-vector` and `in-string` simply
make explicit the use of a list, vector, or string as a sequence. Along
with `in-range`, these constructors raise an exception when given the
wrong kind of value, and since they otherwise avoid a run-time dispatch
to determine the sequence type, they enable more efficient code
generation; see Iteration Performance for more information.

Examples:

```racket
> (for ([i (in-string "abc")])   
    (display i))                 
abc                              
> (for ([i (in-string '(1 2 3))])
    (display i))                 
in-string: contract violation    
  expected: string               
  given: '(1 2 3)                
```

> +\[missing\] in \[missing\] provides more on sequences.

### 11.2. `for` and `for*`

A more complete syntax of `for` is

```racket
(for (clause ...)             
  body ...+)                  
                              
clause = [id sequence-expr]   
       | #:when boolean-expr  
       | #:unless boolean-expr
```

When multiple `[id sequence-expr]` clauses are provided in a `for` form,
the corresponding sequences are traversed in parallel:

```racket
> (for ([i (in-range 1 4)]                          
        [chapter '("Intro" "Details" "Conclusion")])
    (printf "Chapter ~a. ~a\n" i chapter))          
Chapter 1. Intro                                    
Chapter 2. Details                                  
Chapter 3. Conclusion                               
```

With parallel sequences, the `for` expression stops iterating when any
sequence ends. This behavior allows `in-naturals`, which creates an
infinite sequence of numbers, to be used for indexing:

```racket
> (for ([i (in-naturals 1)]                         
        [chapter '("Intro" "Details" "Conclusion")])
    (printf "Chapter ~a. ~a\n" i chapter))          
Chapter 1. Intro                                    
Chapter 2. Details                                  
Chapter 3. Conclusion                               
```

The `for*` form, which has the same syntax as `for`, nests multiple
sequences instead of running them in parallel:

```racket
> (for* ([book '("Guide" "Reference")]               
         [chapter '("Intro" "Details" "Conclusion")])
    (printf "~a ~a\n" book chapter))                 
Guide Intro                                          
Guide Details                                        
Guide Conclusion                                     
Reference Intro                                      
Reference Details                                    
Reference Conclusion                                 
```

Thus, `for*` is a shorthand for nested `for`s in the same way that
`let*` is a shorthand for nested `let`s.

The `#:when boolean-expr` form of a `clause` is another shorthand. It
allows the `body`s to evaluate only when the `boolean-expr` produces a
true value:

```racket
> (for* ([book '("Guide" "Reference")]              
         [chapter '("Intro" "Details" "Conclusion")]
         #:when (not (equal? chapter "Details")))   
    (printf "~a ~a\n" book chapter))                
Guide Intro                                         
Guide Conclusion                                    
Reference Intro                                     
Reference Conclusion                                
```

A `boolean-expr` with `#:when` can refer to any of the preceding
iteration bindings. In a `for` form, this scoping makes sense only if
the test is nested in the iteration of the preceding bindings; thus,
bindings separated by `#:when` are mutually nested, instead of in
parallel, even with `for`.

```racket
> (for ([book '("Guide" "Reference" "Notes")]              
        #:when (not (equal? book "Notes"))                 
        [i (in-naturals 1)]                                
        [chapter '("Intro" "Details" "Conclusion" "Index")]
        #:when (not (equal? chapter "Index")))             
    (printf "~a Chapter ~a. ~a\n" book i chapter))         
Guide Chapter 1. Intro                                     
Guide Chapter 2. Details                                   
Guide Chapter 3. Conclusion                                
Reference Chapter 1. Intro                                 
Reference Chapter 2. Details                               
Reference Chapter 3. Conclusion                            
```

An `#:unless` clause is analogus to a `#:when` clause, but the `body`s
evaluate only when the `boolean-expr` produces a false value.

### 11.3. `for/list` and `for*/list`

The `for/list` form, which has the same syntax as `for`, evaluates the
`body`s to obtain values that go into a newly constructed list:

```racket
> (for/list ([i (in-naturals 1)]                         
             [chapter '("Intro" "Details" "Conclusion")])
    (string-append (number->string i) ". " chapter))     
'("1. Intro" "2. Details" "3. Conclusion")               
```

A `#:when` clause in a `for-list` form prunes the result list along with
evaluations of the `body`s:

```racket
> (for/list ([i (in-naturals 1)]                        
             [chapter '("Intro" "Details" "Conclusion")]
             #:when (odd? i))                           
    chapter)                                            
'("Intro" "Conclusion")                                 
```

This pruning behavior of `#:when` is more useful with `for/list` than
`for`. Whereas a plain `when` form normally suffices with `for`, a
`when` expression form in a `for/list` would cause the result list to
contain `#<void>`s instead of omitting list elements.

The `for*/list` form is like `for*`, nesting multiple iterations:

```racket
> (for*/list ([book '("Guide" "Ref.")]                      
              [chapter '("Intro" "Details")])               
    (string-append book " " chapter))                       
'("Guide Intro" "Guide Details" "Ref. Intro" "Ref. Details")
```

A `for*/list` form is not quite the same thing as nested `for/list`
forms. Nested `for/list`s would produce a list of lists, instead of one
flattened list. Much like `#:when`, then, the nesting of `for*/list` is
more useful than the nesting of `for*`.

### 11.4. `for/vector` and `for*/vector`

The `for/vector` form can be used with the same syntax as the `for/list`
form, but the evaluated `body`s go into a newly-constructed vector
instead of a list:

```racket
> (for/vector ([i (in-naturals 1)]                         
               [chapter '("Intro" "Details" "Conclusion")])
    (string-append (number->string i) ". " chapter))       
'#("1. Intro" "2. Details" "3. Conclusion")                
```

The `for*/vector` form behaves similarly, but the iterations are nested
as in `for*`.

The `for/vector` and `for*/vector` forms also allow the length of the
vector to be constructed to be supplied in advance.  The resulting
iteration can be performed more efficiently than plain `for/vector` or
`for*/vector`:

```racket
> (let ([chapters '("Intro" "Details" "Conclusion")])          
    (for/vector #:length (length chapters) ([i (in-naturals 1)]
                                            [chapter chapters])
      (string-append (number->string i) ". " chapter)))        
'#("1. Intro" "2. Details" "3. Conclusion")                    
```

If a length is provided, the iteration stops when the vector is filled
or the requested iterations are complete, whichever comes first.  If the
provided length exceeds the requested number of iterations, then the
remaining slots in the vector are initialized to the default argument of
`make-vector`.

### 11.5. `for/and` and `for/or`

The `for/and` form combines iteration results with `and`, stopping as
soon as it encounters `#f`:

```racket
> (for/and ([chapter '("Intro" "Details" "Conclusion")])
    (equal? chapter "Intro"))                           
#f                                                      
```

The `for/or` form combines iteration results with `or`, stopping as soon
as it encounters a true value:

```racket
> (for/or ([chapter '("Intro" "Details" "Conclusion")])
    (equal? chapter "Intro"))                          
#t                                                     
```

As usual, the `for*/and` and `for*/or` forms provide the same facility
with nested iterations.

### 11.6. `for/first` and `for/last`

The `for/first` form returns the result of the first time that the
`body`s are evaluated, skipping further iterations. This form is most
useful with a `#:when` clause.

```racket
> (for/first ([chapter '("Intro" "Details" "Conclusion" "Index")]
              #:when (not (equal? chapter "Intro")))             
    chapter)                                                     
"Details"                                                        
```

If the `body`s are evaluated zero times, then the result is `#f`.

The `for/last` form runs all iterations, returning the value of the last
iteration \(or `#f` if no iterations are run\):

```racket
> (for/last ([chapter '("Intro" "Details" "Conclusion" "Index")]
              #:when (not (equal? chapter "Index")))            
    chapter)                                                    
"Conclusion"                                                    
```

As usual, the `for*/first` and `for*/last` forms provide the same
facility with nested iterations:

```racket
> (for*/first ([book '("Guide" "Reference")]                      
               [chapter '("Intro" "Details" "Conclusion" "Index")]
               #:when (not (equal? chapter "Intro")))             
    (list book chapter))                                          
'("Guide" "Details")                                              
> (for*/last ([book '("Guide" "Reference")]                       
              [chapter '("Intro" "Details" "Conclusion" "Index")] 
              #:when (not (equal? chapter "Index")))              
    (list book chapter))                                          
'("Reference" "Conclusion")                                       
```

### 11.7. `for/fold` and `for*/fold`

The `for/fold` form is a very general way to combine iteration results.
Its syntax is slightly different than the syntax of `for`, because
accumulation variables must be declared at the beginning:

```racket
(for/fold ([accum-id init-expr] ...)
          (clause ...)              
  body ...+)                        
```

In the simple case, only one `[accum-id init-expr]` is provided, and the
result of the `for/fold` is the final value for `accum-id`, which starts
out with the value of `init-expr`. In the `clause`s and `body`s,
`accum-id` can be referenced to get its current value, and the last
`body` provides the value of `accum-id` for the next iteration.

Examples:

```racket
> (for/fold ([len 0])                                             
            ([chapter '("Intro" "Conclusion")])                   
    (+ len (string-length chapter)))                              
15                                                                
> (for/fold ([prev #f])                                           
            ([i (in-naturals 1)]                                  
             [chapter '("Intro" "Details" "Details" "Conclusion")]
             #:when (not (equal? chapter prev)))                  
    (printf "~a. ~a\n" i chapter)                                 
    chapter)                                                      
1. Intro                                                          
2. Details                                                        
4. Conclusion                                                     
"Conclusion"                                                      
```

When multiple `accum-id`s are specified, then the last `body` must
produce multiple values, one for each `accum-id`. The `for/fold`
expression itself produces multiple values for the results.

Example:

```racket
> (for/fold ([prev #f]                                            
             [counter 1])                                         
            ([chapter '("Intro" "Details" "Details" "Conclusion")]
             #:when (not (equal? chapter prev)))                  
    (printf "~a. ~a\n" counter chapter)                           
    (values chapter                                               
            (add1 counter)))                                      
1. Intro                                                          
2. Details                                                        
3. Conclusion                                                     
"Conclusion"                                                      
4                                                                 
```

### 11.8. Multiple-Valued Sequences

In the same way that a function or expression can produce multiple
values, individual iterations of a sequence can produce multiple
elements.  For example, a hash table as a sequence generates two values
for each iteration: a key and a value.

In the same way that `let-values` binds multiple results to multiple
identifiers, `for` can bind multiple sequence elements to multiple
iteration identifiers:

> While `let` must be changed to `let-values` to bind multiple
> identifiers, `for` simply allows a parenthesized list of identifiers
> instead of a single identifier in any clause.

```racket
> (for ([(k v) #hash(("apple" . 1) ("banana" . 3))])
    (printf "~a count: ~a\n" k v))                  
banana count: 3                                     
apple count: 1                                      
```

This extension to multiple-value bindings works for all `for` variants.
For example, `for*/list` nests iterations, builds a list, and also works
with multiple-valued sequences:

```racket
> (for*/list ([(k v) #hash(("apple" . 1) ("banana" . 3))]
              [(i) (in-range v)])                        
    k)                                                   
'("banana" "banana" "banana" "apple")                    
```

### 11.9. Breaking an Iteration

An even more complete syntax of `for` is

```racket
(for (clause ...)                    
  body-or-break ... body)            
                                     
clause        = [id sequence-expr]   
              | #:when boolean-expr  
              | #:unless boolean-expr
              | break                
                                     
body-or-break = body                 
              | break                
                                     
break         = #:break boolean-expr 
              | #:final boolean-expr 
```

That is, a `#:break` or `#:final` clause can be included among the
binding clauses and body of the iteration. Among the binding clauses,
`#:break` is like `#:unless` but when its `boolean-expr` is true, all
sequences within the `for` are stopped. Among the `body`s, `#:break` has
the same effect on sequences when its `boolean-expr` is true, and it
also prevents later `body`s from evaluation in the current iteration.

For example, while using `#:unless` between clauses effectively skips
later sequences as well as the body,

```racket
> (for ([book '("Guide" "Story" "Reference")]       
        #:unless (equal? book "Story")              
        [chapter '("Intro" "Details" "Conclusion")])
    (printf "~a ~a\n" book chapter))                
Guide Intro                                         
Guide Details                                       
Guide Conclusion                                    
Reference Intro                                     
Reference Details                                   
Reference Conclusion                                
```

using `#:break` causes the entire `for` iteration to terminate:

```racket
> (for ([book '("Guide" "Story" "Reference")]        
        #:break (equal? book "Story")                
        [chapter '("Intro" "Details" "Conclusion")]) 
    (printf "~a ~a\n" book chapter))                 
Guide Intro                                          
Guide Details                                        
Guide Conclusion                                     
> (for* ([book '("Guide" "Story" "Reference")]       
         [chapter '("Intro" "Details" "Conclusion")])
    #:break (and (equal? book "Story")               
                 (equal? chapter "Conclusion"))      
    (printf "~a ~a\n" book chapter))                 
Guide Intro                                          
Guide Details                                        
Guide Conclusion                                     
Story Intro                                          
Story Details                                        
```

A `#:final` clause is similar to `#:break`, but it does not immediately
terminate the iteration. Instead, it allows at most one more element to
be drawn for each sequence and at most one more evaluation of the
`body`s.

```racket
> (for* ([book '("Guide" "Story" "Reference")]       
         [chapter '("Intro" "Details" "Conclusion")])
    #:final (and (equal? book "Story")               
                 (equal? chapter "Conclusion"))      
    (printf "~a ~a\n" book chapter))                 
Guide Intro                                          
Guide Details                                        
Guide Conclusion                                     
Story Intro                                          
Story Details                                        
Story Conclusion                                     
> (for ([book '("Guide" "Story" "Reference")]        
        #:final (equal? book "Story")                
        [chapter '("Intro" "Details" "Conclusion")]) 
    (printf "~a ~a\n" book chapter))                 
Guide Intro                                          
Guide Details                                        
Guide Conclusion                                     
Story Intro                                          
```

### 11.10. Iteration Performance

Ideally, a `for` iteration should run as fast as a loop that you write
by hand as a recursive-function invocation. A hand-written loop,
however, is normally specific to a particular kind of data, such as
lists. In that case, the hand-written loop uses selectors like `car` and
`cdr` directly, instead of handling all forms of sequences and
dispatching to an appropriate iterator.

The `for` forms can provide the performance of hand-written loops when
enough information is apparent about the sequences to iterate.
Specifically, the clause should have one of the following `fast-clause`
forms:

  `fast-clause`` `=` ``[id` `fast-seq]`                 
` `            ` `|` ``[(id)` `fast-seq]`               
` `            ` `|` ``[(id` `id)` `fast-indexed-seq]`  
` `            ` `|` ``[(id` `...)` `fast-parallel-seq]`

  `fast-seq`` `=` ``(in-range` `expr)`                        
` `         ` `|` ``(in-range` `expr` `expr)`                 
` `         ` `|` ``(in-range` `expr` `expr` `expr)`          
` `         ` `|` ``(in-naturals)`                            
` `         ` `|` ``(in-naturals` `expr)`                     
` `         ` `|` ``(in-list` `expr)`                         
` `         ` `|` ``(in-vector` `expr)`                       
` `         ` `|` ``(in-string` `expr)`                       
` `         ` `|` ``(in-bytes` `expr)`                        
` `         ` `|` ``(in-value` `expr)`                        
` `         ` `|` ``(stop-before` `fast-seq` `predicate-expr)`
` `         ` `|` ``(stop-after` `fast-seq` `predicate-expr)` 

  `fast-indexed-seq`` `=` ``(in-indexed` `fast-seq)`                          
` `                 ` `|` ``(stop-before` `fast-indexed-seq` `predicate-expr)`
` `                 ` `|` ``(stop-after` `fast-indexed-seq` `predicate-expr)` 

  `fast-parallel-seq`` `=` ``(in-parallel` `fast-seq` `...)`                    
` `                  ` `|` ``(stop-before` `fast-parallel-seq` `predicate-expr)`
` `                  ` `|` ``(stop-after` `fast-parallel-seq` `predicate-expr)` 

Examples:

```racket
> (time (for ([i (in-range 100000)])                         
          (for ([elem (in-list '(a b c d e f g h))]) ; fast  
            (void))))                                        
cpu time: 2 real time: 2 gc time: 0                          
> (time (for ([i (in-range 100000)])                         
          (for ([elem '(a b c d e f g h)])           ; slower
            (void))))                                        
cpu time: 39 real time: 38 gc time: 0                        
> (time (let ([seq (in-list '(a b c d e f g h))])            
          (for ([i (in-range 100000)])                       
            (for ([elem seq])                        ; slower
              (void)))))                                     
cpu time: 45 real time: 44 gc time: 0                        
```

The grammars above are not complete, because the set of syntactic
patterns that provide good performance is extensible, just like the set
of sequence values. The documentation for a sequence constructor should
indicate the performance benefits of using it directly in a `for`
`clause`.

> +\[missing\] in \[missing\] provides more on iterations and
> comprehensions.

## 12. Pattern Matching

The `match` form supports pattern matching on arbitrary Racket values,
as opposed to functions like `regexp-match` that compare regular
expressions to byte and character sequences \(see Regular Expressions\).

```racket
(match target-expr        
  [pattern expr ...+] ...)
```

The `match` form takes the result of `target-expr` and tries to match
each `pattern` in order. As soon as it finds a match, it evaluates the
corresponding `expr` sequence to obtain the result for the `match` form.
If `pattern` includes _pattern variables_, they are treated like
wildcards, and each variable is bound in the `expr` to the input
fragments that it matched.

Most Racket literal expressions can be used as patterns:

```racket
> (match 2           
    [1 'one]         
    [2 'two]         
    [3 'three])      
'two                 
> (match #f          
    [#t 'yes]        
    [#f 'no])        
'no                  
> (match "apple"     
    ['apple 'symbol] 
    ["apple" 'string]
    [#f 'boolean])   
'string              
```

Constructors like `cons`, `list`, and `vector` can be used to create
patterns that match pairs, lists, and vectors:

```racket
> (match '(1 2)            
    [(list 0 1) 'one]      
    [(list 1 2) 'two])     
'two                       
> (match '(1 . 2)          
    [(list 1 2) 'list]     
    [(cons 1 2) 'pair])    
'pair                      
> (match #(1 2)            
    [(list 1 2) 'list]     
    [(vector 1 2) 'vector])
'vector                    
```

A constructor bound with `struct` also can be used as a pattern
constructor:

```racket
> (struct shoe (size color))  
> (struct hat (size style))   
> (match (hat 23 'bowler)     
   [(shoe 10 'white) "bottom"]
   [(hat 23 'bowler) "top"])  
"top"                         
```

Unquoted, non-constructor identifiers in a pattern are pattern variables
that are bound in the result expressions, except `_`, which does not
bind \(and thus is usually used as a catch-all\):

```racket
> (match '(1)                      
    [(list x) (+ x 1)]             
    [(list x y) (+ x y)])          
2                                  
> (match '(1 2)                    
    [(list x) (+ x 1)]             
    [(list x y) (+ x y)])          
3                                  
> (match (hat 23 'bowler)          
    [(shoe sz col) sz]             
    [(hat sz stl) sz])             
23                                 
> (match (hat 11 'cowboy)          
    [(shoe sz 'black) 'a-good-shoe]
    [(hat sz 'bowler) 'a-good-hat] 
    [_ 'something-else])           
'something-else                    
```

An ellipsis, written `...`, acts like a Kleene star within a list or
vector pattern: the preceding sub-pattern can be used to match any
number of times for any number of consecutive elements of the list or
vector. If a sub-pattern followed by an ellipsis includes a pattern
variable, the variable matches multiple times, and it is bound in the
result expression to a list of matches:

```racket
> (match '(1 1 1)                                  
    [(list 1 ...) 'ones]                           
    [_ 'other])                                    
'ones                                              
> (match '(1 1 2)                                  
    [(list 1 ...) 'ones]                           
    [_ 'other])                                    
'other                                             
> (match '(1 2 3 4)                                
    [(list 1 x ... 4) x])                          
'(2 3)                                             
> (match (list (hat 23 'bowler) (hat 22 'pork-pie))
    [(list (hat sz styl) ...) (apply + sz)])       
45                                                 
```

Ellipses can be nested to match nested repetitions, and in that case,
pattern variables can be bound to lists of lists of matches:

```racket
> (match '((! 1) (! 2 2) (! 3 3 3))
    [(list (list '! x ...) ...) x])
'((1) (2 2) (3 3 3))               
```

The `quasiquote` form  \(see Quasiquoting: `quasiquote` and `‘` for more
about it\) can also be used to build patterns. While unquoted portions
of a normal quasiquoted form mean regular racket evaluation, here
unquoted portions mean go back to regular pattern matching.

So, in the example below, the with expression is the pattern and it gets
rewritten into the application expression, using quasiquote as a pattern
in the first instance and quasiquote to build an expression in the
second.

```racket
> (match `{with {x 1} {+ x 1}}     
    [`{with {,id ,rhs} ,body}      
     `{{lambda {,id} ,body} ,rhs}])
'((lambda (x) (+ x 1)) 1)          
```

For information on many more pattern forms, see `racket/match`.

Forms like `match-let` and `match-lambda` support patterns in positions
that otherwise must be identifiers. For example, `match-let` generalizes
`let` to a destructing bind:

```racket
> (match-let ([(list x y z) '(1 2 3)])
    (list z y x))                     
'(3 2 1)                              
```

For information on these additional forms, see `racket/match`.

> +\[missing\] in \[missing\] provides more on pattern matching.

## 13. Classes and Objects

> This chapter is based on a paper \[Flatt06\].

A `class` expression denotes a first-class value, just like a `lambda`
expression:

```racket
(class superclass-expr decl-or-expr ...)
```

The `superclass-expr` determines the superclass for the new class. Each
`decl-or-expr` is either a declaration related to methods, fields, and
initialization arguments, or it is an expression that is evaluated each
time that the class is instantiated. In other words, instead of a
method-like constructor, a class has initialization expressions
interleaved with field and method declarations.

By convention, class names end with `%`. The built-in root class is
`object%`. The following expression creates a class with public methods
`get-size`, `grow`, and `eat`:

```racket
(class object%                                          
  (init size)                ; initialization argument  
                                                        
  (define current-size size) ; field                    
                                                        
  (super-new)                ; superclass initialization
                                                        
  (define/public (get-size)                             
    current-size)                                       
                                                        
  (define/public (grow amt)                             
    (set! current-size (+ amt current-size)))           
                                                        
  (define/public (eat other-fish)                       
    (grow (send other-fish get-size))))                 
```

The `size` initialization argument must be supplied via a named
argument when instantiating the class through the `new` form:

`(new` `(class` `object%` `(init` `size)` `....)` `[size` `10])`

Of course, we can also name the class and its instance:

```racket
(define fish% (class object% (init size) ....))
(define charlie (new fish% [size 10]))         
```

In the definition of `fish%`, `current-size` is a private field that
starts out with the value of the `size` initialization argument.
Initialization arguments like `size` are available only during class
instantiation, so they cannot be referenced directly from a method. The
`current-size` field, in contrast, is available to methods.

The `(super-new)` expression in `fish%` invokes the initialization of
the superclass. In this case, the superclass is `object%`, which takes
no initialization arguments and performs no work; `super-new` must be
used, anyway, because a class must always invoke its superclass’s
initialization.

Initialization arguments, field declarations, and expressions such as
`(super-new)` can appear in any order within a `class`, and they can be
interleaved with method declarations. The relative order of expressions
in the class determines the order of evaluation during instantiation.
For example, if a field’s initial value requires calling a method that
works only after superclass initialization, then the field declaration
must be placed after the `super-new` call. Ordering field and
initialization declarations in this way helps avoid imperative
assignment. The relative order of method declarations makes no
difference for evaluation, because methods are fully defined before a
class is instantiated.

### 13.1. Methods

Each of the three `define/public` declarations in `fish%` introduces a
new method. The declaration uses the same syntax as a Racket function,
but a method is not accessible as an independent function.  A call to
the `grow` method of a `fish%` object requires the `send` form:

```racket
> (send charlie grow 6)  
> (send charlie get-size)
16                       
```

Within `fish%`, self methods can be called like functions, because the
method names are in scope.  For example, the `eat` method within `fish%`
directly invokes the `grow` method.  Within a class, attempting to use a
method name in any way other than a method call results in a syntax
error.

In some cases, a class must call methods that are supplied by the
superclass but not overridden. In that case, the class can use `send`
with `this` to access the method:

```racket                                                   
(define hungry-fish% (class fish% (super-new)               
                       (define/public (eat-more fish1 fish2)
                         (send this eat fish1)              
                         (send this eat fish2))))           
```                                                         
                                                            
                                                            

Alternately, the class can declare the existence of a method using
`inherit`, which brings the method name into scope for a direct call:

```racket                                                   
(define hungry-fish% (class fish% (super-new)               
                       (inherit eat)                        
                       (define/public (eat-more fish1 fish2)
                         (eat fish1) (eat fish2))))         
```                                                         
                                                            
                                                            

With the `inherit` declaration, if `fish%` had not provided an `eat`
method, an error would be signaled in the evaluation of the `class` form
for `hungry-fish%`. In contrast, with `(send this ....)`, an error would
not be signaled until the `eat-more` method is called and the `send`
form is evaluated. For this reason, `inherit` is preferred.

Another drawback of `send` is that it is less efficient than `inherit`.
Invocation of a method via `send` involves finding a method in the
target object’s class at run time, making `send` comparable to an
interface-based method call in Java. In contrast, `inherit`-based method
invocations use an offset within the class’s method table that is
computed when the class is created.

To achieve performance similar to `inherit`-based method calls when
invoking a method from outside the method’s class, the programmer must
use the `generic` form, which produces a class- and method-specific
_generic method_ to be invoked with `send-generic`:

`(define` `get-fish-size` `(generic` `fish%` `get-size))`   
                                                            
```racket                                                   
> (send-generic charlie get-fish-size)                      
16                                                          
> (send-generic (new hungry-fish% [size 32]) get-fish-size) 
32                                                          
> (send-generic (new object%) get-fish-size)                
generic:get-size: target is not an instance of the generic's
class                                                       
  target: (object)                                          
  class name: fish%                                         
```                                                         

Roughly speaking, the form translates the class and the external method
name to a location in the class’s method table. As illustrated by the
last example, sending through a generic method checks that its argument
is an instance of the generic’s class.

Whether a method is called directly within a `class`, through a generic
method, or through `send`, method overriding works in the usual way:

```racket                                          
(define picky-fish% (class fish% (super-new)       
                      (define/override (grow amt)  
                                                   
                        (super grow (* 3/4 amt)))))
(define daisy (new picky-fish% [size 20]))         
```                                                
                                                   
```racket                                          
> (send daisy eat charlie)                         
> (send daisy get-size)                            
32                                                 
```                                                

The `grow` method in `picky-fish%` is declared with `define/override`
instead of `define/public`, because `grow` is meant as an overriding
declaration. If `grow` had been declared with `define/public`, an error
would have been signaled when evaluating the `class` expression, because
`fish%` already supplies `grow`.

Using `define/override` also allows the invocation of the overridden
method via a `super` call. For example, the `grow` implementation in
`picky-fish%` uses `super` to delegate to the superclass implementation.

### 13.2. Initialization Arguments

Since `picky-fish%` declares no initialization arguments, any
initialization values supplied in `(new picky-fish% ....)`  are
propagated to the superclass initialization, i.e., to `fish%`. A
subclass can supply additional initialization arguments for its
superclass in a `super-new` call, and such initialization arguments take
precedence over arguments supplied to `new`. For example, the following
`size-10-fish%` class always generates fish of size 10:

`(define` `size-10-fish%` `(class` `fish%` `(super-new` `[size` `10])))`
                                                                        
```racket                                                               
> (send (new size-10-fish%) get-size)                                   
10                                                                      
```                                                                     

In the case of `size-10-fish%`, supplying a `size` initialization
argument with `new` would result in an initialization error; because the
`size` in `super-new` takes precedence, a `size` supplied to `new` would
have no target declaration.

An initialization argument is optional if the `class` form declares a
default value. For example, the following `default-10-fish%` class
accepts a `size` initialization argument, but its value defaults to 10
if no value is supplied on instantiation:

```racket                                           
(define default-10-fish% (class fish%               
                           (init [size 10])         
                           (super-new [size size])))
```                                                 
                                                    
```racket                                           
> (new default-10-fish%)                            
(object:default-10-fish% ...)                       
> (new default-10-fish% [size 20])                  
(object:default-10-fish% ...)                       
```                                                 

In this example, the `super-new` call propagates its own `size` value as
the `size` initialization argument to the superclass.

### 13.3. Internal and External Names

The two uses of `size` in `default-10-fish%` expose the double life of
class-member identifiers. When `size` is the first identifier of a
bracketed pair in `new` or `super-new`, `size` is an _external name_
that is symbolically matched to an initialization argument in a class.
When `size` appears as an expression within `default-10-fish%`, `size`
is an _internal name_ that is lexically scoped. Similarly, a call to an
inherited `eat` method uses `eat` as an internal name, whereas a `send`
of `eat` uses `eat` as an external name.

The full syntax of the `class` form allows a programmer to specify
distinct internal and external names for a class member. Since internal
names are local, they can be renamed to avoid shadowing or conflicts.
Such renaming is not frequently necessary, but workarounds in the
absence of renaming can be especially cumbersome.

### 13.4. Interfaces

Interfaces are useful for checking that an object or a class implements
a set of methods with a particular \(implied\) behavior. This use of
interfaces is helpful even without a static type system \(which is the
main reason that Java has interfaces\).

An interface in Racket is created using the `interface` form, which
merely declares the method names required to implement the interface. An
interface can extend other interfaces, which means that implementations
of the interface automatically implement the extended interfaces.

```racket
(interface (superinterface-expr ...) id ...)
```

To declare that a class implements an interface, the `class*` form must
be used instead of `class`:

```racket
(class* superclass-expr (interface-expr ...) decl-or-expr ...)
```

For example, instead of forcing all fish classes to be derived from
`fish%`, we can define `fish-interface` and change the `fish%` class to
declare that it implements `fish-interface`:

```racket
(define fish-interface (interface () get-size grow eat))
(define fish% (class* object% (fish-interface) ....))   
```

If the definition of `fish%` does not include `get-size`, `grow`, and
`eat` methods, then an error is signaled in the evaluation of the
`class*` form, because implementing the `fish-interface` interface
requires those methods.

The `is-a?` predicate accepts an object as its first argument and either
a class or interface as its second argument. When given a class, `is-a?`
checks whether the object is an instance of that class or a derived
class.  When given an interface, `is-a?` checks whether the object’s
class implements the interface. In addition, the `implementation?`
predicate checks whether a given class implements a given interface.

### 13.5. Final, Augment, and Inner

As in Java, a method in a `class` form can be specified as _final_,
which means that a subclass cannot override the method.  A final method
is declared using `public-final` or `override-final`, depending on
whether the declaration is for a new method or an overriding
implementation.

Between the extremes of allowing arbitrary overriding and disallowing
overriding entirely, the class system also supports Beta-style
_augmentable_ methods \[Goldberg04\]. A method declared with `pubment`
is like `public`, but the method cannot be overridden in subclasses; it
can be augmented only. A `pubment` method must explicitly invoke an
augmentation \(if any\) using `inner`; a subclass augments the method
using `augment`, instead of `override`.

In general, a method can switch between augment and override modes in a
class derivation. The `augride` method specification indicates an
augmentation to a method where the augmentation is itself overrideable
in subclasses \(though the superclass’s implementation cannot be
overridden\). Similarly, `overment` overrides a method and makes the
overriding implementation augmentable.

### 13.6. Controlling the Scope of External Names

> Java’s access modifiers \(like `protected`\) play a role similar to
> `define-member-name`, but unlike in Java, Racket’s mechanism for
> controlling access is based on lexical scope, not the inheritance
> hierarchy.

As noted in Internal and External Names, class members have both
internal and external names. A member definition binds an internal name
locally, and this binding can be locally renamed.  External names, in
contrast, have global scope by default, and a member definition does not
bind an external name. Instead, a member definition refers to an
existing binding for an external name, where the member name is bound to
a _member key_; a class ultimately maps member keys to methods, fields,
and initialization arguments.

Recall the `hungry-fish%` `class` expression:

```racket
(define hungry-fish% (class fish% ....                      
                       (inherit eat)                        
                       (define/public (eat-more fish1 fish2)
                         (eat fish1) (eat fish2))))         
```

During its evaluation, the `hungry-fish%` and `fish%` classes refer to
the same global binding of `eat`.  At run time, calls to `eat` in
`hungry-fish%` are matched with the `eat` method in `fish%` through the
shared method key that is bound to `eat`.

The default binding for an external name is global, but a programmer can
introduce an external-name binding with the `define-member-name` form.

```racket
(define-member-name id member-key-expr)
```

In particular, by using `(generate-member-key)` as the
`member-key-expr`, an external name can be localized for a particular
scope, because the generated member key is inaccessible outside the
scope. In other words, `define-member-name` gives an external name a
kind of package-private scope, but generalized from packages to
arbitrary binding scopes in Racket.

For example, the following `fish%` and `pond%` classes cooperate via a
`get-depth` method that is only accessible to the cooperating classes:

```racket
(define-values (fish% pond%) ; two mutually recursive classes
  (let ()                                                    
    (define-member-name get-depth (generate-member-key))     
    (define fish%                                            
      (class ....                                            
        (define my-depth ....)                               
        (define my-pond ....)                                
        (define/public (dive amt)                            
        (set! my-depth                                       
              (min (+ my-depth amt)                          
                   (send my-pond get-depth))))))             
    (define pond%                                            
      (class ....                                            
        (define current-depth ....)                          
        (define/public (get-depth) current-depth)))          
    (values fish% pond%)))                                   
```

External names are in a namespace that separates them from other Racket
names. This separate namespace is implicitly used for the method name in
`send`, for initialization-argument names in `new`, or for the external
name in a member definition.  The special form `member-name-key`
provides access to the binding of an external name in an arbitrary
expression position: `(member-name-key id)` produces the member-key
binding of `id` in the current scope.

A member-key value is primarily used with a `define-member-name` form.
Normally, then, `(member-name-key id)` captures the method key of `id`
so that it can be communicated to a use of `define-member-name` in a
different scope. This capability turns out to be useful for generalizing
mixins, as discussed next.

### 13.7. Mixins

Since `class` is an expression form instead of a top-level declaration
as in Smalltalk and Java, a `class` form can be nested inside any
lexical scope, including `lambda`. The result is a _mixin_, i.e., a
class extension that is parameterized with respect to its superclass.

For example, we can parameterize the `picky-fish%` class over its
superclass to define `picky-mixin`:

```racket
(define (picky-mixin %)                                    
  (class % (super-new)                                     
    (define/override (grow amt) (super grow (* 3/4 amt)))))
(define picky-fish% (picky-mixin fish%))                   
```

Many small differences between Smalltalk-style classes and Racket
classes contribute to the effective use of mixins. In particular, the
use of `define/override` makes explicit that `picky-mixin` expects a
class with a `grow` method. If `picky-mixin` is applied to a class
without a `grow` method, an error is signaled as soon as `picky-mixin`
is applied.

Similarly, a use of `inherit` enforces a “method existence” requirement
when the mixin is applied:

```racket
(define (hungry-mixin %)                 
  (class % (super-new)                   
    (inherit eat)                        
    (define/public (eat-more fish1 fish2)
      (eat fish1)                        
      (eat fish2))))                     
```

The advantage of mixins is that we can easily combine them to create new
classes whose implementation sharing does not fit into a
single-inheritance hierarchy—without the ambiguities associated with
multiple inheritance. Equipped with `picky-mixin` and `hungry-mixin`,
creating a class for a hungry, yet picky fish is straightforward:

```racket
(define picky-hungry-fish%           
  (hungry-mixin (picky-mixin fish%)))
```

The use of keyword initialization arguments is critical for the easy use
of mixins. For example, `picky-mixin` and `hungry-mixin` can augment any
class with suitable `eat` and `grow` methods, because they do not
specify initialization arguments and add none in their `super-new`
expressions:

```racket
(define person%                                     
  (class object%                                    
    (init name age)                                 
    ....                                            
    (define/public (eat food) ....)                 
    (define/public (grow amt) ....)))               
(define child% (hungry-mixin (picky-mixin person%)))
(define oliver (new child% [name "Oliver"] [age 6]))
```

Finally, the use of external names for class members \(instead of
lexically scoped identifiers\) makes mixin use convenient. Applying
`picky-mixin` to `person%` works because the names `eat` and `grow`
match, without any a priori declaration that `eat` and `grow` should be
the same method in `fish%` and `person%`. This feature is a potential
drawback when member names collide accidentally; some accidental
collisions can be corrected by limiting the scope external names, as
discussed in Controlling the Scope of External Names.

#### 13.7.1. Mixins and Interfaces

Using `implementation?`, `picky-mixin` could require that its base class
implements `grower-interface`, which could be implemented by both
`fish%` and `person%`:

```racket
(define grower-interface (interface () grow))           
(define (picky-mixin %)                                 
  (unless (implementation? % grower-interface)          
    (error "picky-mixin: not a grower-interface class"))
  (class % ....))                                       
```

Another use of interfaces with a mixin is to tag classes generated by
the mixin, so that instances of the mixin can be recognized. In other
words, `is-a?` cannot work on a mixin represented as a function, but it
can recognize an interface \(somewhat like a _specialization
interface_\) that is consistently implemented by the mixin.  For
example, classes generated by `picky-mixin` could be tagged with
`picky-interface`, enabling the `is-picky?` predicate:

```racket
(define picky-interface (interface ()))                 
(define (picky-mixin %)                                 
  (unless (implementation? % grower-interface)          
    (error "picky-mixin: not a grower-interface class"))
  (class* % (picky-interface) ....))                    
(define (is-picky? o)                                   
  (is-a? o picky-interface))                            
```

#### 13.7.2. The `mixin` Form

To codify the `lambda`-plus-`class` pattern for implementing mixins,
including the use of interfaces for the domain and range of the mixin,
the class system provides a `mixin` macro:

```racket
(mixin (interface-expr ...) (interface-expr ...)
  decl-or-expr ...)                             
```

The first set of `interface-expr`s determines the domain of the mixin,
and the second set determines the range. That is, the expansion is a
function that tests whether a given base class implements the first
sequence of `interface-expr`s and produces a class that implements the
second sequence of `interface-expr`s. Other requirements, such as the
presence of `inherit`ed methods in the superclass, are then checked for
the `class` expansion of the `mixin` form.  For example:

```racket
> (define choosy-interface (interface () choose?))           
> (define hungry-interface (interface () eat))               
> (define choosy-eater-mixin                                 
    (mixin (choosy-interface) (hungry-interface)             
      (inherit choose?)                                      
      (super-new)                                            
      (define/public (eat x)                                 
        (cond                                                
          [(choose? x)                                       
           (printf "chomp chomp chomp on ~a.\n" x)]          
          [else                                              
           (printf "I'm not crazy about ~a.\n" x)]))))       
> (define herring-lover%                                     
    (class* object% (choosy-interface)                       
      (super-new)                                            
      (define/public (choose? x)                             
        (regexp-match #px"^herring" x))))                    
> (define herring-eater% (choosy-eater-mixin herring-lover%))
> (define eater (new herring-eater%))                        
> (send eater eat "elderberry")                              
I'm not crazy about elderberry.                              
> (send eater eat "herring")                                 
chomp chomp chomp on herring.                                
> (send eater eat "herring ice cream")                       
chomp chomp chomp on herring ice cream.                      
```

Mixins not only override methods and introduce public methods, they can
also augment methods, introduce augment-only methods, add an
overrideable augmentation, and add an augmentable override — all of the
things that a class can do \(see Final, Augment, and Inner\).

#### 13.7.3. Parameterized Mixins

As noted in Controlling the Scope of External Names, external names can
be bound with `define-member-name`. This facility allows a mixin to be
generalized with respect to the methods that it defines and uses.  For
example, we can parameterize `hungry-mixin` with respect to the external
member key for `eat`:

```racket
(define (make-hungry-mixin eat-method-key)          
  (define-member-name eat eat-method-key)           
  (mixin () () (super-new)                          
    (inherit eat)                                   
    (define/public (eat-more x y) (eat x) (eat y))))
```

To obtain a particular hungry-mixin, we must apply this function to a
member key that refers to a suitable `eat` method, which we can obtain
using `member-name-key`:

```racket
((make-hungry-mixin (member-name-key eat))         
 (class object% .... (define/public (eat x) 'yum)))
```

Above, we apply `hungry-mixin` to an anonymous class that provides
`eat`, but we can also combine it with a class that provides `chomp`,
instead:

```racket
((make-hungry-mixin (member-name-key chomp))         
 (class object% .... (define/public (chomp x) 'yum)))
```

### 13.8. Traits

A _trait_ is similar to a mixin, in that it encapsulates a set of
methods to be added to a class. A trait is different from a mixin in
that its individual methods can be manipulated with trait operators such
as `trait-sum` \(merge the methods of two traits\), `trait-exclude`
\(remove a method from a trait\), and `trait-alias` \(add a copy of a
method with a new name; do not redirect any calls to the old name\).

The practical difference between mixins and traits is that two traits
can be combined, even if they include a common method and even if
neither method can sensibly override the other. In that case, the
programmer must explicitly resolve the collision, usually by aliasing
methods, excluding methods, and merging a new trait that uses the
aliases.

Suppose our `fish%` programmer wants to define two class extensions,
`spots` and `stripes`, each of which includes a `get-color` method. The
fish’s spot color should not override the stripe color nor vice versa;
instead, a `spots+stripes-fish%` should combine the two colors, which is
not possible if `spots` and `stripes` are implemented as plain mixins.
If, however, `spots` and `stripes` are implemented as traits, they can
be combined. First, we alias `get-color` in each trait to a
non-conflicting name. Second, the `get-color` methods are removed from
both and the traits with only aliases are merged. Finally, the new trait
is used to create a class that introduces its own `get-color` method
based on the two aliases, producing the desired `spots+stripes`
extension.

#### 13.8.1. Traits as Sets of Mixins

One natural approach to implementing traits in Racket is as a set of
mixins, with one mixin per trait method.  For example, we might attempt
to define the spots and stripes traits as follows, using association
lists to represent sets:

```racket
(define spots-trait                                    
  (list (cons 'get-color                               
               (lambda (%) (class % (super-new)        
                             (define/public (get-color)
                               'black))))))            
(define stripes-trait                                  
  (list (cons 'get-color                               
              (lambda (%) (class % (super-new)         
                            (define/public (get-color) 
                              'red))))))               
```

A set representation, such as the above, allows `trait-sum` and
`trait-exclude` as simple manipulations; unfortunately, it does not
support the `trait-alias` operator. Although a mixin can be duplicated
in the association list, the mixin has a fixed method name, e.g.,
`get-color`, and mixins do not support a method-rename operation. To
support `trait-alias`, we must parameterize the mixins over the external
method name in the same way that `eat` was parameterized in
Parameterized Mixins.

To support the `trait-alias` operation, `spots-trait` should be
represented as:

```racket
(define spots-trait                                         
  (list (cons (member-name-key get-color)                   
              (lambda (get-color-key %)                     
                (define-member-name get-color get-color-key)
                (class % (super-new)                        
                  (define/public (get-color) 'black))))))   
```

When the `get-color` method in `spots-trait` is aliased to
`get-trait-color` and the `get-color` method is removed, the resulting
trait is the same as

```racket
(list (cons (member-name-key get-trait-color)             
            (lambda (get-color-key %)                     
              (define-member-name get-color get-color-key)
              (class % (super-new)                        
                (define/public (get-color) 'black)))))    
```

To apply a trait `T` to a class `C` and obtain a derived class, we use
`((trait->mixin T) C)`. The `trait->mixin` function supplies each mixin
of `T` with the key for the mixin’s method and a partial extension of
`C`:

```racket
(define ((trait->mixin T) C)                     
  (foldr (lambda (m %) ((cdr m) (car m) %)) C T))
```

Thus, when the trait above is combined with other traits and then
applied to a class, the use of `get-color` becomes a reference to the
external name `get-trait-color`.

#### 13.8.2. Inherit and Super in Traits

This first implementation of traits supports `trait-alias`, and it
supports a trait method that calls itself, but it does not support
trait methods that call each other. In particular, suppose that a
spot-fish’s  market value depends on the color of its spots:

```racket
(define spots-trait                            
  (list (cons (member-name-key get-color) ....)
        (cons (member-name-key get-price)      
              (lambda (get-price %) ....       
                (class % ....                  
                  (define/public (get-price)   
                    .... (get-color) ....))))))
```

In this case, the definition of `spots-trait` fails, because `get-color`
is not in scope for the `get-price` mixin. Indeed, depending on the
order of mixin application when the trait is applied to a class, the
`get-color` method may not be available when `get-price` mixin is
applied to the class. Therefore adding an `(inherit get-color)`
declaration to the `get-price` mixin does not solve the problem.

One solution is to require the use of `(send this get-color)` in methods
such as `get-price`. This change works because `send` always delays the
method lookup until the method call is evaluated. The delayed lookup is
more expensive than a direct call, however. Worse, it also delays
checking whether a `get-color` method even exists.

A second, effective, and efficient solution is to change the encoding of
traits. Specifically, we represent each method as a pair of mixins: one
that introduces the method and one that implements it. When a trait is
applied to a class, all of the method-introducing mixins are applied
first. Then the method-implementing mixins can use `inherit` to directly
access any introduced method.

```racket
(define spots-trait                                      
  (list (list (local-member-name-key get-color)          
              (lambda (get-color get-price %) ....       
                (class % ....                            
                  (define/public (get-color) (void))))   
              (lambda (get-color get-price %) ....       
                (class % ....                            
                  (define/override (get-color) 'black))))
        (list (local-member-name-key get-price)          
              (lambda (get-price get-color %) ....       
                (class % ....                            
                  (define/public (get-price) (void))))   
              (lambda (get-color get-price %) ....       
                (class % ....                            
                  (inherit get-color)                    
                  (define/override (get-price)           
                    .... (get-color) ....))))))          
```

With this trait encoding, `trait-alias` adds a new method with a new
name, but it does not change any references to the old method.

#### 13.8.3. The `trait` Form

The general-purpose trait pattern is clearly too complex for a
programmer to use directly, but it is easily codified in a `trait`
macro:

```racket
(trait trait-clause ...)
```

The `id`s in the optional `inherit` clause are available for direct
reference in the method `expr`s, and they must be supplied either by
other traits or the base class to which the trait is ultimately applied.

Using this form in conjunction with trait operators such as `trait-sum`,
`trait-exclude`, `trait-alias`, and `trait->mixin`, we can implement
`spots-trait` and `stripes-trait` as desired.

```racket
(define spots-trait                                            
  (trait                                                       
    (define/public (get-color) 'black)                         
    (define/public (get-price) ... (get-color) ...)))          
                                                               
(define stripes-trait                                          
  (trait                                                       
    (define/public (get-color) 'red)))                         
                                                               
(define spots+stripes-trait                                    
  (trait-sum                                                   
   (trait-exclude (trait-alias spots-trait                     
                               get-color get-spots-color)      
                  get-color)                                   
   (trait-exclude (trait-alias stripes-trait                   
                               get-color get-stripes-color)    
                  get-color)                                   
   (trait                                                      
     (inherit get-spots-color get-stripes-color)               
     (define/public (get-color)                                
       .... (get-spots-color) .... (get-stripes-color) ....))))
```

### 13.9. Class Contracts

As classes are values, they can flow across contract boundaries, and we
may wish to protect parts of a given class with contracts.  For this,
the `class/c` form is used.  The `class/c` form has many subforms, which
describe two types of contracts on fields and methods: those that affect
uses via instantiated objects and those that affect subclasses.

#### 13.9.1. External Class Contracts

In its simplest form, `class/c` protects the public fields and methods
of objects instantiated from the contracted class.  There is also an
`object/c` form that can be used to similarly protect the public fields
and methods of a particular object. Take the following definition of
`animal%`, which uses a public field for its `size` attribute:

```racket
(define animal%                                    
  (class object%                                   
    (super-new)                                    
    (field [size 10])                              
    (define/public (eat food)                      
      (set! size (+ size (get-field size food))))))
```

For any instantiated `animal%`, accessing the `size` field should return
a positive number.  Also, if the `size` field is set, it should be
assigned a positive number.  Finally, the `eat` method should receive an
argument which is an object with a `size` field that contains a positive
number. To ensure these conditions, we will define the `animal%` class
with an appropriate contract:

```racket
(define positive/c (and/c number? positive?))         
(define edible/c (object/c (field [size positive/c])))
(define/contract animal%                              
  (class/c (field [size positive/c])                  
           [eat (->m edible/c void?)])                
  (class object%                                      
    (super-new)                                       
    (field [size 10])                                 
    (define/public (eat food)                         
      (set! size (+ size (get-field size food))))))   
```

Here we use `->m` to describe the behavior of `eat` since we do not need
to describe any requirements for the `this` parameter. Now that we have
our contracted class, we can see that the contracts on both `size` and
`eat` are enforced:

```racket
> (define bob (new animal%))                                            
> (set-field! size bob 3)                                               
> (get-field size bob)                                                  
3                                                                       
> (set-field! size bob 'large)                                          
animal%: contract violation                                             
  expected: positive/c                                                  
  given: 'large                                                         
  in: the size field in                                                 
      (class/c                                                          
       (eat                                                             
        (->m                                                            
         (object/c (field (size positive/c)))                           
         void?))                                                        
       (field (size positive/c)))                                       
  contract from: (definition animal%)                                   
  blaming: top-level                                                    
   (assuming the contract is correct)                                   
  at: eval:31.0                                                         
> (define richie (new animal%))                                         
> (send bob eat richie)                                                 
> (get-field size bob)                                                  
13                                                                      
> (define rock (new object%))                                           
> (send bob eat rock)                                                   
eat: contract violation;                                                
 no public field size                                                   
  in: the 1st argument of                                               
      the eat method in                                                 
      (class/c                                                          
       (eat                                                             
        (->m                                                            
         (object/c (field (size positive/c)))                           
         void?))                                                        
       (field (size positive/c)))                                       
  contract from: (definition animal%)                                   
  contract on: animal%                                                  
  blaming: top-level                                                    
   (assuming the contract is correct)                                   
  at: eval:31.0                                                         
> (define giant (new (class object% (super-new) (field [size 'large]))))
> (send bob eat giant)                                                  
eat: contract violation                                                 
  expected: positive/c                                                  
  given: 'large                                                         
  in: the size field in                                                 
      the 1st argument of                                               
      the eat method in                                                 
      (class/c                                                          
       (eat                                                             
        (->m                                                            
         (object/c (field (size positive/c)))                           
         void?))                                                        
       (field (size positive/c)))                                       
  contract from: (definition animal%)                                   
  contract on: animal%                                                  
  blaming: top-level                                                    
   (assuming the contract is correct)                                   
  at: eval:31.0                                                         
```

There are two important caveats for external class contracts. First,
external method contracts are only enforced when the target of dynamic
dispatch is the method implementation of the contracted class, which
lies within the contract boundary.  Overriding that implementation, and
thus changing the target of dynamic dispatch, will mean that the
contract is no longer enforced for clients, since accessing the method
no longer crosses the contract boundary.  Unlike external method
contracts, external field contracts are always enforced for clients of
subclasses, since fields cannot be overridden or shadowed.

Second, these contracts do not restrict subclasses of `animal%` in any
way.  Fields and methods that are inherited and used by subclasses are
not checked by these contracts, and uses of the superclass’s methods via
`super` are also unchecked.  The following example illustrates both
caveats:

```racket                                    
(define large-animal%                        
  (class animal%                             
    (super-new)                              
    (inherit-field size)                     
    (set! size 'large)                       
    (define/override (eat food)              
      (display "Nom nom nom") (newline))))   
```                                          
                                             
```racket                                    
> (define elephant (new large-animal%))      
> (send elephant eat (new object%))          
Nom nom nom                                  
> (get-field size elephant)                  
animal%: broke its own contract              
  promised: positive/c                       
  produced: 'large                           
  in: the size field in                      
      (class/c                               
       (eat                                  
        (->m                                 
         (object/c (field (size positive/c)))
         void?))                             
       (field (size positive/c)))            
  contract from: (definition animal%)        
  blaming: (definition animal%)              
   (assuming the contract is correct)        
  at: eval:31.0                              
```                                          

#### 13.9.2. Internal Class Contracts

Notice that retrieving the `size` field from the object `elephant`
blames `animal%` for the contract violation. This blame is correct, but
unfair to the `animal%` class, as we have not yet provided it with a
method for protecting itself from subclasses.  To this end we add
internal class contracts, which provide directives to subclasses for how
they may access and override features of the superclass.  This
distinction between external and internal class contracts allows for
weaker contracts within the class hierarchy, where invariants may be
broken internally by subclasses but should be enforced for external uses
via instantiated objects.

As a simple example of what kinds of protection are available, we
provide an example aimed at the `animal%` class that uses all the
applicable forms:

```racket
(class/c (field [size positive/c])             
         (inherit-field [size positive/c])     
         [eat (->m edible/c void?)]            
         (inherit [eat (->m edible/c void?)])  
         (super [eat (->m edible/c void?)])    
         (override [eat (->m edible/c void?)]))
```

This class contract not only ensures that objects of class `animal%` are
protected as before, but also ensure that subclasses of `animal%` only
store appropriate values within the `size` field and use the
implementation of `size` from `animal%` appropriately. These contract
forms only affect uses within the class hierarchy, and only for method
calls that cross the contract boundary.

That means that `inherit` will only affect subclass uses of a method
until a subclass overrides that method, and that `override` only affects
calls from the superclass into a subclass’s overriding implementation of
that method.  Since these only affect internal uses, the `override` form
does not automatically enter subclasses into obligations when objects of
those classes are used.  Also, use of `override` only makes sense, and
thus can only be used, for methods where no Beta-style augmentation has
taken place. The following example shows this difference:

```racket
(define/contract sloppy-eater%                       
  (class/c [eat (->m edible/c edible/c)])            
  (begin                                             
    (define/contract glutton%                        
      (class/c (override [eat (->m edible/c void?)]))
      (class animal%                                 
        (super-new)                                  
        (inherit eat)                                
        (define/public (gulp food-list)              
          (for ([f food-list])                       
            (eat f)))))                              
    (class glutton%                                  
      (super-new)                                    
      (inherit-field size)                           
      (define/override (eat f)                       
        (let ([food-size (get-field size f)])        
          (set! size (/ food-size 2))                
          (set-field! size f (/ food-size 2))        
          f)))))                                     
```

```racket
> (define pig (new sloppy-eater%))            
> (define slop1 (new animal%))                
> (define slop2 (new animal%))                
> (define slop3 (new animal%))                
> (send pig eat slop1)                        
(object:animal% ...)                          
> (get-field size slop1)                      
5                                             
> (send pig gulp (list slop1 slop2 slop3))    
eat: broke its own contract                   
  promised: void?                             
  produced: (object:animal% ...)              
  in: the range of                            
      the eat method in                       
      (class/c                                
       (override (eat                         
                  (->m                        
                   (object/c                  
                    (field (size positive/c)))
                   void?))))                  
  contract from: (definition glutton%)        
  contract on: glutton%                       
  blaming: (definition sloppy-eater%)         
   (assuming the contract is correct)         
  at: eval:47.0                               
```

In addition to the internal class contract forms shown here, there are
similar forms for Beta-style augmentable methods.  The `inner` form
describes to the subclass what is expected from augmentations of a given
method.  Both `augment` and `augride` tell the subclass that the given
method is a method which has been augmented and that any calls to the
method in the subclass will dynamically dispatch to the appropriate
implementation in the superclass.  Such calls will be checked according
to the given contract.  The two forms differ in that  use of `augment`
signifies that subclasses can augment the given method, whereas use of
`augride` signifies that subclasses must override the current
augmentation instead.

This means that not all forms can be used at the same time.  Only one of
the `override`, `augment`, and `augride` forms can be used for a given
method, and none of these forms can be used if the given method has been
finalized.  In addition, `super` can be specified for a given method
only if `augride` or `override` can be specified. Similarly, `inner` can
be specified only if `augment` or `augride` can be specified.

## 14. Units \(Components\)

_Units_ organize a program into separately compilable and reusable
_components_. A unit resembles a procedure in that both are first-class
values that are used for abstraction. While procedures abstract over
values in expressions, units abstract over names in collections of
definitions. Just as a procedure is called to evaluate its expressions
given actual arguments for its formal parameters, a unit is _invoked_ to
evaluate its definitions given actual references for its imported
variables. Unlike a procedure, however, a unit’s imported variables can
be partially linked with the exported variables of another unit _prior
to invocation_. Linking merges multiple units together into a single
compound unit. The compound unit itself imports variables that will be
propagated to unresolved imported variables in the linked units, and
re-exports some variables from the linked units for further linking.

    14.1 Signatures and Units               
    14.2 Invoking Units                     
    14.3 Linking Units                      
    14.4 First-Class Units                  
    14.5 Whole-`module` Signatures and Units
    14.6 Contracts for Units                
      14.6.1 Adding Contracts to Signatures 
      14.6.2 Adding Contracts to Units      
    14.7 `unit` versus `module`             

### 14.1. Signatures and Units

The interface of a unit is described in terms of _signatures_. Each
signature is defined \(normally within a `module`\) using
`define-signature`.  For example, the following signature, placed in a
`"toy-factory-sig.rkt"` file, describes the exports of a component that
implements a toy factory:

> By convention, signature names end with `^`.

`"toy-factory-sig.rkt"`
```racket
#lang racket                                
                                            
(define-signature toy-factory^              
  (build-toys  ; (integer? -> (listof toy?))
   repaint     ; (toy? symbol? -> toy?)     
   toy?        ; (any/c -> boolean?)        
   toy-color)) ; (toy? -> symbol?)          
                                            
(provide toy-factory^)                      
```

An implementation of the `toy-factory^` signature is written using
`define-unit` with an `export` clause that names `toy-factory^`:

> By convention, unit names end with `@`.

`"simple-factory-unit.rkt"`
```racket
#lang racket                               
                                           
(require "toy-factory-sig.rkt")            
                                           
(define-unit simple-factory@               
  (import)                                 
  (export toy-factory^)                    
                                           
  (printf "Factory started.\n")            
                                           
  (define-struct toy (color) #:transparent)
                                           
  (define (build-toys n)                   
    (for/list ([i (in-range n)])           
      (make-toy 'blue)))                   
                                           
  (define (repaint t col)                  
    (make-toy col)))                       
                                           
(provide simple-factory@)                  
```

The `toy-factory^` signature also could be referenced by a unit that
needs a toy factory to implement something else. In that case,
`toy-factory^` would be named in an `import` clause. For example, a toy
store would get toys from a toy factory. \(Suppose, for the sake of an
example with interesting features, that the store is willing to sell
only toys in a particular color.\)

`"toy-store-sig.rkt"`
```racket
#lang racket                            
                                        
(define-signature toy-store^            
  (store-color     ; (-> symbol?)       
   stock!          ; (integer? -> void?)
   get-inventory)) ; (-> (listof toy?)) 
                                        
(provide toy-store^)                    
```

`"toy-store-unit.rkt"`
```racket
#lang racket                             
                                         
(require "toy-store-sig.rkt"             
         "toy-factory-sig.rkt")          
                                         
(define-unit toy-store@                  
  (import toy-factory^)                  
  (export toy-store^)                    
                                         
  (define inventory null)                
                                         
  (define (store-color) 'green)          
                                         
  (define (maybe-repaint t)              
    (if (eq? (toy-color t) (store-color))
        t                                
        (repaint t (store-color))))      
                                         
  (define (stock! n)                     
    (set! inventory                      
          (append inventory              
                  (map maybe-repaint     
                       (build-toys n)))))
                                         
  (define (get-inventory) inventory))    
                                         
(provide toy-store@)                     
```

Note that `"toy-store-unit.rkt"` imports `"toy-factory-sig.rkt"`, but
not `"simple-factory-unit.rkt"`.  Consequently, the `toy-store@` unit
relies only on the specification of a toy factory, not on a specific
implementation.

### 14.2. Invoking Units

The `simple-factory@` unit has no imports, so it can be invoked directly
using `invoke-unit`:

```racket
> (require "simple-factory-unit.rkt")
> (invoke-unit simple-factory@)      
Factory started.                     
```

The `invoke-unit` form does not make the body definitions available,
however, so we cannot build any toys with this factory. The
`define-values/invoke-unit` form binds the identifiers of a signature to
the values supplied by a unit \(to be invoked\) that implements the
signature:

```racket
> (define-values/invoke-unit/infer simple-factory@)
Factory started.                                   
> (build-toys 3)                                   
(list (toy 'blue) (toy 'blue) (toy 'blue))         
```

Since `simple-factory@` exports the `toy-factory^` signature, each
identifier in `toy-factory^` is defined by the
`define-values/invoke-unit/infer` form. The `/infer` part of the form
name indicates that the identifiers bound by the declaration are
inferred from `simple-factory@`.

Now that the identifiers in `toy-factory^` are defined, we can also
invoke `toy-store@`, which imports `toy-factory^` to produce
`toy-store^`:

```racket
> (require "toy-store-unit.rkt")              
> (define-values/invoke-unit/infer toy-store@)
> (get-inventory)                             
'()                                           
> (stock! 2)                                  
> (get-inventory)                             
(list (toy 'green) (toy 'green))              
```

Again, the `/infer` part `define-values/invoke-unit/infer` determines
that `toy-store@` imports `toy-factory^`, and so it supplies the
top-level bindings that match the names in `toy-factory^` as imports to
`toy-store@`.

### 14.3. Linking Units

We can make our toy economy more efficient by having toy factories that
cooperate with stores, creating toys that do not have to be repainted.
Instead, the toys are always created using the store’s color, which the
factory gets by importing `toy-store^`:

`"store-specific-factory-unit.rkt"`
```racket
#lang racket                          
                                      
(require "toy-store-sig.rkt"          
         "toy-factory-sig.rkt")       
                                      
(define-unit store-specific-factory@  
  (import toy-store^)                 
  (export toy-factory^)               
                                      
  (define-struct toy () #:transparent)
                                      
  (define (toy-color t) (store-color))
                                      
  (define (build-toys n)              
    (for/list ([i (in-range n)])      
      (make-toy)))                    
                                      
  (define (repaint t col)             
    (error "cannot repaint")))        
                                      
(provide store-specific-factory@)     
```

To invoke `store-specific-factory@`, we need `toy-store^` bindings to
supply to the unit. But to get `toy-store^` bindings by invoking
`toy-store@`, we will need a toy factory! The unit implementations are
mutually dependent, and we cannot invoke either before the other.

The solution is to _link_ the units together, and then we can invoke the
combined units. The `define-compound-unit/infer` form links any number
of units to form a combined unit. It can propagate imports and exports
from the linked units, and it can satisfy each unit’s imports using the
exports of other linked units.

```racket
> (require "toy-factory-sig.rkt")               
> (require "toy-store-sig.rkt")                 
> (require "store-specific-factory-unit.rkt")   
> (define-compound-unit/infer toy-store+factory@
    (import)                                    
    (export toy-factory^ toy-store^)            
    (link store-specific-factory@               
          toy-store@))                          
```

The overall result above is a unit `toy-store+factory@` that exports
both `toy-factory^` and `toy-store^`. The connection between
`store-specific-factory@` and `toy-store@` is inferred from the
signatures that each imports and exports.

This unit has no imports, so we can always invoke it:

```racket
> (define-values/invoke-unit/infer toy-store+factory@)
> (stock! 2)                                          
> (get-inventory)                                     
(list (toy) (toy))                                    
> (map toy-color (get-inventory))                     
'(green green)                                        
```

### 14.4. First-Class Units

The `define-unit` form combines `define` with a `unit` form, similar to
the way that `(define (f x) ....)`  combines `define` followed by an
identifier with an implicit `lambda`.

Expanding the shorthand, the definition of `toy-store@` could almost be
written as

```racket
(define toy-store@              
  (unit                         
   (import toy-factory^)        
   (export toy-store^)          
                                
   (define inventory null)      
                                
   (define (store-color) 'green)
   ....))                       
```

A difference between this expansion and `define-unit` is that the
imports and exports of `toy-store@` cannot be inferred. That is, besides
combining `define` and `unit`, `define-unit` attaches static information
to the defined identifier so that its signature information is available
statically to `define-values/invoke-unit/infer` and other forms.

Despite the drawback of losing static signature information, `unit` can
be useful in combination with other forms that work with first-class
values. For example, we could wrap a `unit` that creates a toy store in
a `lambda` to supply the store’s color:

`"toy-store-maker.rkt"`
```racket
#lang racket                                
                                            
(require "toy-store-sig.rkt"                
         "toy-factory-sig.rkt")             
                                            
(define toy-store@-maker                    
  (lambda (the-color)                       
    (unit                                   
     (import toy-factory^)                  
     (export toy-store^)                    
                                            
     (define inventory null)                
                                            
     (define (store-color) the-color)       
                                            
     ; the rest is the same as before       
                                            
     (define (maybe-repaint t)              
       (if (eq? (toy-color t) (store-color))
           t                                
           (repaint t (store-color))))      
                                            
     (define (stock! n)                     
       (set! inventory                      
             (append inventory              
                     (map maybe-repaint     
                          (build-toys n)))))
                                            
     (define (get-inventory) inventory))))  
                                            
(provide toy-store@-maker)                  
```

To invoke a unit created by `toy-store@-maker`, we must use
`define-values/invoke-unit`, instead of the `/infer` variant:

```racket
> (require "simple-factory-unit.rkt")                  
> (define-values/invoke-unit/infer simple-factory@)    
Factory started.                                       
> (require "toy-store-maker.rkt")                      
> (define-values/invoke-unit (toy-store@-maker 'purple)
    (import toy-factory^)                              
    (export toy-store^))                               
> (stock! 2)                                           
> (get-inventory)                                      
(list (toy 'purple) (toy 'purple))                     
```

In the `define-values/invoke-unit` form, the `(import toy-factory^)`
line takes bindings from the current context that match the names in
`toy-factory^` \(the ones that we created by invoking
`simple-factory@`\), and it supplies them as imports to `toy-store@`.
The `(export toy-store^)` clause indicates that the unit produced by
`toy-store@-maker` will export `toy-store^`, and the names from that
signature are defined after invoking the unit.

To link a unit from `toy-store@-maker`, we can use the `compound-unit`
form:

```racket
> (require "store-specific-factory-unit.rkt")                
> (define toy-store+factory@                                 
    (compound-unit                                           
     (import)                                                
     (export TF TS)                                          
     (link [((TF : toy-factory^)) store-specific-factory@ TS]
           [((TS : toy-store^)) toy-store@ TF])))            
```

This `compound-unit` form packs a lot of information into one place. The
left-hand-side `TF` and `TS` in the `link` clause are binding
identifiers. The identifier `TF` is essentially bound to the elements of
`toy-factory^` as implemented by `store-specific-factory@`.  The
identifier `TS` is similarly bound to the elements of `toy-store^` as
implemented by `toy-store@`. Meanwhile, the elements bound to `TS` are
supplied as imports for `store-specific-factory@`, since `TS` follows
`store-specific-factory@`. The elements bound to `TF` are similarly
supplied to `toy-store@`. Finally, `(export TF TS)` indicates that the
elements bound to `TF` and `TS` are exported from the compound unit.

The above `compound-unit` form uses `store-specific-factory@` as a
first-class unit, even though its information could be inferred. Every
unit can be used as a first-class unit, in addition to its use in
inference contexts. Also, various forms let a programmer bridge the gap
between inferred and first-class worlds. For example,
`define-unit-binding` binds a new identifier to the unit produced by an
arbitrary expression; it statically associates signature information to
the identifier, and it dynamically checks the signatures against the
first-class unit produced by the expression.

### 14.5. Whole-`module` Signatures and Units

In programs that use units, modules like `"toy-factory-sig.rkt"` and
`"simple-factory-unit.rkt"` are common. The `racket/signature` and
`racket/unit` module names can be used as languages to avoid much of the
boilerplate module, signature, and unit declaration text.

For example, `"toy-factory-sig.rkt"` can be written as

```racket
#lang racket/signature                   
                                         
build-toys  ; (integer? -> (listof toy?))
repaint     ; (toy? symbol? -> toy?)     
toy?        ; (any/c -> boolean?)        
toy-color   ; (toy? -> symbol?)          
```

The signature `toy-factory^` is automatically provided from the module,
inferred from the filename `"toy-factory-sig.rkt"` by replacing the
`"-sig.rkt"` suffix with `^`.

Similarly, `"simple-factory-unit.rkt"` module can be written

```racket
#lang racket/unit                        
                                         
(require "toy-factory-sig.rkt")          
                                         
(import)                                 
(export toy-factory^)                    
                                         
(printf "Factory started.\n")            
                                         
(define-struct toy (color) #:transparent)
                                         
(define (build-toys n)                   
  (for/list ([i (in-range n)])           
    (make-toy 'blue)))                   
                                         
(define (repaint t col)                  
  (make-toy col))                        
```

The unit `simple-factory@` is automatically provided from the module,
inferred from the filename `"simple-factory-unit.rkt"` by replacing the
`"-unit.rkt"` suffix with `@`.

### 14.6. Contracts for Units

There are a couple of ways of protecting units with contracts.  One way
is useful when writing new signatures, and the other handles the case
when a unit must conform to an already existing signature.

#### 14.6.1. Adding Contracts to Signatures

When contracts are added to a signature, then all units which implement
that signature are protected by those contracts.  The following version
of the `toy-factory^` signature adds the contracts previously written in
comments:

`"contracted-toy-factory-sig.rkt"`
```racket
#lang racket                                
                                            
(define-signature contracted-toy-factory^   
  ((contracted                              
    [build-toys (-> integer? (listof toy?))]
    [repaint    (-> toy? symbol? toy?)]     
    [toy?       (-> any/c boolean?)]        
    [toy-color  (-> toy? symbol?)])))       
                                            
(provide contracted-toy-factory^)           
```

Now we take the previous implementation of `simple-factory@` and
implement this version of `toy-factory^` instead:

`"contracted-simple-factory-unit.rkt"`
```racket
#lang racket                               
                                           
(require "contracted-toy-factory-sig.rkt") 
                                           
(define-unit contracted-simple-factory@    
  (import)                                 
  (export contracted-toy-factory^)         
                                           
  (printf "Factory started.\n")            
                                           
  (define-struct toy (color) #:transparent)
                                           
  (define (build-toys n)                   
    (for/list ([i (in-range n)])           
      (make-toy 'blue)))                   
                                           
  (define (repaint t col)                  
    (make-toy col)))                       
                                           
(provide contracted-simple-factory@)       
```

As before, we can invoke our new unit and bind the exports so that we
can use them.  This time, however, misusing the exports causes the
appropriate contract errors.

```racket
> (require "contracted-simple-factory-unit.rkt")              
> (define-values/invoke-unit/infer contracted-simple-factory@)
Factory started.                                              
> (build-toys 3)                                              
(list (toy 'blue) (toy 'blue) (toy 'blue))                    
> (build-toys #f)                                             
build-toys: contract violation                                
  expected: integer?                                          
  given: #f                                                   
  in: the 1st argument of                                     
      (-> integer? (listof toy?))                             
  contract from:                                              
      (unit contracted-simple-factory@)                       
  blaming: top-level                                          
   (assuming the contract is correct)                         
  at: eval:34.0                                               
> (repaint 3 'blue)                                           
repaint: contract violation                                   
  expected: toy?                                              
  given: 3                                                    
  in: the 1st argument of                                     
      (-> toy? symbol? toy?)                                  
  contract from:                                              
      (unit contracted-simple-factory@)                       
  blaming: top-level                                          
   (assuming the contract is correct)                         
  at: eval:34.0                                               
```

#### 14.6.2. Adding Contracts to Units

However, sometimes we may have a unit that must conform to an already
existing signature that is not contracted.  In this case, we can create
a unit contract with `unit/c` or use the `define-unit/contract` form,
which defines a unit which has been wrapped with a unit contract.

For example, here’s a version of `toy-factory@` which still implements
the regular `toy-factory^`, but whose exports have been protected with
an appropriate unit contract.

`"wrapped-simple-factory-unit.rkt"`
```racket
#lang racket                                       
                                                   
(require "toy-factory-sig.rkt")                    
                                                   
(define-unit/contract wrapped-simple-factory@      
  (import)                                         
  (export (toy-factory^                            
           [build-toys (-> integer? (listof toy?))]
           [repaint    (-> toy? symbol? toy?)]     
           [toy?       (-> any/c boolean?)]        
           [toy-color  (-> toy? symbol?)]))        
                                                   
  (printf "Factory started.\n")                    
                                                   
  (define-struct toy (color) #:transparent)        
                                                   
  (define (build-toys n)                           
    (for/list ([i (in-range n)])                   
      (make-toy 'blue)))                           
                                                   
  (define (repaint t col)                          
    (make-toy col)))                               
                                                   
(provide wrapped-simple-factory@)                  
```

```racket
> (require "wrapped-simple-factory-unit.rkt")              
> (define-values/invoke-unit/infer wrapped-simple-factory@)
Factory started.                                           
> (build-toys 3)                                           
(list (toy 'blue) (toy 'blue) (toy 'blue))                 
> (build-toys #f)                                          
wrapped-simple-factory@: contract violation                
  expected: integer?                                       
  given: #f                                                
  in: the 1st argument of                                  
      (unit/c                                              
       (import)                                            
       (export (toy-factory^                               
                (build-toys                                
                 (-> integer? (listof toy?)))              
                (repaint (-> toy? symbol? toy?))           
                (toy? (-> any/c boolean?))                 
                (toy-color (-> toy? symbol?))))            
       (init-depend))                                      
  contract from:                                           
      (unit wrapped-simple-factory@)                       
  blaming: top-level                                       
   (assuming the contract is correct)                      
  at: <collects>/racket/unit.rkt                           
> (repaint 3 'blue)                                        
wrapped-simple-factory@: contract violation                
  expected: toy?                                           
  given: 3                                                 
  in: the 1st argument of                                  
      (unit/c                                              
       (import)                                            
       (export (toy-factory^                               
                (build-toys                                
                 (-> integer? (listof toy?)))              
                (repaint (-> toy? symbol? toy?))           
                (toy? (-> any/c boolean?))                 
                (toy-color (-> toy? symbol?))))            
       (init-depend))                                      
  contract from:                                           
      (unit wrapped-simple-factory@)                       
  blaming: top-level                                       
   (assuming the contract is correct)                      
  at: <collects>/racket/unit.rkt                           
```

### 14.7. `unit` versus `module`

As a form for modularity, `unit` complements `module`:

* The `module` form is primarily for managing a universal namespace. For
  example, it allows a code fragment to refer specifically to the `car`
  operation from `racket/base`—the one that extracts the first element
  of an instance of the built-in pair datatype—as opposed to any number
  of other functions with the name `car`. In other words, the `module`
  construct lets you refer to _the_ binding that you want.

* The `unit` form is for parameterizing a code fragment with respect to
  most any kind of run-time value. For example, it allows a code
  fragment to work with a `car` function that accepts a single argument,
  where the specific function is determined later by linking the
  fragment to another. In other words, the `unit` construct lets you
  refer to _a_ binding that meets some specification.

The `lambda` and `class` forms, among others, also allow
parameterization of code with respect to values that are chosen later.
In principle, any of those could be implemented in terms of any of the
others. In practice, each form offers certain conveniences—such as
allowing overriding of methods or especially simple application to
values—that make them suitable for different purposes.

The `module` form is more fundamental than the others, in a sense. After
all, a program fragment cannot reliably refer to a `lambda`, `class`, or
`unit` form without the namespace management provided by `module`. At
the same time, because namespace management is closely related to
separate expansion and compilation, `module` boundaries end up as
separate-compilation boundaries in a way that prohibits mutual
dependencies among fragments. For similar reasons, `module` does not
separate interface from implementation.

Use `unit` when `module` by itself almost works, but when separately
compiled pieces must refer to each other, or when you want a stronger
separation between _interface_ \(i.e., the parts that need to be known
at expansion and compilation time\) and _implementation_ \(i.e., the
run-time parts\). More generally, use `unit` when you need to
parameterize code over functions, datatypes, and classes, and when the
parameterized code itself provides definitions to be linked with other
parameterized code.

## 15. Reflection and Dynamic Evaluation

Racket is a _dynamic_ language. It offers numerous facilities for
loading, compiling, and even constructing new code at run time.

    15.1 `eval`                                     
      15.1.1 Local Scopes                           
      15.1.2 Namespaces                             
      15.1.3 Namespaces and Modules                 
    15.2 Manipulating Namespaces                    
      15.2.1 Creating and Installing Namespaces     
      15.2.2 Sharing Data and Code Across Namespaces
    15.3 Scripting Evaluation and Using `load`      

### 15.1. `eval`

> This example will not work within a module or in DrRacket’s definitions
> window, but it will work in the interactions window, for reasons that
> are explained by the end of Namespaces.

The `eval` function takes a representation of an expression or
definition \(as a “quoted” form or syntax object\) and evaluates it:

```racket
> (eval '(+ 1 2))
3                
```

The power of `eval` is that an expression can be constructed
dynamically:

```racket
> (define (eval-formula formula)
    (eval `(let ([x 2]          
                 [y 3])         
             ,formula)))        
> (eval-formula '(+ x y))       
5                               
> (eval-formula '(+ (* x y) y)) 
9                               
```

Of course, if we just wanted to evaluate expressions with given values
for `x` and `y`, we do not need `eval`. A more direct approach is to use
first-class functions:

```racket
> (define (apply-formula formula-proc)        
    (formula-proc 2 3))                       
> (apply-formula (lambda (x y) (+ x y)))      
5                                             
> (apply-formula (lambda (x y) (+ (* x y) y)))
9                                             
```

However, if expressions like `(+ x y)` and `(+ (* x y) y)` are read from
a file supplied by a user, for example, then `eval` might be
appropriate. Similarly, the REPL reads expressions that are typed by a
user and uses `eval` to evaluate them.

Also, `eval` is often used directly or indirectly on whole modules. For
example, a program might load a module on demand using
`dynamic-require`, which is essentially a wrapper around `eval` to
dynamically load the module code.

#### 15.1.1. Local Scopes

The `eval` function cannot see local bindings in the context where it is
called. For example, calling `eval` inside an unquoted `let` form to
evaluate a formula does not make values visible for `x` and `y`:

```racket
> (define (broken-eval-formula formula)              
    (let ([x 2]                                      
          [y 3])                                     
      (eval formula)))                               
> (broken-eval-formula '(+ x y))                     
x: undefined;                                        
 cannot reference an identifier before its definition
  in module: top-level                               
```

The `eval` function cannot see the `x` and `y` bindings precisely
because it is a function, and Racket is a lexically scoped language.
Imagine if `eval` were implemented as

```racket
(define (eval x)                   
  (eval-expanded (macro-expand x)))
```

then at the point when `eval-expanded` is called, the most recent
binding of `x` is to the expression to evaluate, not the `let` binding
in `broken-eval-formula`. Lexical scope prevents such confusing and
fragile behavior, and consequently prevents `eval` from seeing local
bindings in the context where it is called.

You might imagine that even though `eval` cannot see the local bindings
in `broken-eval-formula`, there must actually be a data structure
mapping `x` to `2` and `y` to `3`, and you would like a way to get that
data structure. In fact, no such data structure exists; the compiler is
free to replace every use of `x` with `2` at compile time, so that the
local binding of `x` does not exist in any concrete sense at run-time.
Even when variables cannot be eliminated by constant-folding, normally
the names of the variables can be eliminated, and the data structures
that hold local values do not resemble a mapping from names to values.

#### 15.1.2. Namespaces

Since `eval` cannot see the bindings from the context where it is
called, another mechanism is needed to determine dynamically available
bindings. A _namespace_ is a first-class value that encapsulates the
bindings available for dynamic evaluation.

> Informally, the term _namespace_ is sometimes used interchangeably with
> _environment_ or _scope_. In Racket, the term _namespace_ has the more
> specific, dynamic meaning given above, and it should not be confused
> with static lexical concepts.

Some functions, such as `eval`, accept an optional namespace argument.
More often, the namespace used by a dynamic operation is the _current
namespace_ as determined by the `current-namespace` parameter.

When `eval` is used in a REPL, the current namespace is the one that the
REPL uses for evaluating expressions. That’s why the following
interaction successfully accesses `x` via `eval`:

```racket
> (define x 3)
> (eval 'x)   
3             
```

In contrast, try the following simple module and running it directly in
DrRacket or supplying the file as a command-line argument to `racket`:

```racket
#lang racket      
                  
(eval '(cons 1 2))
```

This fails because the initial current namespace is empty. When you run
`racket` in interactive mode \(see Interactive Mode\), the initial
namespace is initialized with the exports of the `racket` module, but
when you run a module directly, the initial namespace starts empty.

In general, it’s a bad idea to use `eval` with whatever namespace
happens to be installed. Instead, create a namespace explicitly and
install it for the call to eval:

```racket
#lang racket                     
                                 
(define ns (make-base-namespace))
(eval '(cons 1 2) ns) ; works    
```

The `make-base-namespace` function creates a namespace that is
initialized with the exports of `racket/base`. The later section
Manipulating Namespaces provides more information on creating and
configuring namespaces.

#### 15.1.3. Namespaces and Modules

As with `let` bindings, lexical scope means that `eval` cannot
automatically see the definitions of a `module` in which it is called.
Unlike `let` bindings, however, Racket provides a way to reflect a
module into a namespace.

The `module->namespace` function takes a quoted module path and produces
a namespace for evaluating expressions and definitions as if they
appeared in the `module` body:

```racket
> (module m racket/base              
    (define x 11))                   
> (require 'm)                       
> (define ns (module->namespace ''m))
> (eval 'x ns)                       
11                                   
```

> The double quoting in `''m` is because `'m` is a module path that refers
> to an interactively declared module, and so `''m` is the quoted form of
> the path.

The `module->namespace` function is mostly useful from outside a module,
where the module’s full name is known. Inside a `module` form, however,
the full name of a module may not be known, because it may depend on
where the module source is located when it is eventually loaded.

From within a `module`, use `define-namespace-anchor` to declare a
reflection hook on the module, and use `namespace-anchor->namespace` to
reel in the module’s namespace:

```racket
#lang racket                               
                                           
(define-namespace-anchor a)                
(define ns (namespace-anchor->namespace a))
                                           
(define x 1)                               
(define y 2)                               
                                           
(eval '(cons x y) ns) ; produces (1 . 2)   
```

### 15.2. Manipulating Namespaces

A namespace encapsulates two pieces of information:

* A mapping from identifiers to bindings. For example, a namespace might
  map the identifier `lambda` to the `lambda` form. An “empty” namespace
  is one that maps every identifier to an uninitialized top-level
  variable.

* A mapping from module names to module declarations and instances.
  \(The distinction between declaration and instance is discussed in
  Module Instantiations and Visits.\)

The first mapping is used for evaluating expressions in a top-level
context, as in `(eval '(lambda (x) (+ x 1)))`. The second mapping is
used, for example, by `dynamic-require` to locate a module. The call
`(eval '(require racket/base))` normally uses both pieces: the
identifier mapping determines the binding of `require`; if it turns out
to mean `require`, then the module mapping is used to locate the
`racket/base` module.

From the perspective of the core Racket run-time system, all evaluation
is reflective. Execution starts with an initial namespace that contains
a few primitive modules, and that is further populated by loading files
and modules as specified on the command line or as supplied in the REPL.
Top-level `require` and `define` forms adjusts the identifier mapping,
and module declarations \(typically loaded on demand for a `require`
form\) adjust the module mapping.

#### 15.2.1. Creating and Installing Namespaces

The function `make-empty-namespace` creates a new, empty namespace.
Since the namespace is truly empty, it cannot at first be used to
evaluate any top-level expression—not even `(require racket)`. In
particular,

```racket
(parameterize ([current-namespace (make-empty-namespace)])
  (namespace-require 'racket))                            
```

fails, because the namespace does not include the primitive modules on
which `racket` is built.

To make a namespace useful, some modules must be _attached_ from an
existing namespace. Attaching a module adjusts the mapping of module
names to instances by transitively copying entries \(the module and all
its imports\) from an existing namespace’s mapping. Normally, instead of
just attaching the primitive modules—whose names and organization are
subject to change—a higher-level module is attached, such as `racket` or
`racket/base`.

The `make-base-empty-namespace` function provides a namespace that is
empty, except that `racket/base` is attached. The resulting namespace is
still “empty” in the sense that the identifiers-to-bindings part of the
namespace has no mappings; only the module mapping has been populated.
Nevertheless, with an initial module mapping, further modules can be
loaded.

A namespace created with `make-base-empty-namespace` is suitable for
many basic dynamic tasks. For example, suppose that a `my-dsl` library
implements a domain-specific language in which you want to execute
commands from a user-specified file. A namespace created with
`make-base-empty-namespace` is enough to get started:

```racket
(define (run-dsl file)                                           
  (parameterize ([current-namespace (make-base-empty-namespace)])
    (namespace-require 'my-dsl)                                  
    (load file)))                                                
```

Note that the `parameterize` of `current-namespace` does not affect the
meaning of identifiers like `namespace-require` within the
`parameterize` body. Those identifiers obtain their meaning from the
enclosing context \(probably a module\). Only expressions that are
dynamic with respect to this code, such as the content of `load`ed
files, are affected by the `parameterize`.

Another subtle point in the above example is the use of
`(namespace-require 'my-dsl)` instead of `(eval '(require my-dsl))`. The
latter would not work, because `eval` needs to obtain a meaning for
`require` in the namespace, and the namespace’s identifier mapping is
initially empty. The `namespace-require` function, in contrast, directly
imports the given module into the current namespace.  Starting with
`(namespace-require 'racket/base)` would introduce a binding for
`require` and make a subsequent `(eval '(require my-dsl))` work. The
above is better, not only because it is more compact, but also because
it avoids introducing bindings that are not part of the domain-specific
languages.

#### 15.2.2. Sharing Data and Code Across Namespaces

Modules not attached to a new namespace will be loaded and instantiated
afresh if they are demanded by evaluation. For example, `racket/base`
does not include `racket/class`, and loading `racket/class` again will
create a distinct class datatype:

```racket
> (require racket/class)                                          
> (class? object%)                                                
#t                                                                
> (class?                                                         
   (parameterize ([current-namespace (make-base-empty-namespace)])
     (namespace-require 'racket/class) ; loads again              
     (eval 'object%)))                                            
#f                                                                
```

For cases when dynamically loaded code needs to share more code and data
with its context, use the `namespace-attach-module` function. The first
argument to `namespace-attach-module` is a source namespace from which
to draw a module instance; in some cases, the current namespace is known
to include the module that needs to be shared:

```racket
> (require racket/class)                                
> (class?                                               
   (let ([ns (make-base-empty-namespace)])              
     (namespace-attach-module (current-namespace)       
                              'racket/class             
                              ns)                       
     (parameterize ([current-namespace ns])             
       (namespace-require 'racket/class) ; uses attached
       (eval 'object%))))                               
#t                                                      
```

Within a module, however, the combination of `define-namespace-anchor`
and `namespace-anchor->empty-namespace` offers a more reliable method
for obtaining a source namespace:

```racket
#lang racket/base                                                 
                                                                  
(require racket/class)                                            
                                                                  
(define-namespace-anchor a)                                       
                                                                  
(define (load-plug-in file)                                       
  (let ([ns (make-base-empty-namespace)])                         
    (namespace-attach-module (namespace-anchor->empty-namespace a)
                             'racket/class                        
                              ns)                                 
    (parameterize ([current-namespace ns])                        
      (dynamic-require file 'plug-in%))))                         
```

The anchor bound by `namespace-attach-module` connects the run time of a
module with the namespace in which a module is loaded \(which might
differ from the current namespace\).  In the above example, since the
enclosing module requires `racket/class`, the namespace produced by
`namespace-anchor->empty-namespace` certainly contains an instance of
`racket/class`. Moreover, that instance is the same as the one imported
into the module, so the class datatype is shared.

### 15.3. Scripting Evaluation and Using `load`

Historically, Lisp implementations did not offer module systems.
Instead, large programs were built by essentially scripting the REPL to
evaluate program fragments in a particular order. While REPL scripting
turns out to be a bad way to structure programs and libraries, it is
still sometimes a useful capability.

> Describing a program via `load` interacts especially badly with
> macro-defined language extensions \[Flatt02\].

The `load` function runs a REPL script by `read`ing S-expressions from a
file, one by one, and passing them to `eval`. If a file `"place.rkts"`
contains

```racket
(define city "Salt Lake City")
(define state "Utah")         
(printf "~a, ~a\n" city state)
```

then it can be loaded in a REPL:

```racket
> (load "place.rkts")
Salt Lake City, Utah 
> city               
"Salt Lake City"     
```

Since `load` uses `eval`, however, a module like the following generally
will not work—for the same reasons described in Namespaces:

```racket
#lang racket           
                       
(define there "Utopia")
                       
(load "here.rkts")     
```

The current namespace for evaluating the content of `"here.rkts"` is
likely to be empty; in any case, you cannot get `there` from
`"here.rkts"`. Also, any definitions in `"here.rkts"` will not become
visible for use within the module; after all, the `load` happens
dynamically, while references to identifiers within the module are
resolved lexically, and therefore statically.

Unlike `eval`, `load` does not accept a namespace argument. To supply a
namespace to `load`, set the `current-namespace` parameter. The
following example evaluates the expressions in `"here.rkts"` using the
bindings of the `racket/base` module:

```racket
#lang racket                                             
                                                         
(parameterize ([current-namespace (make-base-namespace)])
  (load "here.rkts"))                                    
```

You can even use `namespace-anchor->namespace` to make the bindings of
the enclosing module accessible for dynamic evaluation. In the following
example, when `"here.rkts"` is `load`ed, it can refer to `there` as well
as the bindings of `racket`:

```racket
#lang racket                                                       
                                                                   
(define there "Utopia")                                            
                                                                   
(define-namespace-anchor a)                                        
(parameterize ([current-namespace (namespace-anchor->namespace a)])
  (load "here.rkts"))                                              
```

Still, if `"here.rkts"` defines any identifiers, the definitions cannot
be directly \(i.e., statically\) referenced by in the enclosing module.

The `racket/load` module language is different from `racket` or
`racket/base`. A module using `racket/load` treats all of its content as
dynamic, passing each form in the module body to `eval` \(using a
namespace that is initialized with `racket`\). As a result, uses of
`eval` and `load` in the module body see the same dynamic namespace as
immediate body forms. For example, if `"here.rkts"` contains

```racket
(define here "Morporkia")       
(define (go!) (set! here there))
```

then running

```racket
#lang racket/load      
                       
(define there "Utopia")
                       
(load "here.rkts")     
                       
(go!)                  
(printf "~a\n" here)   
```

prints “Utopia”.

Drawbacks of using `racket/load` include reduced error checking, tool
support, and performance. For example, with the program

```racket
#lang racket/load   
                    
(define good 5)     
(printf "running\n")
good                
bad                 
```

DrRacket’s Check Syntax tool cannot tell that the second `good` is a
reference to the first, and the unbound reference to `bad` is reported
only at run time instead of rejected syntactically.

## 16. Macros

A _macro_ is a syntactic form with an associated _transformer_ that
_expands_ the original form into existing forms. To put it another way,
a macro is an extension to the Racket compiler. Most of the syntactic
forms of `racket/base` and `racket` are actually macros that expand into
a small set of core constructs.

Like many languages, Racket provides pattern-based macros that make
simple transformations easy to implement and reliable to use. Racket
also supports arbitrary macro transformers that are implemented in
Racket—or in a macro-extended variant of Racket.

This chapter provides an introduction to Racket macros, but see [_Fear
of Macros_](http://www.greghendershott.com/fear-of-macros/) for an
introduction from a different perspective.

    16.1 Pattern-Based Macros                              
      16.1.1 `define-syntax-rule`                          
      16.1.2 Lexical Scope                                 
      16.1.3 `define-syntax` and `syntax-rules`            
      16.1.4 Matching Sequences                            
      16.1.5 Identifier Macros                             
      16.1.6 `set!` Transformers                           
      16.1.7 Macro-Generating Macros                       
      16.1.8 Extended Example: Call-by-Reference Functions 
    16.2 General Macro Transformers                        
      16.2.1 Syntax Objects                                
      16.2.2 Macro Transformer Procedures                  
      16.2.3 Mixing Patterns and Expressions: `syntax-case`
      16.2.4 `with-syntax` and `generate-temporaries`      
      16.2.5 Compile and Run-Time Phases                   
      16.2.6 General Phase Levels                          
        16.2.6.1 Phases and Bindings                       
        16.2.6.2 Phases and Modules                        
      16.2.7 Syntax Taints                                 
    16.3 Module Instantiations and Visits                  
      16.3.1 Declaration versus Instantiation              
      16.3.2 Compile-Time Instantiation                    
      16.3.3 Visiting Modules                              
      16.3.4 Lazy Visits via Available Modules             

### 16.1. Pattern-Based Macros

A _pattern-based macro_ replaces any code that matches a pattern to an
expansion that uses parts of the original syntax that match parts of the
pattern.

#### 16.1.1. `define-syntax-rule`

The simplest way to create a macro is to use `define-syntax-rule`:

```racket
(define-syntax-rule pattern template)
```

As a running example, consider the `swap` macro, which swaps the values
stored in two variables. It can be implemented using
`define-syntax-rule` as follows:

> The macro is “un-Rackety” in the sense that it involves side effects on
> variables—but the point of macros is to let you add syntactic forms that
> some other language designer might not approve.

```racket
(define-syntax-rule (swap x y)
  (let ([tmp x])              
    (set! x y)                
    (set! y tmp)))            
```

The `define-syntax-rule` form binds a macro that matches a single
pattern. The pattern must always start with an open parenthesis followed
by an identifier, which is `swap` in this case. After the initial
identifier, other identifiers are _macro pattern variables_ that can
match anything in a use of the macro. Thus, this macro matches the form
`(swap form1 form2)` for any `form1` and `form2`.

> Macro pattern variables are similar to pattern variables for `match`.
> See Pattern Matching.

After the pattern in `define-syntax-rule` is the _template_. The
template is used in place of a form that matches the pattern, except
that each instance of a pattern variable in the template is replaced
with the part of the macro use the pattern variable matched. For
example, in

`(swap` `first` `last)`

the pattern variable `x` matches `first` and `y` matches `last`, so that
the expansion is

```racket
(let ([tmp first]) 
  (set! first last)
  (set! last tmp)) 
```

#### 16.1.2. Lexical Scope

Suppose that we use the `swap` macro to swap variables named `tmp` and
`other`:

```racket
(let ([tmp 5]      
      [other 6])   
  (swap tmp other) 
  (list tmp other))
```

The result of the above expression should be `(6 5)`. The naive
expansion of this use of `swap`, however, is

```racket
(let ([tmp 5]        
      [other 6])     
  (let ([tmp tmp])   
    (set! tmp other) 
    (set! other tmp))
  (list tmp other))  
```

whose result is `(5 6)`. The problem is that the naive expansion
confuses the `tmp` in the context where `swap` is used with the `tmp`
that is in the macro template.

Racket doesn’t produce the naive expansion for the above use of `swap`.
Instead, it produces

```racket
(let ([tmp 5]          
      [other 6])       
  (let ([tmp_1 tmp])   
    (set! tmp other)   
    (set! other tmp_1))
  (list tmp other))    
```

with the correct result in `(6 5)`. Similarly, in the example

```racket
(let ([set! 5]      
      [other 6])    
  (swap set! other) 
  (list set! other))
```

the expansion is

```racket
(let ([set!_1 5]       
      [other 6])       
  (let ([tmp_1 set!_1])
    (set! set!_1 other)
    (set! other tmp_1))
  (list set!_1 other)) 
```

so that the local `set!` binding doesn’t interfere with the assignments
introduced by the macro template.

In other words, Racket’s pattern-based macros automatically maintain
lexical scope, so macro implementors can reason about variable reference
in macros and macro uses in the same way as for functions and function
calls.

#### 16.1.3. `define-syntax` and `syntax-rules`

The `define-syntax-rule` form binds a macro that matches a single
pattern, but Racket’s macro system supports transformers that match
multiple patterns starting with the same identifier. To write such
macros, the programmer must use the more general `define-syntax` form
along with the `syntax-rules` transformer form:

```racket
(define-syntax id               
  (syntax-rules (literal-id ...)
    [pattern template]          
    ...))                       
```

> The `define-syntax-rule` form is itself a macro that expands into
> `define-syntax` with a `syntax-rules` form that contains only one
> pattern and template.

For example, suppose we would like a `rotate` macro that generalizes
`swap` to work on either two or three identifiers, so that

```racket
(let ([red 1] [green 2] [blue 3])       
  (rotate red green)      ; swaps       
  (rotate red green blue) ; rotates left
  (list red green blue))                
```

produces `(1 3 2)`. We can implement `rotate` using `syntax-rules`:

```racket
(define-syntax rotate              
  (syntax-rules ()                 
    [(rotate a b) (swap a b)]      
    [(rotate a b c) (begin         
                     (swap a b)    
                     (swap b c))]))
```

The expression `(rotate red green)` matches the first pattern in the
`syntax-rules` form, so it expands to `(swap red green)`. The expression
`(rotate red green blue)` matches the second pattern, so it expands to
`(begin (swap red green) (swap green blue))`.

#### 16.1.4. Matching Sequences

A better `rotate` macro would allow any number of identifiers, instead
of just two or three. To match a use of `rotate` with any number of
identifiers, we need a pattern form that has something like a Kleene
star. In a Racket macro pattern, a star is written as `...`.

To implement `rotate` with `...`, we need a base case to handle a single
identifier, and an inductive case to handle more than one identifier:

```racket
(define-syntax rotate                         
  (syntax-rules ()                            
    [(rotate a) (void)]                       
    [(rotate a b c ...) (begin                
                          (swap a b)          
                          (rotate b c ...))]))
```

When a pattern variable like `c` is followed by `...` in a pattern, then
it must be followed by `...` in a template, too. The pattern variable
effectively matches a sequence of zero or more forms, and it is replaced
in the template by the same sequence.

Both versions of `rotate` so far are a bit inefficient, since pairwise
swapping keeps moving the value from the first variable into every
variable in the sequence until it arrives at the last one. A more
efficient `rotate` would move the first value directly to the last
variable. We can use `...` patterns to implement the more efficient
variant using a helper macro:

```racket
(define-syntax rotate                        
  (syntax-rules ()                           
    [(rotate a c ...)                        
     (shift-to (c ... a) (a c ...))]))       
                                             
(define-syntax shift-to                      
  (syntax-rules ()                           
    [(shift-to (from0 from ...) (to0 to ...))
     (let ([tmp from0])                      
       (set! to from) ...                    
       (set! to0 tmp))]))                    
```

In the `shift-to` macro, `...` in the template follows `(set! to from)`,
which causes the `(set! to from)` expression to be duplicated as many
times as necessary to use each identifier matched in the `to` and `from`
sequences. \(The number of `to` and `from` matches must be the same,
otherwise the macro expansion fails with an error.\)

#### 16.1.5. Identifier Macros

Given our macro definitions, the `swap` or `rotate` identifiers must be
used after an open parenthesis, otherwise a syntax error is reported:

```racket
> (+ swap 3)              
eval:2:0: swap: bad syntax
  in: swap                
```

An _identifier macro_ is a pattern-matching macro that works when used
by itself without parentheses. For example, we can define `val` as an
identifier macro that expands to `(get-val)`, so `(+ val 3)` would
expand to `(+ (get-val) 3)`.

```racket
> (define-syntax val                                          
    (lambda (stx)                                             
      (syntax-case stx ()                                     
        [val (identifier? (syntax val)) (syntax (get-val))])))
> (define-values (get-val put-val!)                           
    (let ([private-val 0])                                    
      (values (lambda () private-val)                         
              (lambda (v) (set! private-val v)))))            
> val                                                         
0                                                             
> (+ val 3)                                                   
3                                                             
```

The `val` macro uses `syntax-case`, which enables defining more powerful
macros and will be explained in the Mixing Patterns and Expressions:
`syntax-case` section. For now it is sufficient to know that to define a
macro, `syntax-case` is used in a `lambda`, and its templates must be
wrapped with an explicit `syntax` constructor. Finally, `syntax-case`
clauses may specify additional guard conditions after the pattern.

Our `val` macro uses an `identifier?` condition to ensure that `val`
_must not_ be used with parentheses. Instead, the macro raises a syntax
error:

```racket
> (val)                  
eval:8:0: val: bad syntax
  in: (val)              
```

#### 16.1.6. `set!` Transformers

With the above `val` macro, we still must call `put-val!` to change the
stored value. It would be more convenient, however, to use `set!`
directly on `val`. To invoke the macro when `val` is used with `set!`,
we create an assignment transformer with `make-set!-transformer`. We
must also declare `set!` as a literal in the `syntax-case` literal list.

```racket
> (define-syntax val2                                         
    (make-set!-transformer                                    
     (lambda (stx)                                            
       (syntax-case stx (set!)                                
         [val2 (identifier? (syntax val2)) (syntax (get-val))]
         [(set! val2 e) (syntax (put-val! e))]))))            
> val2                                                        
0                                                             
> (+ val2 3)                                                  
3                                                             
> (set! val2 10)                                              
> val2                                                        
10                                                            
```

#### 16.1.7. Macro-Generating Macros

Suppose that we have many identifiers like `val` and `val2` that we’d
like to redirect to accessor and mutator functions like `get-val` and
`put-val!`. We’d like to be able to just write:

`(define-get/put-id` `val` `get-val` `put-val!)`

Naturally, we can implement `define-get/put-id` as a macro:

```racket
> (define-syntax-rule (define-get/put-id id get put!)   
    (define-syntax id                                   
      (make-set!-transformer                            
       (lambda (stx)                                    
         (syntax-case stx (set!)                        
           [id (identifier? (syntax id)) (syntax (get))]
           [(set! id e) (syntax (put! e))])))))         
> (define-get/put-id val3 get-val put-val!)             
> (set! val3 11)                                        
> val3                                                  
11                                                      
```

The `define-get/put-id` macro is a _macro-generating macro_.

#### 16.1.8. Extended Example: Call-by-Reference Functions

We can use pattern-matching macros to add a form to Racket for defining
first-order _call-by-reference_ functions. When a call-by-reference
function body mutates its formal argument, the mutation applies to
variables that are supplied as actual arguments in a call to the
function.

For example, if `define-cbr` is like `define` except that it defines a
call-by-reference function, then

```racket
(define-cbr (f a b)
  (swap a b))      
                   
(let ([x 1] [y 2]) 
  (f x y)          
  (list x y))      
```

produces `(2 1)`.

We will implement call-by-reference functions by having function calls
supply accessor and mutators for the arguments, instead of supplying
argument values directly. In particular, for the function `f` above,
we’ll generate

```racket
(define (do-f get-a get-b put-a! put-b!)
  (define-get/put-id a get-a put-a!)    
  (define-get/put-id b get-b put-b!)    
  (swap a b))                           
```

and redirect a function call `(f x y)` to

```racket
(do-f (lambda () x)           
      (lambda () y)           
      (lambda (v) (set! x v)) 
      (lambda (v) (set! y v)))
```

Clearly, then `define-cbr` is a macro-generating macro, which binds `f`
to a macro that expands to a call of `do-f`. That is, `(define-cbr (f a
b) (swap a b))` needs to generate the definition

```racket
(define-syntax f             
  (syntax-rules ()           
    [(id actual ...)         
     (do-f (lambda () actual)
           ...               
           (lambda (v)       
             (set! actual v))
           ...)]))           
```

At the same time, `define-cbr` needs to define `do-f` using the body of
`f`, this second part is slightly more complex, so we defer most of it
to a `define-for-cbr` helper module, which lets us write `define-cbr`
easily enough:

```racket
(define-syntax-rule (define-cbr (id arg ...) body)
  (begin                                          
    (define-syntax id                             
      (syntax-rules ()                            
        [(id actual (... ...))                    
         (do-f (lambda () actual)                 
               (... ...)                          
               (lambda (v)                        
                 (set! actual v))                 
               (... ...))]))                      
    (define-for-cbr do-f (arg ...)                
      () ; explained below...                     
      body)))                                     
```

Our remaining task is to define `define-for-cbr` so that it converts

`(define-for-cbr` `do-f` `(a` `b)` `()` `(swap` `a` `b))`

to the function definition `do-f` above. Most of the work is generating
a `define-get/put-id` declaration for each argument, `a` and `b`, and
putting them before the body. Normally, that’s an easy task for `...` in
a pattern and template, but this time there’s a catch: we need to
generate the names `get-a` and `put-a!` as well as `get-b` and `put-b!`,
and the pattern language provides no way to synthesize identifiers based
on existing identifiers.

As it turns out, lexical scope gives us a way around this problem. The
trick is to iterate expansions of `define-for-cbr` once for each
argument in the function, and that’s why `define-for-cbr` starts with an
apparently useless `()` after the argument list. We need to keep track
of all the arguments seen so far and the `get` and `put` names generated
for each, in addition to the arguments left to process. After we’ve
processed all the identifiers, then we have all the names we need.

Here is the definition of `define-for-cbr`:

```racket
(define-syntax define-for-cbr            
  (syntax-rules ()                       
    [(define-for-cbr do-f (id0 id ...)   
       (gens ...) body)                  
     (define-for-cbr do-f (id ...)       
       (gens ... (id0 get put)) body)]   
    [(define-for-cbr do-f ()             
       ((id get put) ...) body)          
     (define (do-f get ... put ...)      
       (define-get/put-id id get put) ...
       body)]))                          
```

Step-by-step, expansion proceeds as follows:

```racket
(define-for-cbr do-f (a b)                        
  () (swap a b))                                  
=> (define-for-cbr do-f (b)                       
     ([a get_1 put_1]) (swap a b))                
=> (define-for-cbr do-f ()                        
     ([a get_1 put_1] [b get_2 put_2]) (swap a b))
=> (define (do-f get_1 get_2 put_1 put_2)         
     (define-get/put-id a get_1 put_1)            
     (define-get/put-id b get_2 put_2)            
     (swap a b))                                  
```

The “subscripts” on `get_1`, `get_2`, `put_1`, and `put_2` are inserted
by the macro expander to preserve lexical scope, since the `get`
generated by each iteration of `define-for-cbr` should not bind the
`get` generated by a different iteration. In other words, we are
essentially tricking the macro expander into generating fresh names for
us, but the technique illustrates some of the surprising power of
pattern-based macros with automatic lexical scope.

The last expression eventually expands to just

```racket
(define (do-f get_1 get_2 put_1 put_2)
  (let ([tmp (get_1)])                
    (put_1 (get_2))                   
    (put_2 tmp)))                     
```

which implements the call-by-name function `f`.

To summarize, then, we can add call-by-reference functions to Racket
with just three small pattern-based macros: `define-cbr`,
`define-for-cbr`, and `define-get/put-id`.

### 16.2. General Macro Transformers

The `define-syntax` form creates a _transformer binding_ for an
identifier, which is a binding that can be used at compile time while
expanding expressions to be evaluated at run time. The compile-time
value associated with a transformer binding can be anything; if it is a
procedure of one argument, then the binding is used as a macro, and the
procedure is the _macro transformer_.

    16.2.1 Syntax Objects                                
    16.2.2 Macro Transformer Procedures                  
    16.2.3 Mixing Patterns and Expressions: `syntax-case`
    16.2.4 `with-syntax` and `generate-temporaries`      
    16.2.5 Compile and Run-Time Phases                   
    16.2.6 General Phase Levels                          
      16.2.6.1 Phases and Bindings                       
      16.2.6.2 Phases and Modules                        
    16.2.7 Syntax Taints                                 

#### 16.2.1. Syntax Objects

The input and output of a macro transformer \(i.e., source and
replacement forms\) are represented as _syntax objects_. A syntax object
contains symbols, lists, and constant values \(such as numbers\) that
essentially correspond to the `quote`d form of the expression. For
example, a representation of the expression `(+ 1 2)` contains the
symbol `'+` and the numbers `1` and `2`, all in a list. In addition to
this quoted content, a syntax object associates source-location and
lexical-binding information with each part of the form. The
source-location information is used when reporting syntax errors \(for
example\), and the lexical-binding information allows the macro system
to maintain lexical scope. To accommodate this extra information, the
represention of the expression `(+ 1 2)` is not merely `'(+ 1 2)`, but a
packaging of `'(+ 1 2)` into a syntax object.

To create a literal syntax object, use the `syntax` form:

```racket
> (syntax (+ 1 2))        
#<syntax:eval:1:0 (+ 1 2)>
```

In the same way that `'` abbreviates `quote`, `#'` abbreviates `syntax`:

```racket
> #'(+ 1 2)               
#<syntax:eval:1:0 (+ 1 2)>
```

A syntax object that contains just a symbol is an _identifier syntax
object_. Racket provides some additional operations specific to
identifier syntax objects, including the `identifier?` operation to
detect identifiers. Most notably, `free-identifier=?`  determines
whether two identifiers refer to the same binding:

```racket
> (identifier? #'car)                           
#t                                              
> (identifier? #'(+ 1 2))                       
#f                                              
> (free-identifier=? #'car #'cdr)               
#f                                              
> (free-identifier=? #'car #'car)               
#t                                              
> (require (only-in racket/base [car also-car]))
> (free-identifier=? #'car #'also-car)          
#t                                              
```

To see the lists, symbols, numbers, etc. within a syntax object, use
`syntax->datum`:

```racket
> (syntax->datum #'(+ 1 2))
'(+ 1 2)                   
```

The `syntax-e` function is similar to `syntax->datum`, but it unwraps a
single layer of source-location and lexical-context information, leaving
sub-forms that have their own information wrapped as syntax objects:

```racket
> (syntax-e #'(+ 1 2))                                           
'(#<syntax:eval:1:0 +> #<syntax:eval:1:0 1> #<syntax:eval:1:0 2>)
```

The `syntax-e` function always leaves syntax-object wrappers around
sub-forms that are represented via symbols, numbers, and other literal
values. The only time it unwraps extra sub-forms is when unwrapping a
pair, in which case the `cdr` of the pair may be recursively unwrapped,
depending on how the syntax object was constructed.

The opposite of `syntax->datum` is, of course, `datum->syntax`.  In
addition to a datum like `'(+ 1 2)`, `datum->syntax` needs an existing
syntax object to donate its lexical context, and optionally another
syntax object to donate its source location:

```racket
> (datum->syntax #'lex    
                 '(+ 1 2) 
                 #'srcloc)
#<syntax:eval:1:0 (+ 1 2)>
```

In the above example, the lexical context of `#'lex` is used for the new
syntax object, while the source location of `#'srcloc` is used.

When the second \(i.e., the “datum”\) argument to `datum->syntax`
includes syntax objects, those syntax objects are preserved intact in
the result. That is, deconstructing the result with `syntax-e`
eventually produces the syntax objects that were given to
`datum->syntax`.

#### 16.2.2. Macro Transformer Procedures

Any procedure of one argument can be a macro transformer.  As it turns
out, the `syntax-rules` form is a macro that expands to a procedure
form. For example, if you evaluate a `syntax-rules` form directly
\(instead of placing on the right-hand of a `define-syntax` form\), the
result is a procedure:

```racket
> (syntax-rules () [(nothing) something])
#<procedure>                             
```

Instead of using `syntax-rules`, you can write your own macro
transformer procedure directly using `lambda`. The argument to the
procedure is a syntax object that represents the source form, and the
result of the procedure must be a syntax object that represents the
replacement form:

```racket
> (define-syntax self-as-string                          
    (lambda (stx)                                        
      (datum->syntax stx                                 
                     (format "~s" (syntax->datum stx)))))
> (self-as-string (+ 1 2))                               
"(self-as-string (+ 1 2))"                               
```

The source form passed to a macro transformer represents an expression
in which its identifier is used in an application position \(i.e., after
a parenthesis that starts an expression\), or it represents the
identifier by itself if it is used as an expression position and not in
an application position.The procedure produced by `syntax-rules` raises
a syntax error if its argument corresponds to a use of the identifier by
itself, which is why `syntax-rules` does not implement an identifier
macro.

```racket
> (self-as-string (+ 1 2))
"(self-as-string (+ 1 2))"
> self-as-string          
"self-as-string"          
```

The `define-syntax` form supports the same shortcut syntax for functions
as `define`, so that the following `self-as-string` definition is
equivalent to the one that uses `lambda` explicitly:

```racket
> (define-syntax (self-as-string stx)                 
    (datum->syntax stx                                
                   (format "~s" (syntax->datum stx))))
> (self-as-string (+ 1 2))                            
"(self-as-string (+ 1 2))"                            
```

#### 16.2.3. Mixing Patterns and Expressions: `syntax-case`

The procedure generated by `syntax-rules` internally uses `syntax-e` to
deconstruct the given syntax object, and it uses `datum->syntax` to
construct the result. The `syntax-rules` form doesn’t provide a way to
escape from pattern-matching and template-construction mode into an
arbitrary Racket expression.

The `syntax-case` form lets you mix pattern matching, template
construction, and arbitrary expressions:

```racket
(syntax-case stx-expr (literal-id ...)
  [pattern expr]                      
  ...)                                
```

Unlike `syntax-rules`, the `syntax-case` form does not produce a
procedure. Instead, it starts with a `stx-expr` expression that
determines the syntax object to match against the `pattern`s. Also, each
`syntax-case` clause has a `pattern` and `expr`, instead of a `pattern`
and `template`. Within an `expr`, the `syntax` form—usually abbreviated
with `#'`—shifts into template-construction mode; if the `expr` of a
clause starts with `#'`, then we have something like a `syntax-rules`
form:

```racket
> (syntax->datum              
   (syntax-case #'(+ 1 2) ()  
    [(op n1 n2) #'(- n1 n2)]))
'(- 1 2)                      
```

We could write the `swap` macro using `syntax-case` instead of
`define-syntax-rule` or `syntax-rules`:

```racket
(define-syntax (swap stx)           
  (syntax-case stx ()               
    [(swap x y) #'(let ([tmp x])    
                    (set! x y)      
                    (set! y tmp))]))
```

One advantage of using `syntax-case` is that we can provide better error
reporting for `swap`. For example, with the `define-syntax-rule`
definition of `swap`, then `(swap x 2)` produces a syntax error in terms
of `set!`, because `2` is not an identifier. We can refine our
`syntax-case` implementation of `swap` to explicitly check the
sub-forms:

```racket
(define-syntax (swap stx)                         
  (syntax-case stx ()                             
    [(swap x y)                                   
     (if (and (identifier? #'x)                   
              (identifier? #'y))                  
         #'(let ([tmp x])                         
             (set! x y)                           
             (set! y tmp))                        
         (raise-syntax-error #f                   
                             "not an identifier"  
                             stx                  
                             (if (identifier? #'x)
                                 #'y              
                                 #'x)))]))        
```

With this definition, `(swap x 2)` provides a syntax error originating
from `swap` instead of `set!`.

In the above definition of `swap`, `#'x` and `#'y` are templates, even
though they are not used as the result of the macro transformer. This
example illustrates how templates can be used to access pieces of the
input syntax, in this case for checking the form of the pieces. Also,
the match for `#'x` or `#'y` is used in the call to
`raise-syntax-error`, so that the syntax-error message can point
directly to the source location of the non-identifier.

#### 16.2.4. `with-syntax` and `generate-temporaries`

Since `syntax-case` lets us compute with arbitrary Racket expressions,
we can more simply solve a problem that we had in writing
`define-for-cbr` \(see Extended Example: Call-by-Reference Functions\),
where we needed to generate a set of names based on a sequence `id ...`:

```racket
(define-syntax (define-for-cbr stx)          
  (syntax-case stx ()                        
    [(_ do-f (id ...) body)                  
     ....                                    
       #'(define (do-f get ... put ...)      
           (define-get/put-id id get put) ...
           body) ....]))                     
```

In place of the `....`s above, we need to bind `get ...` and `put ...`
to lists of generated identifiers. We cannot use `let` to bind `get` and
`put`, because we need bindings that count as pattern variables, instead
of normal local variables. The `with-syntax` form lets us bind pattern
variables:

```racket
(define-syntax (define-for-cbr stx)          
  (syntax-case stx ()                        
    [(_ do-f (id ...) body)                  
     (with-syntax ([(get ...) ....]          
                   [(put ...) ....])         
       #'(define (do-f get ... put ...)      
           (define-get/put-id id get put) ...
           body))]))                         
```

Now we need an expression in place of `....` that generates as many
identifiers as there are `id` matches in the original pattern. Since
this is a common task, Racket provides a helper function,
`generate-temporaries`, that takes a sequence of identifiers and returns
a sequence of generated identifiers:

```racket
(define-syntax (define-for-cbr stx)                              
  (syntax-case stx ()                                            
    [(_ do-f (id ...) body)                                      
     (with-syntax ([(get ...) (generate-temporaries #'(id ...))] 
                   [(put ...) (generate-temporaries #'(id ...))])
       #'(define (do-f get ... put ...)                          
           (define-get/put-id id get put) ...                    
           body))]))                                             
```

This way of generating identifiers is normally easier to think about
than tricking the macro expander into generating names with purely
pattern-based macros.

In general, the left-hand side of a `with-syntax` binding is a pattern,
just like in `syntax-case`. In fact, a `with-syntax` form is just a
`syntax-case` form turned partially inside-out.

#### 16.2.5. Compile and Run-Time Phases

As sets of macros get more complicated, you might want to write your own
helper functions, like `generate-temporaries`. For example, to provide
good syntax error messsages, `swap`, `rotate`, and `define-cbr` all
should check that certain sub-forms in the source form are identifiers.
We could use a `check-ids` function to perform this checking everywhere:

```racket
(define-syntax (swap stx)                  
  (syntax-case stx ()                      
    [(swap x y) (begin                     
                  (check-ids stx #'(x y))  
                  #'(let ([tmp x])         
                      (set! x y)           
                      (set! y tmp)))]))    
                                           
(define-syntax (rotate stx)                
  (syntax-case stx ()                      
    [(rotate a c ...)                      
     (begin                                
       (check-ids stx #'(a c ...))         
       #'(shift-to (c ... a) (a c ...)))]))
```

The `check-ids` function can use the `syntax->list` function to convert
a syntax-object wrapping a list into a list of syntax objects:

```racket
(define (check-ids stx forms)                 
  (for-each                                   
   (lambda (form)                             
     (unless (identifier? form)               
       (raise-syntax-error #f                 
                           "not an identifier"
                           stx                
                           form)))            
   (syntax->list forms)))                     
```

If you define `swap` and `check-ids` in this way, however, it doesn’t
work:

```racket
> (let ([a 1] [b 2]) (swap a b))                     
check-ids: undefined;                                
 cannot reference an identifier before its definition
  in module: top-level                               
```

The problem is that `check-ids` is defined as a run-time expression, but
`swap` is trying to use it at compile time. In interactive mode, compile
time and run time are interleaved, but they are not interleaved within
the body of a module, and they are not interleaved across modules that
are compiled ahead-of-time. To help make all of these modes treat code
consistently, Racket separates the binding spaces for different phases.

To define a `check-ids` function that can be referenced at compile time,
use `begin-for-syntax`:

```racket
(begin-for-syntax                               
  (define (check-ids stx forms)                 
    (for-each                                   
     (lambda (form)                             
       (unless (identifier? form)               
         (raise-syntax-error #f                 
                             "not an identifier"
                             stx                
                             form)))            
     (syntax->list forms))))                    
```

With this for-syntax definition, then `swap` works:

```racket
> (let ([a 1] [b 2]) (swap a b) (list a b))
'(2 1)                                     
> (swap a 1)                               
eval:13:0: swap: not an identifier         
  at: 1                                    
  in: (swap a 1)                           
```

When organizing a program into modules, you may want to put helper
functions in one module to be used by macros that reside on other
modules. In that case, you can write the helper function using `define`:

`"utils.rkt"`
```racket
#lang racket                                  
                                              
(provide check-ids)                           
                                              
(define (check-ids stx forms)                 
  (for-each                                   
   (lambda (form)                             
     (unless (identifier? form)               
       (raise-syntax-error #f                 
                           "not an identifier"
                           stx                
                           form)))            
   (syntax->list forms)))                     
```

Then, in the module that implements macros, import the helper function
using `(require (for-syntax "utils.rkt"))` instead of `(require
"utils.rkt")`:

```racket
#lang racket                             
                                         
(require (for-syntax "utils.rkt"))       
                                         
(define-syntax (swap stx)                
  (syntax-case stx ()                    
    [(swap x y) (begin                   
                  (check-ids stx #'(x y))
                  #'(let ([tmp x])       
                      (set! x y)         
                      (set! y tmp)))]))  
```

Since modules are separately compiled and cannot have circular
dependencies, the `"utils.rkt"` module’s run-time body can be compiled
before the compiling the module that implements `swap`.  Thus, the
run-time definitions in `"utils.rkt"` can be used to implement `swap`,
as long as they are explicitly shifted into compile time by `(require
(for-syntax ....))`.

The `racket` module provides `syntax-case`, `generate-temporaries`,
`lambda`, `if`, and more for use in both the run-time and compile-time
phases. That is why we can use `syntax-case` in the `racket` REPL both
directly and in the right-hand side of a `define-syntax` form.

The `racket/base` module, in contrast, exports those bindings only in
the run-time phase. If you change the module above that defines `swap`
so that it uses the `racket/base` language instead of `racket`, then it
no longer works. Adding `(require (for-syntax racket/base))` imports
`syntax-case` and more into the compile-time phase, so that the module
works again.

Suppose that `define-syntax` is used to define a local macro in the
right-hand side of a `define-syntax` form. In that case, the right-hand
side of the inner `define-syntax` is in the _meta-compile phase level_,
also known as _phase level 2_. To import `syntax-case` into that phase
level, you would have to use `(require (for-syntax (for-syntax
racket/base)))` or, equivalently, `(require (for-meta 2 racket/base))`.
For example,

```racket
#lang racket/base                                         
(require  ;; This provides the bindings for the definition
          ;; of shell-game.                               
          (for-syntax racket/base)                        
                                                          
          ;; And this for the definition of               
          ;; swap.                                        
          (for-syntax (for-syntax racket/base)))          
                                                          
(define-syntax (shell-game stx)                           
                                                          
  (define-syntax (swap stx)                               
    (syntax-case stx ()                                   
      [(_ a b)                                            
       #'(let ([tmp a])                                   
           (set! a b)                                     
           (set! b tmp))]))                               
                                                          
  (syntax-case stx ()                                     
    [(_ a b c)                                            
     (let ([a #'a] [b #'b] [c #'c])                       
       (when (= 0 (random 2)) (swap a b))                 
       (when (= 0 (random 2)) (swap b c))                 
       (when (= 0 (random 2)) (swap a c))                 
       #`(list #,a #,b #,c))]))                           
                                                          
(shell-game 3 4 5)                                        
(shell-game 3 4 5)                                        
(shell-game 3 4 5)                                        
```

Negative phase levels also exist. If a macro uses a helper function that
is imported `for-syntax`, and if the helper function returns
syntax-object constants generated by `syntax`, then identifiers in the
syntax will need bindings at _phase level -1_, also known as the
_template phase level_, to have any binding at the run-time phase level
relative to the module that defines the macro.

For instance, the `swap-stx` helper function in the example below is not
a syntax transformer—it’s just an ordinary function—but it produces
syntax objects that get spliced into the result of `shell-game`.
Therefore, its containing `helper` submodule needs to be imported at
`shell-game`’s phase 1 with `(require (for-syntax 'helper))`.

But from the perspective of `swap-stx`, its results will ultimately be
evaluated at phase level -1, when the syntax returned by `shell-game` is
evaluated. In other words, a negative phase level is a positive phase
level from the opposite direction: `shell-game`’s phase 1 is
`swap-stx`’s phase 0, so `shell-game`’s phase 0 is `swap-stx`’s phase
-1. And that’s why this example won’t work—the `'helper` submodule has
no bindings at phase -1.

```racket
#lang racket/base                 
(require (for-syntax racket/base))
                                  
(module helper racket/base        
  (provide swap-stx)              
  (define (swap-stx a-stx b-stx)  
    #`(let ([tmp #,a-stx])        
          (set! #,a-stx #,b-stx)  
          (set! #,b-stx tmp))))   
                                  
(require (for-syntax 'helper))    
                                  
(define-syntax (shell-game stx)   
  (syntax-case stx ()             
    [(_ a b c)                    
     #`(begin                     
         #,(swap-stx #'a #'b)     
         #,(swap-stx #'b #'c)     
         #,(swap-stx #'a #'c)     
         (list a b c))]))         
                                  
(define x 3)                      
(define y 4)                      
(define z 5)                      
(shell-game x y z)                
```

To repair this example, we add `(require (for-template racket/base))` to
the `'helper` submodule.

```racket
#lang racket/base                                                          
(require (for-syntax racket/base))                                         
                                                                           
(module helper racket/base                                                 
  (require (for-template racket/base)) ; binds `let` and `set!` at phase -1
  (provide swap-stx)                                                       
  (define (swap-stx a-stx b-stx)                                           
    #`(let ([tmp #,a-stx])                                                 
          (set! #,a-stx #,b-stx)                                           
          (set! #,b-stx tmp))))                                            
                                                                           
(require (for-syntax 'helper))                                             
                                                                           
(define-syntax (shell-game stx)                                            
  (syntax-case stx ()                                                      
    [(_ a b c)                                                             
     #`(begin                                                              
         #,(swap-stx #'a #'b)                                              
         #,(swap-stx #'b #'c)                                              
         #,(swap-stx #'a #'c)                                              
         (list a b c))]))                                                  
                                                                           
(define x 3)                                                               
(define y 4)                                                               
(define z 5)                                                               
(shell-game x y z)                                                         
(shell-game x y z)                                                         
(shell-game x y z)                                                         
```

#### 16.2.6. General Phase Levels

A _phase_ can be thought of as a way to separate computations in a
pipeline of processes where one produces code that is used by the next.
\(E.g., a pipeline that consists of a preprocessor process, a compiler,
and an assembler.\)

Imagine starting two Racket processes for this purpose.  If you ignore
inter-process communication channels like sockets and files, the
processes will have no way to share anything other than the text that is
piped from the standard output of one process into the standard input of
the other.  Similarly, Racket effectively allows multiple invocations of
a module to exist in the same process but separated by phase.  Racket
enforces _separation_ of such phases, where different phases cannot
communicate in any way other than via the protocol of macro expansion,
where the output of one phases is the code used in the next.

##### 16.2.6.1. Phases and Bindings

Every binding of an identifier exists in a particular phase.  The link
between a binding and its phase is represented by an integer _phase
level_.  Phase level 0 is the phase used for “plain” \(or “runtime”\)
definitions, so

`(define` `age` `5)`

adds a binding for `age` into phase level 0.  The identifier `age` can
be defined at a higher phase level using `begin-for-syntax`:

```racket
(begin-for-syntax
  (define age 5))
```

With a single `begin-for-syntax` wrapper, `age` is defined at phase
level 1.  We can easily mix these two definitions in the same module or
in a top-level namespace, and there is no clash between the two `age`s
that are defined at different phase levels:

```racket
> (define age 3)   
> (begin-for-syntax
    (define age 9))
```

The `age` binding at phase level 0 has a value of 3, and the `age`
binding at phase level 1 has a value of 9.

Syntax objects capture binding information as a first-class value. Thus,

`#'age`

is a syntax object that represents the `age` binding—but since there are
two `age`s \(one at phase level 0 and one at phase level 1\), which one
does it capture?  In fact, Racket imbues `#'age` with lexical
information for all phase levels, so the answer is that `#'age` captures
both.

The relevant binding of `age` captured by `#'age` is determined when
`#'age` is eventually used.  As an example, we bind `#'age` to a pattern
variable so we can use it in a template, and then we `eval`uate the
template: We use `eval` here to demonstrate phases, but see Reflection
and Dynamic Evaluation for caveats about `eval`.

```racket
> (eval (with-syntax ([age #'age])
          #'(displayln age)))     
3                                 
```

The result is `3` because `age` is used at phase 0 level. We can try
again with the use of `age` inside `begin-for-syntax`:

```racket
> (eval (with-syntax ([age #'age])
          #'(begin-for-syntax     
              (displayln age))))  
9                                 
```

In this case, the answer is `9`, because we are using `age` at phase
level 1 instead of 0 \(i.e., `begin-for-syntax` evaluates its
expressions at phase level 1\). So, you can see that we started with the
same syntax object, `#'age`, and we were able to use it in two different
ways: at phase level 0 and at phase level 1.

A syntax object has a lexical context from the moment it first exists. A
syntax object that is provided from a module retains its lexical
context, and so it references bindings in the context of its source
module, not the context of its use.  The following example defines
`button` at phase level 0 and binds it to `0`, while `see-button` binds
the syntax object for `button` in module `a`:

```racket
> (module a racket                                           
    (define button 0)                                        
    (provide (for-syntax see-button))                        
    ; Why not (define see-button #'button)? We explain later.
    (define-for-syntax see-button #'button))                 
> (module b racket                                           
    (require 'a)                                             
    (define button 8)                                        
    (define-syntax (m stx)                                   
      see-button)                                            
    (m))                                                     
> (require 'b)                                               
0                                                            
```

The result of the `m` macro is the value of `see-button`, which is
`#'button` with the lexical context of the `a` module.  Even though
there is another `button` in `b`, the second `button` will not confuse
Racket, because the lexical context of `#'button` \(the value bound to
`see-button`\) is `a`.

Note that `see-button` is bound at phase level 1 by virtue of defining
it with `define-for-syntax`.  Phase level 1 is needed because `m` is a
macro, so its body executes at one phase higher than the context of its
definition.  Since `m` is defined at phase level 0, its body is at phase
level 1, so any bindings referenced by the body must be at phase level
1.

##### 16.2.6.2. Phases and Modules

A phase level is a module-relative concept.  When importing from another
module via `require`, Racket lets us shift imported bindings to a phase
level that is different from the original one:

```racket
(require "a.rkt")                ; import with no phase shift
(require (for-syntax "a.rkt"))   ; shift phase by +1         
(require (for-template "a.rkt")) ; shift phase by -1         
(require (for-meta 5 "a.rkt"))   ; shift phase by +5         
```

That is, using `for-syntax` in `require` means that all of the bindings
from that module will have their phase levels increased by one.  A
binding that is `define`d at phase level 0 and imported with
`for-syntax` becomes a phase-level 1 binding:

```racket
> (module c racket                          
    (define x 0) ; defined at phase level 0 
    (provide x))                            
> (module d racket                          
    (require (for-syntax 'c))               
    ; has a binding at phase level 1, not 0:
    #'x)                                    
```

Let’s see what happens if we try to create a binding for the `#'button`
syntax object at phase level 0:

```racket
> (define button 0)           
> (define see-button #'button)
```

Now both `button` and `see-button` are defined at phase 0.  The lexical
context of `#'button` will know that there is a binding for `button` at
phase 0.  In fact, it seems like things are working just fine if we try
to `eval` `see-button`:

```racket
> (eval see-button)
0                  
```

Now, let’s use `see-button` in a macro:

```racket
> (define-syntax (m stx)                             
    see-button)                                      
> (m)                                                
see-button: undefined;                               
 cannot reference an identifier before its definition
  in module: top-level                               
```

Clearly, `see-button` is not defined at phase level 1, so we cannot
refer to it inside the macro body.  Let’s try to use `see-button` in
another module by putting the button definitions in a module and
importing it at phase level 1.  Then, we will get `see-button` at phase
level 1:

```racket
> (module a racket                                              
    (define button 0)                                           
    (define see-button #'button)                                
    (provide see-button))                                       
> (module b racket                                              
    (require (for-syntax 'a)) ; gets see-button at phase level 1
    (define-syntax (m stx)                                      
      see-button)                                               
    (m))                                                        
eval:1:0: button: unbound identifier;                           
 also, no #%top syntax transformer is bound                     
  in: button                                                    
```

Racket says that `button` is unbound now!  When `a` is imported at phase
level 1, we have the following bindings:

```racket
button     at phase level 1
see-button at phase level 1
```

So the macro `m` can see a binding for `see-button` at phase level 1 and
will return the `#'button` syntax object, which refers to `button`
binding at phase level 1.  But the use of `m` is at phase level 0, and
there is no `button` at phase level 0 in `b`.  That is why `see-button`
needs to be bound at phase level 1, as in the original `a`.  In the
original `b`, then, we have the following bindings:

```racket
button     at phase level 0
see-button at phase level 1
```

In this scenario, we can use `see-button` in the macro, since
`see-button` is bound at phase level 1.  When the macro expands, it will
refer to a `button` binding at phase level 0.

Defining `see-button` with `(define see-button #'button)` isn’t
inherently wrong; it depends on how we intend to use `see-button`.  For
example, we can arrange for `m` to sensibly use `see-button` because it
puts it in a phase level 1 context using `begin-for-syntax`:

```racket
> (module a racket                 
    (define button 0)              
    (define see-button #'button)   
    (provide see-button))          
> (module b racket                 
    (require (for-syntax 'a))      
    (define-syntax (m stx)         
      (with-syntax ([x see-button])
        #'(begin-for-syntax        
            (displayln x))))       
    (m))                           
0                                  
```

In this case, module `b` has both `button` and `see-button` bound at
phase level 1.  The expansion of the macro is

```racket
(begin-for-syntax    
  (displayln button))
```

which works, because `button` is bound at phase level 1.

Now, you might try to cheat the phase system by importing `a` at both
phase level 0 and phase level 1.  Then you would have the following
bindings

```racket
button     at phase level 0
see-button at phase level 0
button     at phase level 1
see-button at phase level 1
```

You might expect now that `see-button` in a macro would work, but it
doesn’t:

```racket
> (module a racket                         
    (define button 0)                      
    (define see-button #'button)           
    (provide see-button))                  
> (module b racket                         
    (require 'a                            
             (for-syntax 'a))              
    (define-syntax (m stx)                 
      see-button)                          
    (m))                                   
eval:1:0: button: unbound identifier;      
 also, no #%top syntax transformer is bound
  in: button                               
```

The `see-button` inside macro `m` comes from the `(for-syntax 'a)`
import.  For macro `m` to work, it needs to have `button` bound at phase
0. That binding exists—it’s implied by `(require 'a)`.  However,
`(require 'a)` and `(require (for-syntax 'a))` are _different
instantiations_ of the same module.  The `see-button` at phase 1 only
refers to the `button` at phase 1, not the `button` bound at phase 0
from a different instantiation—even from the same source module.

This kind of phase-level mismatch between instantiations can be repaired
with `syntax-shift-phase-level`. Recall that a syntax object like
`#'button` captures lexical information at _all_ phase levels. The
problem here is that `see-button` is invoked at phase 1, but needs to
return a syntax object that can be evaluated at phase 0. By default,
`see-button` is bound to `#'button` at the same phase level. But with
`syntax-shift-phase-level`, we can make `see-button` refer to `#'button`
at a different relative phase level. In this case, we use a phase shift
of `-1` to make `see-button` at phase 1 refer to `#'button` at phase 0.
\(Because the phase shift happens at every level, it will also make
`see-button` at phase 0 refer to `#'button` at phase -1.\)

Note that `syntax-shift-phase-level` merely creates a reference across
phases. To make that reference work, we still need to instantiate our
module at both phases so the reference and its target have their
bindings available. Thus, in module `'b`, we still import module `'a` at
both phase 0 and phase 1—using `(require 'a (for-syntax 'a))`—so we have
a phase-1 binding for `see-button` and a phase-0 binding for `button`.
Now macro `m` will work.

```racket
> (module a racket                                            
    (define button 0)                                         
    (define see-button (syntax-shift-phase-level #'button -1))
    (provide see-button))                                     
> (module b racket                                            
    (require 'a (for-syntax 'a))                              
    (define-syntax (m stx)                                    
      see-button)                                             
    (m))                                                      
> (require 'b)                                                
0                                                             
```

By the way, what happens to the `see-button` that’s bound at phase 0?
Its `#'button` binding has likewise been shifted, but to phase -1. Since
`button` itself isn’t bound at phase -1, if we try to evaluate
`see-button` at phase 0, we get an error. In other words, we haven’t
permanently cured our mismatch problem—we’ve just shifted it to a less
bothersome location.

```racket
> (module a racket                                            
    (define button 0)                                         
    (define see-button (syntax-shift-phase-level #'button -1))
    (provide see-button))                                     
> (module b racket                                            
    (require 'a (for-syntax 'a))                              
    (define-syntax (m stx)                                    
      see-button)                                             
    (m))                                                      
> (module b2 racket                                           
    (require 'a)                                              
    (eval see-button))                                        
> (require 'b2)                                               
button: undefined;                                            
 cannot reference an identifier before its definition         
  in module: top-level                                        
```

Mismatches like the one above can also arise when a macro tries to match
literal bindings—using `syntax-case` or `syntax-parse`.

```racket
> (module x racket                                 
    (require (for-syntax syntax/parse)             
             (for-template racket/base))           
                                                   
    (provide (all-defined-out))                    
                                                   
    (define button 0)                              
    (define (make) #'button)                       
    (define-syntax (process stx)                   
      (define-literal-set locals (button))         
      (syntax-parse stx                            
        [(_ (n (~literal button))) #'#''ok])))     
> (module y racket                                 
    (require (for-meta 1 'x)                       
             (for-meta 2 'x racket/base))          
                                                   
    (begin-for-syntax                              
      (define-syntax (m stx)                       
        (with-syntax ([out (make)])                
          #'(process (0 out)))))                   
                                                   
    (define-syntax (p stx)                         
      (m))                                         
                                                   
    (p))                                           
eval:2.0: process: expected the identifier `button'
  at: button                                       
  in: (process (0 button))                         
```

In this example, `make` is being used in `y` at phase level 2, and it
returns the `#'button` syntax object—which refers to `button` bound at
phase level 0 inside `x` and at phase level 2 in `y` from `(for-meta 2
'x)`.  The `process` macro is imported at phase level 1 from `(for-meta
1 'x)`, and it knows that `button` should be bound at phase level 1.
When the `syntax-parse` is executed inside `process`, it is looking for
`button` bound at phase level 1 but it sees only a phase level 2 binding
and doesn’t match.

To fix the example, we can provide `make` at phase level 1 relative to
`x`, and then we import it at phase level 1 in `y`:

```racket
> (module x racket                            
    (require (for-syntax syntax/parse)        
             (for-template racket/base))      
                                              
    (provide (all-defined-out))               
                                              
    (define button 0)                         
                                              
    (provide (for-syntax make))               
    (define-for-syntax (make) #'button)       
    (define-syntax (process stx)              
      (define-literal-set locals (button))    
      (syntax-parse stx                       
        [(_ (n (~literal button))) #'#''ok])))
> (module y racket                            
    (require (for-meta 1 'x)                  
             (for-meta 2 racket/base))        
                                              
    (begin-for-syntax                         
      (define-syntax (m stx)                  
        (with-syntax ([out (make)])           
          #'(process (0 out)))))              
                                              
    (define-syntax (p stx)                    
      (m))                                    
                                              
    (p))                                      
> (require 'y)                                
'ok                                           
```

#### 16.2.7. Syntax Taints

A use of a macro can expand into a use of an identifier that is not
exported from the module that binds the macro. In general, such an
identifier must not be extracted from the expanded expression and used
in a different context, because using the identifier in a different
context may break invariants of the macro’s module.

For example, the following module exports a macro `go` that expands to a
use of `unchecked-go`:

`"m.rkt"`
```racket
#lang racket                             
(provide go)                             
                                         
(define (unchecked-go n x)               
  ; to avoid disaster, n must be a number
  (+ n 17))                              
                                         
(define-syntax (go stx)                  
  (syntax-case stx ()                    
   [(_ x)                                
    #'(unchecked-go 8 x)]))              
```

If the reference to `unchecked-go` is extracted from the expansion of
`(go 'a)`, then it might be inserted into a new expression,
`(unchecked-go #f 'a)`, leading to disaster. The `datum->syntax`
procedure can be used similarly to construct references to an unexported
identifier, even when no macro expansion includes a reference to the
identifier.

To prevent such abuses of unexported identifiers, the `go` macro must
explicitly protect its expansion by using `syntax-protect`:

```racket
(define-syntax (go stx)                     
  (syntax-case stx ()                       
   [(_ x)                                   
    (syntax-protect #'(unchecked-go 8 x))]))
```

The `syntax-protect` function causes any syntax object that is extracted
from the result of `go` to be _tainted_.  The macro expander rejects
tainted identifiers, so attempting to extract `unchecked-go` from the
expansion of `(go 'a)` produces an identifier that cannot be used to
construct a new expression \(or, at least, not one that the macro
expander will accept\). The `syntax-rules`, `syntax-id-rule`, and
`define-syntax-rule` forms automatically protect their expansion
results.

More precisely, `syntax-protect` _arms_ a syntax object with a _dye
pack_. When a syntax object is armed, then `syntax-e` taints any syntax
object in its result. Similarly, `datum->syntax` taints its result when
its first argument is armed. Finally, if any part of a quoted syntax
object is armed, then the corresponding part is tainted in the resulting
syntax constant.

Of course, the macro expander itself must be able to _disarm_ a taint on
a syntax object, so that it can further expand an expression or its
sub-expressions. When a syntax object is armed with a dye pack, the dye
pack has an associated inspector that can be used to disarm the dye
pack. A `(syntax-protect stx)` function call is actually a shorthand for
`(syntax-arm stx #f #t)`, which arms `stx` using a suitable inspector.
The expander uses `syntax-disarm` and with its inspector on every
expression before trying to expand or compile it.

In much the same way that the macro expander copies properties from a
syntax transformer’s input to its output \(see \[missing\]\), the
expander copies dye packs from a transformer’s input to its output.
Building on the previous example,

`"n.rkt"`
```racket
#lang racket                
(require "m.rkt")           
                            
(provide go-more)           
                            
(define y 'hello)           
                            
(define-syntax (go-more stx)
  (syntax-protect #'(go y)))
```

the expansion of `(go-more)` introduces a reference to the unexported
`y` in `(go y)`, and the expansion result is armed so that `y` cannot be
extracted from the expansion.  Even if `go` did not use `syntax-protect`
for its result \(perhaps because it does not need to protect
`unchecked-go` after all\), the dye pack on `(go y)` is propagated to
the final expansion `(unchecked-go 8 y)`. The macro expander uses
`syntax-rearm` to propagate dye packs from a transformer’s input to its
output.

##### 16.2.7.1. Tainting Modes

In some cases, a macro implementor intends to allow limited
destructuring of a macro result without tainting the result. For
example, given the following `define-like-y` macro,

`"q.rkt"`
```racket
#lang racket                                            
                                                        
(provide define-like-y)                                 
                                                        
(define y 'hello)                                       
                                                        
(define-syntax (define-like-y stx)                      
  (syntax-case stx ()                                   
    [(_ id) (syntax-protect #'(define-values (id) y))]))
```

someone may use the macro in an internal definition:

```racket
(let ()            
  (define-like-y x)
  x)               
```

The implementor of the `"q.rkt"` module most likely intended to allow
such uses of `define-like-y`. To convert an internal definition into a
`letrec` binding, however, the `define` form produced by `define-like-y`
must be deconstructed, which would normally taint both the binding `x`
and the reference to `y`.

Instead, the internal use of `define-like-y` is allowed, because
`syntax-protect` treats specially a syntax list that begins with
`define-values`. In that case, instead of arming the overall expression,
each individual element of the syntax list is armed, pushing dye packs
further into the second element of the list so that they are attached to
the defined identifiers. Thus, `define-values`, `x`, and `y` in the
expansion result `(define-values (x) y)` are individually armed, and the
definition can be deconstructed for conversion to `letrec`.

Just like `syntax-protect`, the expander rearms a transformer result
that starts with `define-values`, by pushing dye packs into the list
elements. As a result, `define-like-y` could have been implemented to
produce `(define id y)`, which uses `define` instead of `define-values`.
In that case, the entire `define` form is at first armed with a dye
pack, but as the `define` form is expanded to `define-values`, the dye
pack is moved to the parts.

The macro expander treats syntax-list results starting with
`define-syntaxes` in the same way that it treats results starting with
`define-values`. Syntax-list results starting with `begin` are treated
similarly, except that the second element of the syntax list is treated
like all the other elements \(i.e., the immediate element is armed,
instead of its content\). Furthermore, the macro expander applies this
special handling recursively, in case a macro produces a `begin` form
that contains nested `define-values` forms.

The default application of dye packs can be overridden by attaching a
`'taint-mode` property \(see \[missing\]\) to the resulting syntax
object of a macro transformer. If the property value is `'opaque`, then
the syntax object is armed and not its parts. If the property value is
`'transparent`, then the syntax object’s parts are armed. If the
property value is `'transparent-binding`, then the syntax object’s parts
and the sub-parts of the second part \(as for `define-values` and
`define-syntaxes`\) are armed. The `'transparent` and
`'transparent-binding` modes trigger recursive property checking at the
parts, so that armings can be pushed arbitrarily deeply into a
transformer’s result.

##### 16.2.7.2. Taints and Code Inspectors

Tools that are intended to be privileged \(such as a debugging
transformer\) must disarm dye packs in expanded programs.  Privilege is
granted through _code inspectors_. Each dye pack records an inspector,
and a syntax object can be disarmed using a sufficiently powerful
inspector.

When a module is declared, the declaration captures the current value of
the `current-code-inspector` parameter.  The captured inspector is used
when `syntax-protect` is applied by a macro transformer that is defined
within the module. A tool can disarm the resulting syntax object by
supplying `syntax-disarm` with an inspector that is the same or a
super-inspector of the module’s inspector. Untrusted code is ultimately
run after setting `current-code-inspector` to a less powerful inspector
\(after trusted code, such as debugging tools, have been loaded\).

With this arrangement, macro-generating macros require some care, since
the generating macro may embed syntax objects in the generated macro
that need to have the generating module’s protection level, rather than
the protection level of the module that contains the generated macro. To
avoid this problem, use the module’s declaration-time inspector, which
is accessible as `(variable-reference->module-declaration-inspector
(#%variable-reference))`, and use it to define a variant of
`syntax-protect`.

For example, suppose that the `go` macro is implemented through a macro:

```racket
#lang racket                                             
(provide def-go)                                         
                                                         
(define (unchecked-go n x)                               
  (+ n 17))                                              
                                                         
(define-syntax (def-go stx)                              
  (syntax-case stx ()                                    
    [(_ go)                                              
     (protect-syntax                                     
      #'(define-syntax (go stx)                          
          (syntax-case stx ()                            
            [(_ x)                                       
             (protect-syntax #'(unchecked-go 8 x))])))]))
```

When `def-go` is used inside another module to define `go`, and when the
`go`-defining module is at a different protection level than the
`def-go`-defining module, the generated macro’s use of `protect-syntax`
is not right.  The use of `unchecked-go` should be protected at the
level of the `def-go`-defining module, not the `go`-defining module.

The solution is to define and use `go-syntax-protect`, instead:

```racket
#lang racket                                                   
(provide def-go)                                               
                                                               
(define (unchecked-go n x)                                     
  (+ n 17))                                                    
                                                               
(define-for-syntax go-syntax-protect                           
  (let ([insp (variable-reference->module-declaration-inspector
               (#%variable-reference))])                       
    (lambda (stx) (syntax-arm stx insp))))                     
                                                               
(define-syntax (def-go stx)                                    
  (syntax-case stx ()                                          
    [(_ go)                                                    
     (protect-syntax                                           
      #'(define-syntax (go stx)                                
          (syntax-case stx ()                                  
           [(_ x)                                              
            (go-syntax-protect #'(unchecked-go 8 x))])))]))    
```

##### 16.2.7.3. Protected Exports

Sometimes, a module needs to export bindings to some modules—other
modules that are at the same trust level as the exporting module—but
prevent access from untrusted modules. Such exports should use the
`protect-out` form in `provide`. For example, `ffi/unsafe` exports all
of its unsafe bindings as _protected_ in this sense.

Code inspectors, again, provide the mechanism for determining which
modules are trusted and which are untrusted. When a module is declared,
the value of `current-code-inspector` is associated to the module
declaration. When a module is instantiated \(i.e., when the body of the
declaration is actually executed\), a sub-inspector is created to guard
the module’s exports. Access to the module’s protected exports requires
a code inspector higher in the inspector hierarchy than the module’s
instantiation inspector; note that a module’s declaration inspector is
always higher than its instantiation inspector, so modules are declared
with the same code inspector can access each other’s exports.

Syntax-object constants within a module, such as literal identifiers in
a template, retain the inspector of their source module. In this way, a
macro from a trusted module can be used within an untrusted module, and
protected identifiers in the macro expansion still work, even through
they ultimately appear in an untrusted module. Naturally, such
identifiers should be armed, so that they cannot be extracted from the
macro expansion and abused by untrusted code.

Compiled code from a `".zo"` file is inherently untrustworthy,
unfortunately, since it can be synthesized by means other than
`compile`. When compiled code is written to a `".zo"` file,
syntax-object constants within the compiled code lose their inspectors.
All syntax-object constants within compiled code acquire the enclosing
module’s declaration-time inspector when the code is loaded.

### 16.3. Module Instantiations and Visits

Modules often contain just function and structure-type definitions, in
which case the module itself behaves in a purely functional way, and the
time when the functions are created is not observable. If a module’s
top-level expressions include side effects, however, then the timing of
the effects can matter. The distinction between module declaration and
instantiation provides some control over that timing. The concept of
module visits further explains the interaction of effects with macro
implementations.

#### 16.3.1. Declaration versus Instantiation

Declaring a module does not immediately evaluate expressions in the
module’s body. For example, evaluating

```racket
> (module number-n racket/base
    (provide n)               
    (define n (random 10))    
    (printf "picked ~a\n" n)) 
```

declares the module `number-n`, but it doesn’t immediately pick a random
number for `n` or display the number. A `require` of `number-n` causes
the module to be _instantiated_ \(i.e., it triggers an
_instantiation_\), which implies that the expressions in the body of the
module are evaluated:

```racket
> (require 'number-n)
picked 5             
> n                  
5                    
```

After a module is instantiated in a particular namespace, further
`require`s of the module use the same instance, as opposed to
instantiating the module again:

```racket
> (require 'number-n)       
> n                         
5                           
> (module use-n racket/base 
    (require 'number-n)     
    (printf "still ~a\n" n))
> (require 'use-n)          
still 5                     
```

The `dynamic-require` function, like `require`, triggers instantion of a
module if it is not already instantiated, so `dynamic-require` with `#f`
as a second argument is useful to just trigger the instantion effects of
a module:

```racket
> (module use-n-again racket/base   
    (require 'number-n)             
    (printf "also still ~a\n" n))   
> (dynamic-require ''use-n-again #f)
also still 5                        
```

Instantiation of modules by `require` is transitive. That is, if
`require` of a module instantiates it, then any module `require`d by
that one is also instantiated \(if it’s not instantiated already\):

```racket
> (module number-m racket/base
    (provide m)               
    (define m (random 10))    
    (printf "picked ~a\n" m)) 
> (module use-m racket/base   
    (require 'number-m)       
    (printf "still ~a\n" m))  
> (require 'use-m)            
picked 0                      
still 0                       
```

#### 16.3.2. Compile-Time Instantiation

In the same way that declaring a module does not by itself instantiate a
module, declaring a module that `require`s another module does not by
itself instantiate the `require`d module, as illustrated in the
preceding example. However, declaring a module _does_ expand and compile
the module. If a module imports another with `(require (for-syntax
....))`, then module that is imported `for-syntax` must be instantiated
during expansion:

```racket
> (module number-p racket/base               
    (provide p)                              
    (define p (random 10))                   
    (printf "picked ~a\n" p))                
> (module use-p-at-compile-time racket/base  
    (require (for-syntax racket/base         
                         'number-p))         
    (define-syntax (pm stx)                  
      #`#,p)                                 
    (printf "was ~a at compile time\n" (pm)))
picked 1                                     
```

Unlike run-time instantiation in a namespace, when a module is used
`for-syntax` for another module expansion in the same namespace, the
`for-syntax`ed module is instantiated separately for each expansion.
Continuing the previous example, if `number-p` is used a second time
`for-syntax`, then a second random number is selected for a new `p`:

```racket
> (module use-p-again-at-compile-time racket/base   
    (require (for-syntax racket/base                
                         'number-p))                
    (define-syntax (pm stx)                         
      #`#,p)                                        
    (printf "was ~a at second compile time\n" (pm)))
picked 3                                            
```

Separate compile-time instantiations of `number-p` helps prevent
accidental propagation of effects from one module’s compilation to
another module’s compilation. Preventing those effects make compilation
reliably separate and more deterministic.

The expanded forms of `use-p-at-compile-time` and
`use-p-again-at-compile-time` record the number that was selected each
time, so those two different numbers are printed when the modules are
instantiated:

```racket
> (dynamic-require ''use-p-at-compile-time #f)      
was 1 at compile time                               
> (dynamic-require ''use-p-again-at-compile-time #f)
was 3 at second compile time                        
```

A namespace’s top level behaves like a separate module, where multiple
interactions in the top level conceptually extend a single expansion of
the module. So, when using `(require (for-syntax ....))` twice in the
top level, the second use does not trigger a new compile-time instance:

```racket
> (begin (require (for-syntax 'number-p)) 'done)      
picked 4                                              
'done                                                 
> (begin (require (for-syntax 'number-p)) 'done-again)
'done-again                                           
```

However, a run-time instance of a module is kept separate from all
compile-time instances, including at the top level, so a
non-`for-syntax` use of `number-p` will pick another random number:

```racket
> (require 'number-p)
picked 5             
```

#### 16.3.3. Visiting Modules

When a module `provide`s a macro for use by other modules, the other
modules use the macro by directly `require`ing the macro provider—i.e.,
without `for-syntax`. That’s because the macro is being imported for use
in a run-time position \(even though the macro’s implementation lives at
compile time\), while `for-syntax` would import a binding for use in
compile-time position.

The module implementing a macro, meanwhile, might `require` another
module `for-syntax` to implement the macro. The `for-syntax` module
needs a compile-time instantiation during any module expansion that
might use the macro. That requirement sets up a kind of transitivity
through `require` that is similar to instantiation transitivity, but
“off by one” at the point where the `for-syntax` shift occurs in the
chain.

Here’s an example to make that scenario concrete:

```racket
> (module number-q racket/base                      
    (provide q)                                     
    (define q (random 10))                          
    (printf "picked ~a\n" q))                       
> (module use-q-at-compile-time racket/base         
    (require (for-syntax racket/base                
                         'number-q))                
    (provide qm)                                    
    (define-syntax (qm stx)                         
      #`#,q)                                        
    (printf "was ~a at compile time\n" (qm)))       
picked 7                                            
> (module use-qm racket/base                        
    (require 'use-q-at-compile-time)                
    (printf "was ~a at second compile time\n" (qm)))
picked 4                                            
> (dynamic-require ''use-qm #f)                     
was 7 at compile time                               
was 4 at second compile time                        
```

In this example, when `use-q-at-compile-time` is expanded and compiled,
`number-q` is instantiated once. In this case, that instantion is needed
to expand the `(qm)` macro, but the module system would proactively
create a compile-time instantiation of `number-q` even if the `qm` macro
turned out not to be used.

Then, as `use-qm` is expanded and compiled, a second compile-time
instantiation of `number-q` is created. That compile-time instantion is
needed to expand the `(qm)` form within `use-qm`.

Instantiating `use-qm` correctly reports the number that was picked
during that second module’s compilation. First, though, the `require` of
`use-q-at-compile-time` in `use-qm` triggers a transitive instantiation
of `use-q-at-compile-time`, which correctly reports the number that was
picked in its compilation.

Overall, the example illustrates a transitive effect of `require` that
we had already seen:

* When a module is instantiated, the run-time expressions      in its
  body are evaluated.

* When a module is instantiated, then any module that it `require`s
  \(without `for-syntax`\) is also instantiated.

This rule does not explain the compile-time instantiations of
`number-q`, however. To explain that, we need a new word, _visit_, for
the concept that we saw in Compile-Time Instantiation:

* When a module is visited, the compile-time expressions      \(such as
  macro definition\) in its body are evaluated.

* As a module is expanded, it is visited.

* When a module is visited, then any module that it `require`s
  \(without `for-syntax`\) is also visited.

* When a module is visited, then any module that it `require`s
  `for-syntax` is instantiated at compile time.

Note that when visiting one module causes a compile-time instantion of
another module, the transitiveness of instantiated through regular
`require`s can trigger more compile-time instantiations. Instantiation
itself won’t trigger further visits, however, because any instantiated
module has already been expanded and compiled.

The compile-time expressions of a module that are evaluated by visiting
include both the right-hand sides of `define-syntax` forms and the body
of `begin-for-syntax` forms. That’s why a randomly selected number is
printed immediately in the following example:

```racket
> (module compile-time-number racket/base
    (require (for-syntax racket/base))   
    (begin-for-syntax                    
      (printf "picked ~a\n" (random)))   
    (printf "running\n"))                
picked 0.25549265186825576               
```

Instantiating the module evaluates only the run-time expressions, which
prints “running” but not a new random number:

```racket
> (dynamic-require ''compile-time-number #f)
running                                     
```

The description of instantiates and visit above is phrased in terms of
normal `require`s and `for-syntax` `require`s, but a more precise
specification is in terms of module phases. For example, if module `A`
has `(require (for-syntax B))` and module `B` has `(require
(for-template C))`, then module `C` is instantiated when module `A` is
instantiated, because the `for-syntax` and `for-template` shifts cancel.
We have not yet specified what happens with `for-meta 2` for when
`for-syntax`es combine; we leave that to the next section, Lazy Visits
via Available Modules.

If you think of the top-level as a kind of module that is continuously
expanded, the above rules imply that `require` of another module at the
top level both instantiates and visits the other module \(if it is not
already instantiated and visited\). That’s roughly true, but the visit
is made lazy in a way that is also explained in the next section, Lazy
Visits via Available Modules.

Meanwhile, `dynamic-require` only instantiates a module; it does not
visit the module. That simplification is why some of the preceding
examples use `dynamic-require` instead of `require`. The extra visits of
a top-level `require` would make the earlier examples less clear.

#### 16.3.4. Lazy Visits via Available Modules

A top-level `require` of a module does not actually visit the module.
Instead, it makes the module _available_. An available module will be
visited when a future expression needs to be expanded in the same
context. The next expression may or may not involve some imported macro
that needs its compile-time helpers evaluated by visiting, but the
module system proactively visits the module, just in case.

In the following example, a random number is picked as a result of
visiting a module’s own body while that module is being expanded. A
`require` of the module instantiates it, printing “running”, and also
makes the module available. Evaluating any other expression implies
expanding the expression, and that expansion triggers a visit of the
available module—which picks another random number:

```racket
> (module another-compile-time-number racket/base
    (require (for-syntax racket/base))           
    (begin-for-syntax                            
      (printf "picked ~a\n" (random)))           
    (printf "running\n"))                        
picked 0.3634379786893492                        
> (require 'another-compile-time-number)         
running                                          
> 'next                                          
picked 0.5057086679589476                        
'next                                            
> 'another                                       
'another                                         
```

> Beware that the expander flattens the content of a top-level `begin`
> into the top level as soon as the `begin` is discovered. So, `(begin
> (require 'another-compile-time-number) 'next)` would still have printed
> “picked” before “next“.

The final evaluation of `'another` also visits any available modules,
but no modules were made newly available by simply evaluating `'next`.

When a module `require`s another module using `for-meta n` for some `n`
greater than 1, the `require`d module is made available at phase `n`. A
module that is available at phase `n` is visited when some expression at
phase `n`_-_1__ is expanded.

To help illustrate, the following examples use
`(variable-reference->module-base-phase (#%variable-reference))`, which
returns a number for the phase at which the enclosing module is
instantiated:

```racket
> (module show-phase racket/base                                            
    (printf "running at ~a\n"                                               
            (variable-reference->module-base-phase (#%variable-reference))))
> (require 'show-phase)                                                     
running at 0                                                                
> (module use-at-phase-1 racket/base                                        
    (require (for-syntax 'show-phase)))                                     
running at 1                                                                
> (module unused-at-phase-2 racket/base                                     
    (require (for-meta 2 'show-phase)))                                     
```

For the last module above, `show-phase` is made available at phase 2,
but no expressions within the module are ever expanded at phase 1, so
there’s no phase-2 printout. The following module includes a phase-1
expression after the phase-2 `require`, so there’s a printout:

```racket
> (module use-at-phase-2 racket/base  
    (require (for-meta 2 'show-phase) 
             (for-syntax racket/base))
    (define-syntax x 'ok))            
running at 2                          
```

If we `require` the module `use-at-phase-1` at the top level, then
`show-phase` is made available at phase 1. Evaluating another expression
causes `use-at-phase-1` to be visited, which in turn instantitates
`show-phase`:

```racket
> (require 'use-at-phase-1)
> 'next                    
running at 1               
'next                      
```

A `require` of `use-at-phase-2` is similar, except that `show-phase` is
made available at phase 2, so it is not instantiated until some
expression is expanded at phase 1:

```racket
> (require 'use-at-phase-2)            
> 'next                                
'next                                  
> (require (for-syntax racket/base))   
> (begin-for-syntax 'compile-time-next)
running at 2                           
```

## 17. Creating Languages

The macro facilities defined in the preceding chapter let a programmer
define syntactic extensions to a language, but a macro is limited in two
ways:

* a macro cannot restrict the syntax available in its context or change
  the meaning of surrounding forms; and

* a macro can extend the syntax of a language only within the parameters
  of the language’s lexical conventions, such as using parentheses to
  group the macro name with its subforms and using the core syntax of
  identifiers, keywords, and literals.

> +The distinction between the reader and expander layer is introduced in
> Lists and Racket Syntax.

That is, a macro can only extend a language, and it can do so only at
the expander layer. Racket offers additional facilities for defining a
starting point of the expander layer, for extending the reader layer,
for defining the starting point of the reader layer, and for packaging a
reader and expander starting point into a conveniently named language.

    17.1 Module Languages                            
      17.1.1 Implicit Form Bindings                  
      17.1.2 Using `#lang s-exp`                     
    17.2 Reader Extensions                           
      17.2.1 Source Locations                        
      17.2.2 Readtables                              
    17.3 Defining new `#lang` Languages              
      17.3.1 Designating a `#lang` Language          
      17.3.2 Using `#lang reader`                    
      17.3.3 Using `#lang s-exp syntax/module-reader`
      17.3.4 Installing a Language                   
      17.3.5 Source-Handling Configuration           
      17.3.6 Module-Handling Configuration           

### 17.1. Module Languages

When using the longhand `module` form for writing modules, the module
path that is specified after the new module’s name provides the initial
imports for the module. Since the initial-import module determines even
the most basic bindings that are available in a module’s body, such as
`require`, the initial import can be called a _module language_.

The most common module languages are `racket` or `racket/base`, but you
can define your own module language by defining a suitable module. For
example, using `provide` subforms like `all-from-out`, `except-out`, and
`rename-out`, you can add, remove, or rename bindings from `racket` to
produce a module language that is a variant of `racket`:

> +The `module` Form introduces the longhand `module` form.

```racket
> (module raquet racket                                    
    (provide (except-out (all-from-out racket) lambda)     
             (rename-out [lambda function])))              
> (module score 'raquet                                    
    (map (function (points) (case points                   
                             [(0) "love"] [(1) "fifteen"]  
                             [(2) "thirty"] [(3) "forty"]))
         (list 0 2)))                                      
> (require 'score)                                         
'("love" "thirty")                                         
```

#### 17.1.1. Implicit Form Bindings

If you try to remove too much from `racket` in defining your own module
language, then the resulting module will no longer work right as a
module language:

```racket
> (module just-lambda racket                               
    (provide lambda))                                      
> (module identity 'just-lambda                            
    (lambda (x) x))                                        
eval:2:0: module: no #%module-begin binding in the module's
language                                                   
  in: (module identity (quote just-lambda) (lambda (x) x)) 
```

The `#%module-begin` form is an implicit form that wraps the body of a
module. It must be provided by a module that is to be used as module
language:

```racket
> (module just-lambda racket        
    (provide lambda #%module-begin))
> (module identity 'just-lambda     
    (lambda (x) x))                 
> (require 'identity)               
#<procedure>                        
```

The other implicit forms provided by `racket/base` are `#%app` for
function calls, `#%datum` for literals, and `#%top` for identifiers that
have no binding:

```racket
> (module just-lambda racket        
    (provide lambda #%module-begin  
             ; ten needs these, too:
             #%app #%datum))        
> (module ten 'just-lambda          
    ((lambda (x) x) 10))            
> (require 'ten)                    
10                                  
```

Implicit forms such as `#%app` can be used explicitly in a module, but
they exist mainly to allow a module language to restrict or change the
meaning of implicit uses. For example, a `lambda-calculus` module
language might restrict functions to a single argument, restrict
function calls to supply a single argument, restrict the module body to
a single expression, disallow literals, and treat unbound identifiers as
uninterpreted symbols:

```racket
> (module lambda-calculus racket                             
    (provide (rename-out [1-arg-lambda lambda]               
                         [1-arg-app #%app]                   
                         [1-form-module-begin #%module-begin]
                         [no-literals #%datum]               
                         [unbound-as-quoted #%top]))         
    (define-syntax-rule (1-arg-lambda (x) expr)              
      (lambda (x) expr))                                     
    (define-syntax-rule (1-arg-app e1 e2)                    
      (#%app e1 e2))                                         
    (define-syntax-rule (1-form-module-begin e)              
      (#%module-begin e))                                    
    (define-syntax (no-literals stx)                         
      (raise-syntax-error #f "no" stx))                      
    (define-syntax-rule (unbound-as-quoted . id)             
      'id))                                                  
> (module ok 'lambda-calculus                                
    ((lambda (x) (x z))                                      
     (lambda (y) y)))                                        
> (require 'ok)                                              
'z                                                           
> (module not-ok 'lambda-calculus                            
    (lambda (x y) x))                                        
eval:4:0: lambda: use does not match pattern: (lambda (x)    
expr)                                                        
  in: (lambda (x y) x)                                       
> (module not-ok 'lambda-calculus                            
    (lambda (x) x)                                           
    (lambda (y) (y y)))                                      
eval:5:0: #%module-begin: use does not match pattern:        
(#%module-begin e)                                           
  in: (#%module-begin (lambda (x) x) (lambda (y) (y y)))     
> (module not-ok 'lambda-calculus                            
    (lambda (x) (x x x)))                                    
eval:6:0: #%app: use does not match pattern: (#%app e1 e2)   
  in: (#%app x x x)                                          
> (module not-ok 'lambda-calculus                            
    10)                                                      
eval:7:0: #%datum: no                                        
  in: (#%datum . 10)                                         
```

Module languages rarely redefine `#%app`, `#%datum`, and `#%top`, but
redefining `#%module-begin` is more frequently useful. For example, when
using modules to construct descriptions of HTML pages where a
description is exported from the module as `page`, an alternate
`#%module-begin` can help eliminate `provide` and quasiquoting
boilerplate, as in `"html.rkt"`:

`"html.rkt"`
```racket
#lang racket                                          
(require racket/date)                                 
                                                      
(provide (except-out (all-from-out racket)            
                     #%module-begin)                  
         (rename-out [module-begin #%module-begin])   
         now)                                         
                                                      
(define-syntax-rule (module-begin expr ...)           
  (#%module-begin                                     
   (define page `(html expr ...))                     
   (provide page)))                                   
                                                      
(define (now)                                         
  (parameterize ([date-display-format 'iso-8601])     
    (date->string (seconds->date (current-seconds)))))
```

Using the `"html.rkt"` module language, a simple web page can be
described without having to explicitly define or export `page` and
starting in `quasiquote`d mode instead of expression mode:

```racket
> (module lady-with-the-spinning-head "html.rkt"                
    (title "Queen of Diamonds")                                 
    (p "Updated: " ,(now)))                                     
> (require 'lady-with-the-spinning-head)                        
> page                                                          
'(html (title "Queen of Diamonds") (p "Updated: " "2019-01-21"))
```

#### 17.1.2. Using `#lang`` ``s-exp`

Implementing a language at the level of `#lang` is more complex than
declaring a single module, because `#lang` lets programmers control
several different facets of a language. The `s-exp` language, however,
acts as a kind of meta-language for using a module language with the
`#lang` shorthand:

```racket
#lang s-exp module-name
form ...               
```

is the same as

```racket
(module name module-name
  form ...)             
```

where `name` is derived from the source file containing the `#lang`
program. The name `s-exp` is short for “S-expression,” which is a
traditional name for Racket’s reader-level lexical conventions:
parentheses, identifiers, numbers, double-quoted strings with certain
backslash escapes, and so on.

Using `#lang s-exp`, the `lady-with-the-spinning-head` example from
before can be written more compactly as:

```racket
#lang s-exp "html.rkt"     
                           
(title "Queen of Diamonds")
(p "Updated: " ,(now))     
```

Later in this guide, Defining new `#lang` Languages explains how to
define your own `#lang` language, but first we explain how you can write
reader-level extensions to Racket.

### 17.2. Reader Extensions

> +\[missing\] in \[missing\] provides more on reader extensions.

The reader layer of the Racket language can be extended through the
`#reader` form. A reader extension is implemented as a module that is
named after `#reader`. The module exports functions that parse raw
characters into a form to be consumed by the expander layer.

The syntax of `#reader` is

`#reader` >_module-path_< >_reader-specific_<

where >_module-path_< names a module that provides `read` and
`read-syntax` functions. The >_reader-specific_< part is a sequence of
characters that is parsed as determined by the `read` and `read-syntax`
functions from >_module-path_<.

For example, suppose that file `"five.rkt"` contains

`"five.rkt"`
```racket
#lang racket/base                                      
                                                       
(provide read read-syntax)                             
                                                       
(define (read in) (list (read-string 5 in)))           
(define (read-syntax src in) (list (read-string 5 in)))
```

Then, the program

```racket
#lang racket/base             
                              
'(1 #reader"five.rkt"234567 8)
```

is equivalent to

```racket
#lang racket/base 
                  
'(1 ("23456") 7 8)
```

because the `read` and `read-syntax` functions of `"five.rkt"` both read
five characters from the input stream and put them into a string and
then a list. The reader functions from `"five.rkt"` are not obliged to
follow Racket lexical conventions and treat the continuous sequence
`234567` as a single number. Since only the `23456` part is consumed by
`read` or `read-syntax`, the `7` remains to be parsed in the usual
Racket way. Similarly, the reader functions from `"five.rkt"` are not
obliged to ignore whitespace, and

```racket
#lang racket/base              
                               
'(1 #reader"five.rkt" 234567 8)
```

is equivalent to

```racket
#lang racket/base  
                   
'(1 (" 2345") 67 8)
```

since the first character immediately after `"five.rkt"` is a space.

A `#reader` form can be used in the REPL, too:

```racket
> '#reader"five.rkt"abcde
'("abcde")               
```

#### 17.2.1. Source Locations

The difference between `read` and `read-syntax` is that `read` is meant
to be used for data while `read-syntax` is meant to be used to parse
programs. More precisely, the `read` function will be used when the
enclosing stream is being parsed by the Racket `read`, and `read-syntax`
is used when the enclosing stream is being parsed by the Racket
`read-syntax` function. Nothing requires `read` and `read-syntax` to
parse input in the same way, but making them different would confuse
programmers and tools.

The `read-syntax` function can return the same kind of value as `read`,
but it should normally return a syntax object that connects the parsed
expression with source locations. Unlike the `"five.rkt"` example, the
`read-syntax` function is typically implemented directly to produce
syntax objects, and then `read` can use `read-syntax` and strip away
syntax object wrappers to produce a raw result.

The following `"arith.rkt"` module implements a reader to parse simple
infix arithmetic expressions into Racket forms. For example, `1*2+3`
parses into the Racket form `(+ (* 1 2) 3)`. The supported operators are
`+`, `-`, `*`, and `/`, while operands can be unsigned integers or
single-letter variables. The implementation uses `port-next-location` to
obtain the current source location, and it uses `datum->syntax` to turn
raw values into syntax objects.

`"arith.rkt"`
```racket
#lang racket                                                
(require syntax/readerr)                                    
                                                            
(provide read read-syntax)                                  
                                                            
(define (read in)                                           
  (syntax->datum (read-syntax #f in)))                      
                                                            
(define (read-syntax src in)                                
  (skip-whitespace in)                                      
  (read-arith src in))                                      
                                                            
(define (skip-whitespace in)                                
  (regexp-match #px"^\\s*" in))                             
                                                            
(define (read-arith src in)                                 
  (define-values (line col pos) (port-next-location in))    
  (define expr-match                                        
    (regexp-match                                           
     ; Match an operand followed by any number of           
     ; operator–operand sequences, and prohibit an          
     ; additional operator from following immediately:      
     #px"^([a-z]|[0-9]+)(?:[-+*/]([a-z]|[0-9]+))*(?![-+*/])"
     in))                                                   
                                                            
  (define (to-syntax v delta span-str)                      
    (datum->syntax #f v (make-srcloc delta span-str)))      
  (define (make-srcloc delta span-str)                      
    (and line                                               
         (vector src line (+ col delta) (+ pos delta)       
                 (string-length span-str))))                
                                                            
  (define (parse-expr s delta)                              
    (match (or (regexp-match #rx"^(.*?)([+-])(.*)$" s)      
               (regexp-match #rx"^(.*?)([*/])(.*)$" s))     
      [(list _ a-str op-str b-str)                          
       (define a-len (string-length a-str))                 
       (define a (parse-expr a-str delta))                  
       (define b (parse-expr b-str (+ delta 1 a-len)))      
       (define op (to-syntax (string->symbol op-str)        
                             (+ delta a-len) op-str))       
       (to-syntax (list op a b) delta s)]                   
      [_ (to-syntax (or (string->number s)                  
                        (string->symbol s))                 
                    delta s)]))                             
                                                            
  (unless expr-match                                        
    (raise-read-error "bad arithmetic syntax"               
                      src line col pos                      
                      (and pos (- (file-position in) pos))))
  (parse-expr (bytes->string/utf-8 (car expr-match)) 0))    
```

If the `"arith.rkt"` reader is used in an expression position, then its
parse result will be treated as a Racket expression. If it is used in a
quoted form, however, then it just produces a number or a list:

```racket
> #reader"arith.rkt" 1*2+3 
5                          
> '#reader"arith.rkt" 1*2+3
'(+ (* 1 2) 3)             
```

The `"arith.rkt"` reader could also be used in positions that make no
sense. Since the `read-syntax` implementation tracks source locations,
syntax errors can at least refer to parts of the input in terms of their
original locations \(at the beginning of the error message\):

```racket
> (let #reader"arith.rkt" 1*2+3 8)                          
repl:1:27: let: bad syntax (not an identifier and expression
for a binding)                                              
  at: +                                                     
  in: (let (+ (* 1 2) 3) 8)                                 
```

#### 17.2.2. Readtables

A reader extension’s ability to parse input characters in an arbitrary
way can be powerful, but many cases of lexical extension call for a less
general but more composable approach. In much the same way that the
expander level of Racket syntax can be extended through macros, the
reader level of Racket syntax can be composably extended through a
_readtable_.

The Racket reader is a recursive-descent parser, and the readtable maps
characters to parsing handlers. For example, the default readtable maps
`(` to a handler that recursively parses subforms until it finds a `)`.
The `current-readtable` parameter determines the readtable that is used
by `read` or `read-syntax`. Rather than parsing raw characters directly,
a reader extension can install an extended readtable and then chain to
`read` or `read-syntax`.

> +See Dynamic Binding: `parameterize` for an introduction to parameters.

The `make-readtable` function constructs a new readtable as an extension
of an existing one. It accepts a sequence of specifications in terms of
a character, a type of mapping for the character, and \(for certain
types of mappings\) a parsing procedure. For example, to extend the
readtable so that `$` can be used to start and end infix expressions,
implement a `read-dollar` function and use:

```racket
(make-readtable (current-readtable)                
                #\$ 'terminating-macro read-dollar)
```

The protocol for `read-dollar` requires the function to accept different
numbers of arguments depending on whether it is being used in `read` or
`read-syntax` mode. In `read` mode, the parser function is given two
arguments: the character that triggered the parser function and the
input port that is being read. In `read-syntax` mode, the function must
accept four additional arguments that provide the source location of the
character.

The following `"dollar.rkt"` module defines a `read-dollar` function in
terms of the `read` and `read-syntax` functions provided by
`"arith.rkt"`, and it puts `read-dollar` together with new `read` and
`read-syntax` functions that install the readtable and chain to Racket’s
`read` or `read-syntax`:

`"dollar.rkt"`
```racket
#lang racket                                            
(require syntax/readerr                                 
         (prefix-in arith: "arith.rkt"))                
                                                        
(provide (rename-out [$-read read]                      
                     [$-read-syntax read-syntax]))      
                                                        
(define ($-read in)                                     
  (parameterize ([current-readtable (make-$-readtable)])
    (read in)))                                         
                                                        
(define ($-read-syntax src in)                          
  (parameterize ([current-readtable (make-$-readtable)])
    (read-syntax src in)))                              
                                                        
(define (make-$-readtable)                              
  (make-readtable (current-readtable)                   
                  #\$ 'terminating-macro read-dollar))  
                                                        
(define read-dollar                                     
  (case-lambda                                          
   [(ch in)                                             
    (check-$-after (arith:read in) in (object-name in))]
   [(ch in src line col pos)                            
    (check-$-after (arith:read-syntax src in) in src)]))
                                                        
(define (check-$-after val in src)                      
  (regexp-match #px"^\\s*" in) ; skip whitespace        
  (let ([ch (peek-char in)])                            
    (unless (equal? ch #\$) (bad-ending ch src in))     
    (read-char in))                                     
  val)                                                  
                                                        
(define (bad-ending ch src in)                          
  (let-values ([(line col pos) (port-next-location in)])
    ((if (eof-object? ch)                               
         raise-read-error                               
         raise-read-eof-error)                          
     "expected a closing `$'"                           
     src line col pos                                   
     (if (eof-object? ch) 0 1))))                       
```

With this reader extension, a single `#reader` can be used at the
beginning of an expression to enable multiple uses of `$` that switch to
infix arithmetic:

```racket
> #reader"dollar.rkt" (let ([a $1*2+3$] [b $5/6$]) $a+b$)
35/6                                                     
```

### 17.3. Defining new `#lang` Languages

When loading a module as a source program that starts

`#lang` `language`

the `language` determines the way that the rest of the module is parsed
at the reader level. The reader-level parse must produce a `module` form
as a syntax object. As always, the second sub-form after `module`
specifies the module language that controls the meaning of the module’s
body forms. Thus, a `language` specified after `#lang` controls both the
reader-level and expander-level parsing of a module.

    17.3.1 Designating a `#lang` Language          
    17.3.2 Using `#lang reader`                    
    17.3.3 Using `#lang s-exp syntax/module-reader`
    17.3.4 Installing a Language                   
    17.3.5 Source-Handling Configuration           
    17.3.6 Module-Handling Configuration           

#### 17.3.1. Designating a `#lang` Language

The syntax of a `language` intentionally overlaps with the syntax of a
module path as used in `require` or as a module language, so that names
like `racket`, `racket/base`, `slideshow`, or `scribble/manual` can be
used both as `#lang` languages and as module paths.

At the same time, the syntax of `language` is far more restricted than a
module path, because only `a`-`z`, `A`-`Z`, `0`-`9`, `/` \(not at the
start or end\), `_`, `-`, and `+` are allowed in a `language` name.
These restrictions keep the syntax of `#lang` as simple as possible.
Keeping the syntax of `#lang` simple, in turn, is important because the
syntax is inherently inflexible and non-extensible; the `#lang` protocol
allows a `language` to refine and define syntax in a practically
unconstrained way, but the `#lang` protocol itself must remain fixed so
that various different tools can “boot” into the extended world.

Fortunately, the `#lang` protocol provides a natural way to refer to
languages in ways other than the rigid `language` syntax: by defining a
`language` that implements its own nested protocol. We have already seen
one example \(in Using `#lang s-exp`\): the `s-exp` `language` allows a
programmer to specify a module language using the general module path
syntax. Meanwhile, `s-exp` takes care of the reader-level
responsibilities of a `#lang` language.

Unlike `racket`, `s-exp` cannot be used as a module path with `require`.
Although the syntax of `language` for `#lang` overlaps with the syntax
of module paths, a `language` is not used directly as a module path.
Instead, a `language` obtains a module path by trying two locations:
first, it looks for a `reader` submodule of the main module for
`language`. If this is not a valid module path, then `language`  is
suffixed with `/lang/reader`. \(If neither is a valid module path, an
error is raised.\) The resulting module supplies `read` and
`read-syntax` functions using a protocol that is similar to the one for
`#reader`.

> +Reader Extensions introduces `#reader`.

A consequence of the way that a `#lang` `language` is turned into a
module path is that the language must be installed in a collection,
similar to the way that `"racket"` or `"slideshow"` are collections that
are distributed with Racket. Again, however, there’s an escape from this
restriction: the `reader` language lets you specify a reader-level
implementation of a language using a general module path.

#### 17.3.2. Using `#lang`` ``reader`

The `reader` language for `#lang` is similar to `s-exp`, in that it acts
as a kind of meta-language. Whereas `s-exp` lets a programmer specify a
module language at the expander layer of parsing, `reader` lets a
programmer specify a language at the reader level.

A `#lang reader` must be followed by a module path, and the specified
module must provide two functions: `read` and `read-syntax`. The
protocol is the same as for a `#reader` implementation, but for `#lang`,
the `read` and `read-syntax` functions must produce a `module` form that
is based on the rest of the input file for the module.

The following `"literal.rkt"` module implements a language that treats
its entire body as literal text and exports the text as a `data` string:

`"literal.rkt"`
```racket
#lang racket                                            
(require syntax/strip-context)                          
                                                        
(provide (rename-out [literal-read read]                
                     [literal-read-syntax read-syntax]))
                                                        
(define (literal-read in)                               
  (syntax->datum                                        
   (literal-read-syntax #f in)))                        
                                                        
(define (literal-read-syntax src in)                    
  (with-syntax ([str (port->string in)])                
    (strip-context                                      
     #'(module anything racket                          
         (provide data)                                 
         (define data 'str)))))                         
```

The `"literal.rkt"` language uses `strip-context` on the generated
`module` expression, because a `read-syntax` function should return a
syntax object with no lexical context. Also, the `"literal.rkt"`
language creates a module named `anything`, which is an arbitrary
choice; the language is intended to be used in a file, and the longhand
module name is ignored when it appears in a `require`d file.

The `"literal.rkt"` language can be used in a module `"tuvalu.rkt"`:

`"tuvalu.rkt"`
```racket
#lang reader "literal.rkt"
Technology!               
System!                   
Perfect!                  
```

Importing `"tuvalu.rkt"` binds `data` to a string version of the module
content:

```racket
> (require "tuvalu.rkt")            
> data                              
"\nTechnology!\nSystem!\nPerfect!\n"
```

#### 17.3.3. Using `#lang`` ``s-exp`` ``syntax/module-reader`

Parsing a module body is usually not as trivial as in `"literal.rkt"`. A
more typical module parser must iterate to parse multiple forms for a
module body. A language is also more likely to extend Racket
syntax—perhaps through a readtable—instead of replacing Racket syntax
completely.

The `syntax/module-reader` module language abstracts over common parts
of a language implementation to simplify the creation of new languages.
In its most basic form, a language implemented with
`syntax/module-reader` simply specifies the module language to be used
for the language, in which case the reader layer of the language is the
same as Racket. For example, with

`"raquet-mlang.rkt"`
```racket
#lang racket                                      
(provide (except-out (all-from-out racket) lambda)
         (rename-out [lambda function]))          
```

and

`"raquet.rkt"`
```racket
#lang s-exp syntax/module-reader
"raquet-mlang.rkt"              
```

then

```racket
#lang reader "raquet.rkt"         
(define identity (function (x) x))
(provide identity)                
```

implements and exports the `identity` function, since
`"raquet-mlang.rkt"` exports `lambda` as `function`.

The `syntax/module-reader` language accepts many optional specifications
to adjust other features of the language. For example, an alternate
`read` and `read-syntax` for parsing the language can be specified with
`#:read` and `#:read-syntax`, respectively. The following
`"dollar-racket.rkt"` language uses `"dollar.rkt"` \(see Readtables\) to
build a language that is like `racket` but with a `$` escape to simple
infix arithmetic:

`"dollar-racket.rkt"`
```racket
#lang s-exp syntax/module-reader     
racket                               
#:read $-read                        
#:read-syntax $-read-syntax          
                                     
(require (prefix-in $- "dollar.rkt"))
```

The `require` form appears at the end of the module, because all of the
keyword-tagged optional specifications for `syntax/module-reader` must
appear before any helper imports or definitions.

The following module uses `"dollar-racket.rkt"` to implement a `cost`
function using a `$` escape:

`"store.rkt"`
```racket
#lang reader "dollar-racket.rkt"        
                                        
(provide cost)                          
                                        
; Cost of ‘n' $1 rackets with 7% sales  
; tax and shipping-and-handling fee ‘h':
(define (cost n h)                      
  $n*107/100+h$)                        
```

#### 17.3.4. Installing a Language

So far, we have used the `reader` meta-language to access languages like
`"literal.rkt"` and `"dollar-racket.rkt"`. If you want to use something
like `#lang literal` directly, then you must move `"literal.rkt"` into a
Racket collection named `"literal"` \(see also Adding Collections\).
Specifically, move `"literal.rkt"` to a `reader` submodule of
`"literal/main.rkt"` for any directory name `"literal"`, like so:

`"literal/main.rkt"`
```racket
#lang racket                                              
                                                          
(module reader racket                                     
  (require syntax/strip-context)                          
                                                          
  (provide (rename-out [literal-read read]                
                       [literal-read-syntax read-syntax]))
                                                          
  (define (literal-read in)                               
    (syntax->datum                                        
     (literal-read-syntax #f in)))                        
                                                          
  (define (literal-read-syntax src in)                    
    (with-syntax ([str (port->string in)])                
      (strip-context                                      
       #'(module anything racket                          
           (provide data)                                 
           (define data 'str))))))                        
```

Then, install the `"literal"` directory as a package:

  `cd /path/to/literal ; raco pkg install`

After moving the file and installing the package, you can use `literal`
directly after `#lang`:

```racket
#lang literal
Technology!  
System!      
Perfect!     
```

> See \[missing\] for more information on using `raco`.

You can also make your language available for others to install by using
the Racket package manager \(see \[missing\]\). After you create a
`"literal"` package and register it with the Racket package catalog
\(see \[missing\]\), others can install it using `raco pkg`:

  `raco pkg install literal`

Once installed, others can invoke the language the same way: by using
`#lang literal` at the top of a source file.

If you use a public source repository \(e.g., GitHub\), you can link
your package to the source. As you improve the package, others can
update their version using `raco pkg`:

  `raco pkg update literal`

> See \[missing\] for more information about the Racket package manager.

#### 17.3.5. Source-Handling Configuration

The Racket distribution includes a Scribble language for writing prose
documents, where Scribble extends the normal Racket to better support
text. Here is an example Scribble document:

  `#lang scribble/base`                            
                                                   
  `@(define (get-name) "Self-Describing Document")`
                                                   
  `@title[(get-name)]`                             
                                                   
  `The title of this document is “@(get-name).”`   

If you put that program in DrRacket’s definitions area and click Run,
then nothing much appears to happen. The `scribble/base` language just
binds and exports `doc` as a description of a document, similar to the
way that `"literal.rkt"` exports a string as `data`.

Simply opening a module with the language `scribble/base` in DrRacket,
however, causes a Scribble HTML button to appear. Furthermore, DrRacket
knows how to colorize Scribble syntax by coloring green those parts of
the document that correspond to literal text. The language name
`scribble/base` is not hard-wired into DrRacket. Instead, the
implementation of the `scribble/base` language provides button and
syntax-coloring information in response to a query from DrRacket.

If you have installed the `literal` language as described in Installing
a Language, then you can adjust `"literal/main.rkt"` so that DrRacket
treats the content of a module in the `literal` language as plain text
instead of \(erroneously\) as Racket syntax:

`"literal/main.rkt"`
```racket
#lang racket                                             
                                                         
(module reader racket                                    
  (require syntax/strip-context)                         
                                                         
  (provide (rename-out [literal-read read]               
                       [literal-read-syntax read-syntax])
           get-info)                                     
                                                         
  (define (literal-read in)                              
    (syntax->datum                                       
     (literal-read-syntax #f in)))                       
                                                         
  (define (literal-read-syntax src in)                   
    (with-syntax ([str (port->string in)])               
      (strip-context                                     
       #'(module anything racket                         
           (provide data)                                
           (define data 'str)))))                        
                                                         
  (define (get-info in mod line col pos)                 
    (lambda (key default)                                
      (case key                                          
        [(color-lexer)                                   
         (dynamic-require 'syntax-color/default-lexer    
                          'default-lexer)]               
        [else default]))))                               
```

This revised `literal` implementation provides a `get-info` function.
The `get-info` function is called by `read-language` \(which DrRacket
calls\) with the source input stream and location information, in case
query results should depend on the content of the module after the
language name \(which is not the case for `literal`\). The result of
`get-info` is a function of two arguments. The first argument is always
a symbol, indicating the kind of information that a tool requests from
the language; the second argument is the default result to be returned
if the language does not recognize the query or has no information for
it.

After DrRacket obtains the result of `get-info` for a language, it calls
the function with a `'color-lexer` query; the result should be a
function that implements syntax-coloring parsing on an input stream. For
`literal`, the `syntax-color/default-lexer` module provides a
`default-lexer` syntax-coloring parser that is suitable for plain text,
so `literal` loads and returns that parser in response to a
`'color-lexer` query.

The set of symbols that a programming tool uses for queries is entirely
between the tool and the languages that choose to cooperate with it. For
example, in addition to `'color-lexer`, DrRacket uses a
`'drracket:toolbar-buttons` query to determine which buttons should be
available in the toolbar to operate on modules using the language.

The `syntax/module-reader` language lets you specify `get-info` handling
through a `#:info` optional specification. The protocol for an `#:info`
function is slightly different from the raw `get-info` protocol; the
revised protocol allows `syntax/module-reader` the possibility of
handling future language-information queries automatically.

#### 17.3.6. Module-Handling Configuration

Suppose that the file `"death-list-5.rkt"` contains

`"death-list-5.rkt"`
```racket
#lang racket         
(list "O-Ren Ishii"  
      "Vernita Green"
      "Budd"         
      "Elle Driver"  
      "Bill")        
```

If you `require` `"death-list-5.rkt"` directly, then it prints the list
in the usual Racket result format:

```racket
> (require "death-list-5.rkt")                              
'("O-Ren Ishii" "Vernita Green" "Budd" "Elle Driver" "Bill")
```

However, if `"death-list-5.rkt"` is required by a `"kiddo.rkt"` that is
implemented with `scheme` instead of `racket`:

`"kiddo.rkt"`
```racket
#lang scheme                
(require "death-list-5.rkt")
```

then, if you run `"kiddo.rkt"` file in DrRacket or if you run it
directly with `racket`, `"kiddo.rkt"` causes `"death-list-5.rkt"` to
print its list in traditional Scheme format, without the leading quote:

`("O-Ren Ishii" "Vernita Green" "Budd" "Elle Driver" "Bill")`

The `"kiddo.rkt"` example illustrates how the format for printing a
result value can depend on the main module of a program instead of the
language that is used to implement it.

More broadly, certain features of a language are only invoked when a
module written in that language is run directly with `racket` \(as
opposed to being imported into another module\). One example is
result-printing style \(as shown above\). Another example is REPL
behavior. These features are part of what’s called the _run-time
configuration_ of a language.

Unlike the syntax-coloring property of a language \(as described in
Source-Handling Configuration\), the run-time configuration is a
property of a _module_ per se as opposed to a property of the _source
text_ representing the module. For that reason, the run-time
configuration for a module needs to be available even if the module is
compiled to bytecode form and the source is unavailable. Therefore,
run-time configuration cannot be handled by the `get-info` function
we’re exporting from the language’s parser module.

Instead, it will be handled by a new `configure-runtime` submodule that
we’ll add inside the parsed `module` form. When a module is run directly
with `racket`, `racket` looks for a `configure-runtime` submodule. If it
exists, `racket` runs it. But if the module is imported into another
module, the `'configure-runtime` submodule is ignored. \(And if the
`configure-runtime` submodule doesn’t exist, `racket` just evaluates the
module as usual.\) That means that the `configure-runtime` submodule can
be used for any special setup tasks that need to happen when the module
is run directly.

Going back to the `literal` language \(see Source-Handling
Configuration\), we can adjust the language so that directly running a
`literal` module causes it to print out its string, while using a
`literal` module in a larger program simply provides `data` without
printing. To make this work, we will need an extra module. \(For clarity
here, we will implement this module as a separate file. But it could
equally well be a submodule of an existing file.\)

```racket
.... (the main installation or the user’s space)   
|- "literal"                                       
   |- "main.rkt"            (with reader submodule)
   |- "show.rkt"            (new)                  
```

* The `"literal/show.rkt"` module will provide a `show` function to be
  applied to the string content of a `literal` module, and also provide
  a `show-enabled` parameter that controls whether `show` actually
  prints the result.

* The new `configure-runtime` submodule in `"literal/main.rkt"` will set
  the `show-enabled` parameter to `#t`. The net effect is that `show`
  will print the strings that it’s given, but only when a module using
  the `literal` language is run directly \(because only then will the
  `configure-runtime` submodule be invoked\).

These changes are implemented in the following revised
`"literal/main.rkt"`:

`"literal/main.rkt"`
```racket
#lang racket                                             
                                                         
(module reader racket                                    
  (require syntax/strip-context)                         
                                                         
  (provide (rename-out [literal-read read]               
                       [literal-read-syntax read-syntax])
           get-info)                                     
                                                         
  (define (literal-read in)                              
    (syntax->datum                                       
     (literal-read-syntax #f in)))                       
                                                         
  (define (literal-read-syntax src in)                   
    (with-syntax ([str (port->string in)])               
      (strip-context                                     
       #'(module anything racket                         
           (module configure-runtime racket              
             (require literal/show)                      
             (show-enabled #t))                          
           (require literal/show)                        
           (provide data)                                
           (define data 'str)                            
           (show data)))))                               
                                                         
  (define (get-info in mod line col pos)                 
    (lambda (key default)                                
      (case key                                          
        [(color-lexer)                                   
         (dynamic-require 'syntax-color/default-lexer    
                          'default-lexer)]               
        [else default]))))                               
```

Then the `"literal/show.rkt"` module must provide the `show-enabled`
parameter and `show` function:

`"literal/show.rkt"`
```racket
#lang racket                             
                                         
(provide show show-enabled)              
                                         
(define show-enabled (make-parameter #f))
                                         
(define (show v)                         
  (when (show-enabled)                   
    (display v)))                        
```

With all of the pieces for `literal` in place, try running the following
variant of `"tuvalu.rkt"` directly and through a `require` from another
module:

`"tuvalu.rkt"`
```racket
#lang literal
Technology!  
System!      
Perfect!     
```

When run directly, we’ll see the result printed like so, because our
`configure-runtime` submodule will have set the `show-enabled` parameter
to `#t`:

`Technology!  System!  Perfect!`

But when imported into another module, printing will be suppressed,
because the `configure-runtime` submodule will not be invoked, and
therefore the `show-enabled` parameter will remain at its default value
of `#f`.

## 18. Concurrency and Synchronization

Racket provides _concurrency_ in the form of _threads_, and it provides
a general `sync` function that can be used to synchronize both threads
and other implicit forms of concurrency, such as ports.

Threads run concurrently in the sense that one thread can preempt
another without its cooperation, but threads do not run in parallel in
the sense of using multiple hardware processors.  See Parallelism for
information on parallelism in Racket.

### 18.1. Threads

To execute a procedure concurrently, use `thread`.  The following
example creates two new threads from the main thread:

```racket
(displayln "This is the original thread")                     
(thread (lambda () (displayln "This is a new thread.")))      
(thread (lambda () (displayln "This is another new thread.")))
```

The next example creates a new thread that would otherwise loop forever,
but the main thread uses `sleep` to pause itself for 2.5 seconds, then
uses `kill-thread` to terminate the worker thread:

```racket
(define worker (thread (lambda ()                  
                         (let loop ()              
                           (displayln "Working...")
                           (sleep 0.2)             
                           (loop)))))              
(sleep 2.5)                                        
(kill-thread worker)                               
```

> In DrRacket, the main thread keeps going until the Stop button is
> clicked, so in DrRacket the `thread-wait` is not necessary.

If the main thread finishes or is killed, the application exits, even if
other threads are still running.  A thread can use `thread-wait` to wait
for another thread to finish.  Here, the main thread uses `thread-wait`
to make sure the worker thread finishes before the main thread exits:

```racket
(define worker (thread                                    
                 (lambda ()                               
                   (for ([i 100])                         
                     (printf "Working hard... ~a~n" i)))))
(thread-wait worker)                                      
(displayln "Worker finished")                             
```

### 18.2. Thread Mailboxes

Each thread has a mailbox for receiving messages.  The `thread-send`
function asynchronously sends a message to another thread’s mailbox,
while `thread-receive` returns the oldest message from the current
thread’s mailbox, blocking to wait for a message if necessary.  In the
following example, the main thread sends data to the worker thread to be
processed, then sends a `'done` message when there is no more data and
waits for the worker thread to finish.

```racket
(define worker-thread (thread                               
                       (lambda ()                           
                         (let loop ()                       
                           (match (thread-receive)          
                             [(? number? num)               
                              (printf "Processing ~a~n" num)
                              (loop)]                       
                             ['done                         
                              (printf "Done~n")])))))       
(for ([i 20])                                               
  (thread-send worker-thread i))                            
(thread-send worker-thread 'done)                           
(thread-wait worker-thread)                                 
```

In the next example, the main thread delegates work to multiple
arithmetic threads, then waits to receive the results.  The arithmetic
threads process work items then send the results to the main thread.

```racket
(define (make-arithmetic-thread operation)                     
  (thread (lambda ()                                           
            (let loop ()                                       
              (match (thread-receive)                          
                [(list oper1 oper2 result-thread)              
                 (thread-send result-thread                    
                              (format "~a + ~a = ~a"           
                                      oper1                    
                                      oper2                    
                                      (operation oper1 oper2)))
                 (loop)])))))                                  
                                                               
(define addition-thread (make-arithmetic-thread +))            
(define subtraction-thread (make-arithmetic-thread -))         
                                                               
(define worklist '((+ 1 1) (+ 2 2) (- 3 2) (- 4 1)))           
(for ([item worklist])                                         
  (match item                                                  
    [(list '+ o1 o2)                                           
     (thread-send addition-thread                              
                  (list o1 o2 (current-thread)))]              
    [(list '- o1 o2)                                           
     (thread-send subtraction-thread                           
                  (list o1 o2 (current-thread)))]))            
                                                               
(for ([i (length worklist)])                                   
  (displayln (thread-receive)))                                
```

### 18.3. Semaphores

Semaphores facilitate synchronized access to an arbitrary shared
resource. Use semaphores when multiple threads must perform non-atomic
operations on a single resource.

In the following example, multiple threads print to standard output
concurrently.  Without synchronization, a line printed by one thread
might appear in the middle of a line printed by another thread.  By
using a semaphore initialized with a count of `1`, only one thread will
print at a time. The `semaphore-wait` function blocks until the
semaphore’s internal counter is non-zero, then decrements the counter
and returns. The `semaphore-post` function increments the counter so
that another thread can unblock and then print.

```racket
(define output-semaphore (make-semaphore 1))       
(define (make-thread name)                         
  (thread (lambda ()                               
            (for [(i 10)]                          
              (semaphore-wait output-semaphore)    
              (printf "thread ~a: ~a~n" name i)    
              (semaphore-post output-semaphore)))))
(define threads                                    
  (map make-thread '(A B C)))                      
(for-each thread-wait threads)                     
```

The pattern of waiting on a semaphore, working, and posting to the
semaphore can also be expressed using `call-with-semaphore`,which has
the advantage of posting to the semaphore if control escapes \(e.g., due
to an exception\):

```racket
(define output-semaphore (make-semaphore 1))           
(define (make-thread name)                             
  (thread (lambda ()                                   
            (for [(i 10)]                              
              (call-with-semaphore                     
               output-semaphore                        
               (lambda ()                              
                (printf "thread ~a: ~a~n" name i)))))))
(define threads                                        
  (map make-thread '(A B C)))                          
(for-each thread-wait threads)                         
```

Semaphores are a low-level technique.  Often, a better solution is to
restrict resource access to a single thread.  For example, synchronizing
access to standard output might be better accomplished by having a
dedicated thread for printing output.

### 18.4. Channels

Channels synchronize two threads while a value is passed from one thread
to the other.  Unlike a thread mailbox, multiple threads can get items
from a single channel, so channels should be used when multiple threads
need to consume items from a single work queue.

In the following example, the main thread adds items to a channel using
`channel-put`, while multiple worker threads consume those items using
`channel-get`.  Each call to either procedure blocks until another
thread calls the other procedure with the same channel.  The workers
process the items and then pass their results to the result thread via
the `result-channel`.

```racket
(define result-channel (make-channel))                      
(define result-thread                                       
        (thread (lambda ()                                  
                  (let loop ()                              
                    (displayln (channel-get result-channel))
                    (loop)))))                              
                                                            
(define work-channel (make-channel))                        
(define (make-worker thread-id)                             
  (thread                                                   
   (lambda ()                                               
     (let loop ()                                           
       (define item (channel-get work-channel))             
       (case item                                           
         [(DONE)                                            
          (channel-put result-channel                       
                       (format "Thread ~a done" thread-id))]
         [else                                              
          (channel-put result-channel                       
                       (format "Thread ~a processed ~a"     
                               thread-id                    
                               item))                       
          (loop)])))))                                      
(define work-threads (map make-worker '(1 2)))              
(for ([item '(A B C D E F G H DONE DONE)])                  
  (channel-put work-channel item))                          
(for-each thread-wait work-threads)                         
```

### 18.5. Buffered Asynchronous Channels

Buffered asynchronous channels are similar to the channels described
above, but the “put” operation of asynchronous channels does not
block—unless the given channel was created with a buffer limit and the
limit has been reached.  The asynchronous-put operation is therefore
somewhat similar to `thread-send`, but unlike thread mailboxes,
asynchronous channels allow multiple threads to consume items from a
single channel.

In the following example, the main thread adds items to the work
channel, which holds a maximum of three items at a time.  The worker
threads process items from this channel and then send results to the
print thread.

```racket
(require racket/async-channel)                                      
                                                                    
(define print-thread                                                
  (thread (lambda ()                                                
            (let loop ()                                            
              (displayln (thread-receive))                          
              (loop)))))                                            
(define (safer-printf . items)                                      
  (thread-send print-thread                                         
               (apply format items)))                               
                                                                    
(define work-channel (make-async-channel 3))                        
(define (make-worker-thread thread-id)                              
  (thread                                                           
   (lambda ()                                                       
     (let loop ()                                                   
       (define item (async-channel-get work-channel))               
       (safer-printf "Thread ~a processing item: ~a" thread-id item)
       (loop)))))                                                   
                                                                    
(for-each make-worker-thread '(1 2 3))                              
(for ([item '(a b c d e f g h i j k l m)])                          
  (async-channel-put work-channel item))                            
```

Note the above example lacks any synchronization to verify that all
items were processed.  If the main thread were to exit without such
synchronization, it is possible that the worker threads will not finish
processing some items or the print thread will not print all items.

### 18.6. Synchronizable Events and `sync`

There are other ways to synchronize threads.  The `sync` function allows
threads to coordinate via synchronizable events. Many values double as
events, allowing a uniform way to synchronize threads using different
types.  Examples of events include channels, ports, threads, and alarms.
This section builds up a number of examples that show how the
combination of events, threads, and `sync` \(along with recursive
functions\) allow you to implement arbitrarily sophisticated
communication protocols to coordinate concurrent parts of a program.

In the next example, a channel and an alarm are used as synchronizable
events. The workers `sync` on both so that they can process channel
items until the alarm is activated.  The channel items are processed,
and then results are sent back to the main thread.

```racket
(define main-thread (current-thread))                             
(define alarm (alarm-evt (+ 3000 (current-inexact-milliseconds))))
(define channel (make-channel))                                   
(define (make-worker-thread thread-id)                            
  (thread                                                         
   (lambda ()                                                     
     (define evt (sync channel alarm))                            
     (cond                                                        
       [(equal? evt alarm)                                        
        (thread-send main-thread 'alarm)]                         
       [else                                                      
        (thread-send main-thread                                  
                     (format "Thread ~a received ~a"              
                             thread-id                            
                             evt))]))))                           
(make-worker-thread 1)                                            
(make-worker-thread 2)                                            
(make-worker-thread 3)                                            
(channel-put channel 'A)                                          
(channel-put channel 'B)                                          
(let loop ()                                                      
  (match (thread-receive)                                         
    ['alarm                                                       
     (displayln "Done")]                                          
    [result                                                       
     (displayln result)                                           
     (loop)]))                                                    
```

The next example shows a function for use in a simple TCP echo server.
The function uses `sync/timeout` to synchronize on input from the given
port or a message in the thread’s mailbox.  The first argument to
`sync/timeout` specifies the maximum number of seconds it should wait on
the given events. The `read-line-evt` function returns an event that is
ready when a line of input is available in the given input port.  The
result of `thread-receive-evt` is ready when `thread-receive` would not
block.  In a real application, the messages received in the thread
mailbox could be used for control messages, etc.

```racket
(define (serve in-port out-port)                          
  (let loop []                                            
    (define evt (sync/timeout 2                           
                              (read-line-evt in-port 'any)
                              (thread-receive-evt)))      
    (cond                                                 
      [(not evt)                                          
       (displayln "Timed out, exiting")                   
       (tcp-abandon-port in-port)                         
       (tcp-abandon-port out-port)]                       
      [(string? evt)                                      
       (fprintf out-port "~a~n" evt)                      
       (flush-output out-port)                            
       (loop)]                                            
      [else                                               
       (printf "Received a message in mailbox: ~a~n"      
               (thread-receive))                          
       (loop)])))                                         
```

The `serve` function is used in the following example, which starts a
server thread and a client thread that communicate over TCP.  The client
prints three lines to the server, which echoes them back.  The client’s
`copy-port` call blocks until EOF is received.  The server times out
after two seconds, closing the ports, which allows `copy-port` to finish
and the client to exit.  The main thread uses `thread-wait` to wait for
the client thread to exit \(since, without `thread-wait`, the main
thread might exit before the other threads are finished\).

```racket
(define port-num 4321)                                                    
(define (start-server)                                                    
  (define listener (tcp-listen port-num))                                 
  (thread                                                                 
    (lambda ()                                                            
      (define-values [in-port out-port] (tcp-accept listener))            
      (serve in-port out-port))))                                         
                                                                          
(start-server)                                                            
                                                                          
(define client-thread                                                     
  (thread                                                                 
   (lambda ()                                                             
     (define-values [in-port out-port] (tcp-connect "localhost" port-num))
     (display "first\nsecond\nthird\n" out-port)                          
     (flush-output out-port)                                              
     ; copy-port will block until EOF is read from in-port                
     (copy-port in-port (current-output-port)))))                         
                                                                          
(thread-wait client-thread)                                               
```

Sometimes, you want to attach result behavior directly to the event
passed to `sync`.  In the following example, the worker thread
synchronizes on three channels, but each channel must be handled
differently.  Using `handle-evt` associates a callback with the given
event.  When `sync` selects the given event, it calls the callback to
generate the synchronization result, rather than using the event’s
normal synchronization result.  Since the event is handled in the
callback, there is no need to dispatch on the return value of `sync`.

```racket
(define add-channel (make-channel))                                      
(define multiply-channel (make-channel))                                 
(define append-channel (make-channel))                                   
                                                                         
(define (work)                                                           
  (let loop ()                                                           
    (sync (handle-evt add-channel                                        
                      (lambda (list-of-numbers)                          
                        (printf "Sum of ~a is ~a~n"                      
                                list-of-numbers                          
                                (apply + list-of-numbers))))             
          (handle-evt multiply-channel                                   
                      (lambda (list-of-numbers)                          
                        (printf "Product of ~a is ~a~n"                  
                                list-of-numbers                          
                                (apply * list-of-numbers))))             
          (handle-evt append-channel                                     
                      (lambda (list-of-strings)                          
                        (printf "Concatenation of ~s is ~s~n"            
                                list-of-strings                          
                                (apply string-append list-of-strings)))))
    (loop)))                                                             
                                                                         
(define worker (thread work))                                            
(channel-put add-channel '(1 2))                                         
(channel-put multiply-channel '(3 4))                                    
(channel-put multiply-channel '(5 6))                                    
(channel-put add-channel '(7 8))                                         
(channel-put append-channel '("a" "b"))                                  
```

The result of `handle-evt` invokes its callback in tail position with
respect to `sync`, so it is safe to use recursion as in the following
example.

```racket
(define control-channel (make-channel))                  
(define add-channel (make-channel))                      
(define subtract-channel (make-channel))                 
(define (work state)                                     
  (printf "Current state: ~a~n" state)                   
  (sync (handle-evt add-channel                          
                    (lambda (number)                     
                      (printf "Adding: ~a~n" number)     
                      (work (+ state number))))          
        (handle-evt subtract-channel                     
                    (lambda (number)                     
                      (printf "Subtracting: ~a~n" number)
                      (work (- state number))))          
        (handle-evt control-channel                      
                    (lambda (kill-message)               
                      (printf "Done~n")))))              
                                                         
(define worker (thread (lambda () (work 0))))            
(channel-put add-channel 2)                              
(channel-put subtract-channel 3)                         
(channel-put add-channel 4)                              
(channel-put add-channel 5)                              
(channel-put subtract-channel 1)                         
(channel-put control-channel 'done)                      
(thread-wait worker)                                     
```

The `wrap-evt` function is like `handle-evt`, except that its handler is
not called in tail position with respect to `sync`. At the same time,
`wrap-evt` disables break exceptions during its handler’s invocation.

### 18.7. Building Your Own Synchronization Patterns

Events also allow you to encode many different communication patterns
between multiple concurrent parts of a program. One common such pattern
is producer-consumer. Here is a way to implement on variation on it
using the above ideas. Generally speaking, these communication patterns
are implemented via a server loops that uses `sync` to wait for any of a
number of different possibilities to occur and then reacts them,
updating some local state.

```racket
(define/contract (produce x)                               
  (-> any/c void?)                                         
  (channel-put producer-chan x))                           
                                                           
(define/contract (consume)                                 
  (-> any/c)                                               
  (channel-get consumer-chan))                             
                                                           
; private state and server loop                            
                                                           
(define producer-chan (make-channel))                      
(define consumer-chan (make-channel))                      
(void                                                      
 (thread                                                   
  (λ ()                                                    
    ; the items variable holds the items that              
    ; have been produced but not yet consumed              
    (let loop ([items '()])                                
      (sync                                                
                                                           
       ; wait for production                               
       (handle-evt                                         
        producer-chan                                      
        (λ (i)                                             
          ; if that event was chosen,                      
          ; we add an item to our list                     
          ; and go back around the loop                    
          (loop (cons i items))))                          
                                                           
       ; wait for consumption, but only                    
       ; if we have something to produce                   
       (handle-evt                                         
        (if (null? items)                                  
            never-evt                                      
            (channel-put-evt consumer-chan (car items)))   
        (λ (_)                                             
          ; if that event was chosen,                      
          ; we know that the first item item               
          ; has been consumed; drop it and                 
          ; and go back around the loop                    
          (loop (cdr items)))))))))                        
                                                           
; an example (non-deterministic) interaction               
> (void                                                    
   (thread (λ () (sleep (/ (random 10) 100)) (produce 1))) 
   (thread (λ () (sleep (/ (random 10) 100)) (produce 2))))
> (list (consume) (consume))                               
'(1 2)                                                     
```

It is possible to build up more complex synchronization patterns. Here
is a silly example where we extend the producer consumer with an
operation to wait until at least a certain number of items have been
produced.

```racket
(define/contract (produce x)                                                
  (-> any/c void?)                                                          
  (channel-put producer-chan x))                                            
                                                                            
(define/contract (consume)                                                  
  (-> any/c)                                                                
  (channel-get consumer-chan))                                              
                                                                            
(define/contract (wait-at-least n)                                          
  (-> natural? void?)                                                       
  (define c (make-channel))                                                 
  ; we send a new channel over to the                                       
  ; main loop so that we can wait here                                      
  (channel-put wait-at-least-chan (cons n c))                               
  (channel-get c))                                                          
                                                                            
(define producer-chan (make-channel))                                       
(define consumer-chan (make-channel))                                       
(define wait-at-least-chan (make-channel))                                  
(void                                                                       
 (thread                                                                    
  (λ ()                                                                     
    (let loop ([items '()]                                                  
               [total-items-seen 0]                                         
               [waiters '()])                                               
      ; instead of waiting on just production/                              
      ; consumption now we wait to learn about                              
      ; threads that want to wait for a certain                             
      ; number of elements to be reached                                    
      (apply                                                                
       sync                                                                 
       (handle-evt                                                          
        producer-chan                                                       
        (λ (i) (loop (cons i items)                                         
                     (+ total-items-seen 1)                                 
                     waiters)))                                             
       (handle-evt                                                          
        (if (null? items)                                                   
            never-evt                                                       
            (channel-put-evt consumer-chan (car items)))                    
        (λ (_) (loop (cdr items) total-items-seen waiters)))                
                                                                            
       ; wait for threads that are interested                               
       ; the number of items produced                                       
       (handle-evt                                                          
        wait-at-least-chan                                                  
        (λ (waiter) (loop items total-items-seen (cons waiter waiters))))   
                                                                            
       ; for each thread that wants to wait,                                
       (for/list ([waiter (in-list waiters)])                               
         ; we check to see if there has been enough                         
         ; production                                                       
         (cond                                                              
           [(>= (car waiter) total-items-seen)                              
            ; if so, we send a mesage back on the channel                   
            ; and continue the loop without that item                       
            (handle-evt                                                     
             (channel-put-evt                                               
              (cdr waiter)                                                  
              (void))                                                       
             (λ (_) (loop items total-items-seen (remove waiter waiters))))]
           [else                                                            
            ; otherwise, we just ignore that one                            
            never-evt])))))))                                               
                                                                            
; an example (non-deterministic) interaction                                
> (define thds                                                              
    (for/list ([i (in-range 10)])                                           
      (thread (λ ()                                                         
                (produce i)                                                 
                (wait-at-least 10)                                          
                (display (format "~a -> ~a\n" i (consume)))))))             
> (for ([thd (in-list thds)])                                               
    (thread-wait thd))                                                      
0 -> 9                                                                      
2 -> 5                                                                      
3 -> 1                                                                      
4 -> 3                                                                      
6 -> 4                                                                      
1 -> 2                                                                      
7 -> 7                                                                      
8 -> 0                                                                      
9 -> 6                                                                      
5 -> 8                                                                      
```

## 19. Performance

Alan Perlis famously quipped “Lisp programmers know the value of
everything and the cost of nothing.” A Racket programmer knows, for
example, that a `lambda` anywhere in a program produces a value that is
closed over its lexical environment—but how much does allocating that
value cost? While most programmers have a reasonable grasp of the cost
of various operations and data structures at the machine level, the gap
between the Racket language model and the underlying computing machinery
can be quite large.

In this chapter, we narrow the gap by explaining details of the Racket
compiler and run-time system and how they affect the run-time and memory
performance of Racket code.

### 19.1. Performance in DrRacket

By default, DrRacket instruments programs for debugging, and debugging
instrumentation \(provided by the \[missing\] library\) can
significantly degrade performance for some programs. Even when debugging
is disabled through the Choose Language... dialog’s Show Details panel,
the Preserve stacktrace checkbox is clicked by default, which also
affects performance. Disabling debugging and stacktrace preservation
provides performance results that are more consistent with running in
plain `racket`.

Even so, DrRacket and programs developed within DrRacket use the same
Racket virtual machine, so garbage collection times \(see Memory
Management\) may be longer in DrRacket than when a program is run by
itself, and DrRacket threads may impede execution of program threads.
**For the most reliable timing results for a program, run in plain
`racket` instead of in the DrRacket development environment.**
Non-interactive mode should be used instead of the REPL to benefit from
the module system. See Modules and Performance for details.

### 19.2. The Bytecode and Just-in-Time \(JIT\) Compilers

Every definition or expression to be evaluated by Racket is compiled to
an internal bytecode format. In interactive mode, this compilation
occurs automatically and on-the-fly. Tools like `raco make` and `raco
setup` marshal compiled bytecode to a file, so that you do not have to
compile from source every time that you run a program. \(Most of the
time required to compile a file is actually in macro expansion;
generating bytecode from fully expanded code is relatively fast.\) See
Compilation and Configuration: `raco` for more information on generating
bytecode files.

The bytecode compiler applies all standard optimizations, such as
constant propagation, constant folding, inlining, and dead-code
elimination. For example, in an environment where `+` has its usual
binding, the expression `(let ([x 1] [y (lambda () 4)]) (+ 1 (y)))` is
compiled the same as the constant `5`.

On some platforms, bytecode is further compiled to native code via a
_just-in-time_ or _JIT_ compiler. The JIT compiler substantially speeds
programs that execute tight loops, arithmetic on small integers, and
arithmetic on inexact real numbers. Currently, JIT compilation is
supported for x86, x86\_64 \(a.k.a. AMD64\), ARM, and 32-bit PowerPC
processors. The JIT compiler can be disabled via the `eval-jit-enabled`
parameter or the `--no-jit`/`-j` command-line flag for `racket`.

The JIT compiler works incrementally as functions are applied, but the
JIT compiler makes only limited use of run-time information when
compiling procedures, since the code for a given module body or `lambda`
abstraction is compiled only once. The JIT’s granularity of compilation
is a single procedure body, not counting the bodies of any lexically
nested procedures. The overhead for JIT compilation is normally so small
that it is difficult to detect.

### 19.3. Modules and Performance

The module system aids optimization by helping to ensure that
identifiers have the usual bindings. That is, the `+` provided by
`racket/base` can be recognized by the compiler and inlined, which is
especially important for JIT-compiled code. In contrast, in a
traditional interactive Scheme system, the top-level `+` binding might
be redefined, so the compiler cannot assume a fixed `+` binding \(unless
special flags or declarations are used to compensate for the lack of a
module system\).

Even in the top-level environment, importing with `require` enables some
inlining optimizations. Although a `+` definition at the top level might
shadow an imported `+`, the shadowing definition applies only to
expressions evaluated later.

Within a module, inlining and constant-propagation optimizations take
additional advantage of the fact that definitions within a module cannot
be mutated when no `set!` is visible at compile time. Such optimizations
are unavailable in the top-level environment. Although this optimization
within modules is important for performance, it hinders some forms of
interactive development and exploration. The
`compile-enforce-module-constants` parameter disables the JIT compiler’s
assumptions about module definitions when interactive exploration is
more important. See Assignment and Redefinition for more information.

The compiler may inline functions or propagate constants across module
boundaries. To avoid generating too much code in the case of function
inlining, the compiler is conservative when choosing candidates for
cross-module inlining; see Function-Call Optimizations for information
on providing inlining hints to the compiler.

The later section `letrec` Performance provides some additional caveats
concerning inlining of module bindings.

### 19.4. Function-Call Optimizations

When the compiler detects a function call to an immediately visible
function, it generates more efficient code than for a generic call,
especially for tail calls. For example, given the program

```racket
(letrec ([odd (lambda (x)              
                (if (zero? x)          
                    #f                 
                    (even (sub1 x))))] 
         [even (lambda (x)             
                 (if (zero? x)         
                     #t                
                     (odd (sub1 x))))])
  (odd 40000000))                      
```

the compiler can detect the `odd`–`even` loop and produce code that runs
much faster via loop unrolling and related optimizations.

Within a module form, `define`d variables are lexically scoped like
`letrec` bindings, and definitions within a module therefore permit call
optimizations, so

```racket
(define (odd x) ....) 
(define (even x) ....)
```

within a module would perform the same as the `letrec` version.

For direct calls to functions with keyword arguments, the compiler can
typically check keyword arguments statically and generate a direct call
to a non-keyword variant of the function, which reduces the run-time
overhead of keyword checking. This optimization applies only for
keyword-accepting procedures that are bound with `define`.

For immediate calls to functions that are small enough, the compiler may
inline the function call by replacing the call with the body of the
function. In addition to the size of the target function’s body, the
compiler’s heuristics take into account the amount of inlining already
performed at the call site and whether the called function itself calls
functions other than simple primitive operations. When a module is
compiled, some functions defined at the module level are determined to
be candidates for inlining into other modules; normally, only trivial
functions are considered candidates for cross-module inlining, but a
programmer can wrap a function definition with `begin-encourage-inline`
to encourage inlining of the function.

Primitive operations like `pair?`, `car`, and `cdr` are inlined at the
machine-code level by the JIT compiler. See also the later section
Fixnum and Flonum Optimizations for information about inlined arithmetic
operations.

### 19.5. Mutation and Performance

Using `set!` to mutate a variable can lead to bad performance. For
example, the microbenchmark

```racket
#lang racket/base                 
                                  
(define (subtract-one x)          
  (set! x (sub1 x))               
  x)                              
                                  
(time                             
  (let loop ([n 4000000])         
    (if (zero? n)                 
        'done                     
        (loop (subtract-one n)))))
```

runs much more slowly than the equivalent

```racket
#lang racket/base                 
                                  
(define (subtract-one x)          
  (sub1 x))                       
                                  
(time                             
  (let loop ([n 4000000])         
    (if (zero? n)                 
        'done                     
        (loop (subtract-one n)))))
```

In the first variant, a new location is allocated for `x` on every
iteration, leading to poor performance. A more clever compiler could
unravel the use of `set!` in the first example, but since mutation is
discouraged \(see Guidelines for Using Assignment\), the compiler’s
effort is spent elsewhere.

More significantly, mutation can obscure bindings where inlining and
constant-propagation might otherwise apply. For example, in

```racket
(let ([minus1 #f])          
  (set! minus1 sub1)        
  (let loop ([n 4000000])   
    (if (zero? n)           
        'done               
        (loop (minus1 n)))))
```

the `set!` obscures the fact that `minus1` is just another name for the
built-in `sub1`.

### 19.6. `letrec` Performance

When `letrec` is used to bind only procedures and literals, then the
compiler can treat the bindings in an optimal manner, compiling uses of
the bindings efficiently. When other kinds of bindings are mixed with
procedures, the compiler may be less able to determine the control flow.

For example,

```racket
(letrec ([loop (lambda (x)            
                (if (zero? x)         
                    'done             
                    (loop (next x))))]
         [junk (display loop)]        
         [next (lambda (x) (sub1 x))])
  (loop 40000000))                    
```

likely compiles to less efficient code than

```racket
(letrec ([loop (lambda (x)            
                (if (zero? x)         
                    'done             
                    (loop (next x))))]
         [next (lambda (x) (sub1 x))])
  (loop 40000000))                    
```

In the first case, the compiler likely does not know that `display` does
not call `loop`. If it did, then `loop` might refer to `next` before the
binding is available.

This caveat about `letrec` also applies to definitions of functions and
constants as internal definitions or in modules. A definition sequence
in a module body is analogous to a sequence of `letrec` bindings, and
non-constant expressions in a module body can interfere with the
optimization of references to later bindings.

### 19.7. Fixnum and Flonum Optimizations

A _fixnum_ is a small exact integer. In this case, “small” depends on
the platform. For a 32-bit machine, numbers that can be expressed in 30
bits plus a sign bit are represented as fixnums. On a 64-bit machine, 62
bits plus a sign bit are available.

A _flonum_ is used to represent any inexact real number. They correspond
to 64-bit IEEE floating-point numbers on all platforms.

Inlined fixnum and flonum arithmetic operations are among the most
important advantages of the JIT compiler. For example, when `+` is
applied to two arguments, the generated machine code tests whether the
two arguments are fixnums, and if so, it uses the machine’s instruction
to add the numbers \(and check for overflow\). If the two numbers are
not fixnums, then it checks whether both are flonums; in that case, the
machine’s floating-point operations are used directly. For functions
that take any number of arguments, such as `+`, inlining works for two
or more arguments \(except for `-`, whose one-argument case is also
inlined\) when the arguments are either all fixnums or all flonums.

Flonums are typically _boxed_, which means that memory is allocated to
hold every result of a flonum computation. Fortunately, the generational
garbage collector \(described later in Memory Management\) makes
allocation for short-lived results reasonably cheap. Fixnums, in
contrast are never boxed, so they are typically cheap to use.

> See Parallelism with Futures for an example use of flonum-specific
> operations.

The `racket/flonum` library provides flonum-specific operations, and
combinations of flonum operations allow the JIT compiler to generate
code that avoids boxing and unboxing intermediate results. Besides
results within immediate combinations, flonum-specific results that are
bound with `let` and consumed by a later flonum-specific operation are
unboxed within temporary storage. Unboxing applies most reliably to uses
of a flonum-specific operation with two arguments. Finally, the compiler
can detect some flonum-valued loop accumulators and avoid boxing of the
accumulator. The bytecode decompiler \(see \[missing\]\) annotates
combinations where the JIT can avoid boxes with `#%flonum`,
`#%as-flonum`, and `#%from-flonum`.

> Unboxing of local bindings and accumulators is not supported by the JIT
> for PowerPC.

The `racket/unsafe/ops` library provides unchecked fixnum- and
flonum-specific operations. Unchecked flonum-specific operations allow
unboxing, and sometimes they allow the compiler to reorder expressions
to improve performance. See also Unchecked, Unsafe Operations,
especially the warnings about unsafety.

### 19.8. Unchecked, Unsafe Operations

The `racket/unsafe/ops` library provides functions that are like other
functions in `racket/base`, but they assume \(instead of checking\) that
provided arguments are of the right type. For example,
`unsafe-vector-ref` accesses an element from a vector without checking
that its first argument is actually a vector and without checking that
the given index is in bounds. For tight loops that use these functions,
avoiding checks can sometimes speed the computation, though the benefits
vary for different unchecked functions and different contexts.

Beware that, as “unsafe” in the library and function names suggest,
misusing the exports of `racket/unsafe/ops` can lead to crashes or
memory corruption.

### 19.9. Foreign Pointers

The `ffi/unsafe` library provides functions for unsafely reading and
writing arbitrary pointer values. The JIT recognizes uses of `ptr-ref`
and `ptr-set!` where the second argument is a direct reference to one of
the following built-in C types: `_int8`, `_int16`, `_int32`, `_int64`,
`_double`, `_float`, and `_pointer`. Then, if the first argument to
`ptr-ref` or `ptr-set!` is a C pointer \(not a byte string\), then the
pointer read or write is performed inline in the generated code.

The bytecode compiler will optimize references to integer abbreviations
like `_int` to C types like `_int32`—where the representation sizes are
constant across platforms—so the JIT can specialize access with those C
types. C types such as `_long` or `_intptr` are not constant across
platforms, so their uses are currently not specialized by the JIT.

Pointer reads and writes using `_float` or `_double` are not currently
subject to unboxing optimizations.

### 19.10. Regular Expression Performance

When a string or byte string is provided to a function like
`regexp-match`, then the string is internally compiled into a regexp
value. Instead of supplying a string or byte string multiple times as a
pattern for matching, compile the pattern once to a regexp value using
`regexp`, `byte-regexp`, `pregexp`, or `byte-pregexp`. In place of a
constant string or byte string, write a constant regexp using an `#rx`
or `#px` prefix.

```racket
(define (slow-matcher str)                
  (regexp-match? "[0-9]+" str))           
                                          
(define (fast-matcher str)                
  (regexp-match? #rx"[0-9]+" str))        
                                          
(define (make-slow-matcher pattern-str)   
  (lambda (str)                           
    (regexp-match? pattern-str str)))     
                                          
(define (make-fast-matcher pattern-str)   
  (define pattern-rx (regexp pattern-str))
  (lambda (str)                           
    (regexp-match? pattern-rx str)))      
```

### 19.11. Memory Management

The Racket implementation is available in three variants: _3m_, _CGC_,
and _CS_. The 3m and CS variants use a modern, _generational garbage
collector_ that makes allocation relatively cheap for short-lived
objects. The CGC variant uses a _conservative garbage collector_ which
facilitates interaction with C code at the expense of both precision and
speed for Racket memory management. The 3m variant is currently the
standard one.

Although memory allocation is reasonably cheap, avoiding allocation
altogether is normally faster. One particular place where allocation can
be avoided sometimes is in _closures_, which are the run-time
representation of functions that contain free variables. For example,

```racket
(let loop ([n 40000000] [prev-thunk (lambda () #f)])
  (if (zero? n)                                     
      (prev-thunk)                                  
      (loop (sub1 n)                                
            (lambda () n))))                        
```

allocates a closure on every iteration, since `(lambda () n)`
effectively saves `n`.

The compiler can eliminate many closures automatically. For example, in

```racket
(let loop ([n 40000000] [prev-val #f]) 
  (let ([prev-thunk (lambda () n)])    
    (if (zero? n)                      
        prev-val                       
        (loop (sub1 n) (prev-thunk)))))
```

no closure is ever allocated for `prev-thunk`, because its only
application is visible, and so it is inlined. Similarly, in

```racket
(let n-loop ([n 400000])         
  (if (zero? n)                  
      'done                      
      (let m-loop ([m 100])      
        (if (zero? m)            
            (n-loop (sub1 n))    
            (m-loop (sub1 m))))))
```

then the expansion of the `let` form to implement `m-loop` involves a
closure over `n`, but the compiler automatically converts the closure to
pass itself `n` as an argument instead.

### 19.12. Reachability and Garbage Collection

In general, Racket re-uses the storage for a value when the garbage
collector can prove that the object is unreachable from any other
\(reachable\) value. Reachability is a low-level, abstraction breaking
concept \(and thus one must understand many details of the runtime
system’s implementation to accurate predicate precisely when values are
reachable from each other\), but generally speaking one value is
reachable from a second one when there is some operation to recover the
original value from the second one.

To help programmers understand when an object is no longer reachable and
its storage can be reused, Racket provides `make-weak-box` and
`weak-box-value`, the creator and accessor for a one-record struct that
the garbage collector treats specially. An object inside a weak box does
not count as reachable, and so `weak-box-value` might return the object
inside the box, but it might also return `#f` to indicate that the
object was otherwise unreachable and garbage collected. Note that unless
a garbage collection actually occurs, the value will remain inside the
weak box, even if it is unreachable.

For example, consider this program:

```racket
#lang racket                              
(struct fish (weight color) #:transparent)
(define f (fish 7 'blue))                 
(define b (make-weak-box f))              
(printf "b has ~s\n" (weak-box-value b))  
(collect-garbage)                         
(printf "b has ~s\n" (weak-box-value b))  
```

It will print `b has #(struct:fish 7 blue)` twice because the definition
of `f` still holds onto the fish. If the program were this, however:

```racket
#lang racket                              
(struct fish (weight color) #:transparent)
(define f (fish 7 'blue))                 
(define b (make-weak-box f))              
(printf "b has ~s\n" (weak-box-value b))  
(set! f #f)                               
(collect-garbage)                         
(printf "b has ~s\n" (weak-box-value b))  
```

the second printout will be `b has #f` because no reference to the fish
exists \(other than the one in the box\).

As a first approximation, all values in Racket must be allocated and
will demonstrate behavior similar to the fish above. There are a number
of exceptions, however:

* Small integers \(recognizable with `fixnum?`\) are always available
  without explicit allocation. From the perspective of the garbage
  collector and weak boxes, their storage is never reclaimed. \(Due to
  clever representation techniques, however, their storage does not
  count towards the space that Racket uses. That is, they are
  effectively free.\)

* Procedures where the compiler can see all of their call sites may
  never be allocated at all \(as discussed above\). Similar
  optimizations may also eliminate the allocation for other kinds of
  values.

* Interned symbols are allocated only once \(per place\). A table inside
  Racket tracks this allocation so a symbol may not become garbage
  because that table holds onto it.

* Reachability is only approximate with the CGC collector \(i.e., a
  value may appear reachable to that collector when there is, in fact,
  no way to reach it anymore\).

### 19.13. Weak Boxes and Testing

One important use of weak boxes is in testing that some abstraction
properly releases storage for data it no longer needs, but there is a
gotcha that can easily cause such test cases to pass improperly.

Imagine you’re designing a data structure that needs to hold onto some
value temporarily but then should clear a field or somehow break a link
to avoid referencing that value so it can be collected. Weak boxes are a
good way to test that your data structure properly clears the value.
This is, you might write a test case that builds a value, extracts some
other value from it \(that you hope becomes unreachable\), puts the
extracted value into a weak-box, and then checks to see if the value
disappears from the box.

This code is one attempt to follow that pattern, but it has a subtle
bug:

```racket
#lang racket                                       
(let* ([fishes (list (fish 8 'red)                 
                     (fish 7 'blue))]              
       [wb (make-weak-box (list-ref fishes 0))])   
  (collect-garbage)                                
  (printf "still there? ~s\n" (weak-box-value wb)))
```

Specifically, it will show that the weak box is empty, but not because
`fishes` no longer holds onto the value, but because `fishes` itself is
not reachable anymore!

Change the program to this one:

```racket
#lang racket                                      
(let* ([fishes (list (fish 8 'red)                
                     (fish 7 'blue))]             
       [wb (make-weak-box (list-ref fishes 0))])  
  (collect-garbage)                               
  (printf "still there? ~s\n" (weak-box-value wb))
  (printf "fishes is ~s\n" fishes))               
```

and now we see the expected result. The difference is that last
occurrence of the variable `fishes`. That constitutes a reference to the
list, ensuring that the list is not itself garbage collected, and thus
the red fish is not either.

### 19.14. Reducing Garbage Collection Pauses

By default, Racket’s generational garbage collector creates brief pauses
for frequent _minor collections_, which inspect only the most recently
allocated objects, and long pauses for infrequent _major collections_,
which re-inspect all memory.

For some applications, such as animations and games, long pauses due to
a major collection can interfere unacceptably with a program’s
operation. To reduce major-collection pauses, the Racket garbage
collector supports _incremental garbage-collection_ mode. In incremental
mode, minor collections create longer \(but still relatively short\)
pauses by performing extra work toward the next major collection. If all
goes well, most of a major collection’s work has been performed by minor
collections the time that a major collection is needed, so the major
collection’s pause is as short as a minor collection’s pause.
Incremental mode tends to run more slowly overall, but it can provide
much more consistent real-time behavior.

If the `PLT_INCREMENTAL_GC` environment variable is set to a value that
starts with `1`, `y`, or `Y` when Racket starts, incremental mode is
permanently enabled. Since incremental mode is only useful for certain
parts of some programs, however, and since the need for incremental mode
is a property of a program rather than its environment, the preferred
way to enable incremental mode is with `(collect-garbage 'incremental)`.

Calling `(collect-garbage 'incremental)` does not perform an immediate
garbage collection, but instead requests that each minor collection
perform incremental work up to the next major collection. The request
expires with the next major collection. Make a call to `(collect-garbage
'incremental)` in any repeating task within an application that needs to
be responsive in real time. Force a full collection with
`(collect-garbage)` just before an initial `(collect-garbage
'incremental)` to initiate incremental mode from an optimal state.

To check whether incremental mode is in use and how it affects pause
times, enable `debug`-level logging output for the `GC` topic. For
example,

  `racket -W "debuG@GC error" main.rkt`

runs `"main.rkt"` with garbage-collection logging to stderr \(while
preserving `error`-level logging for all topics\). Minor collections are
reported by `min` lines, increment-mode minor collection are reported
with `mIn` lines, and major collections are reported with `MAJ` lines.

## 20. Parallelism

Racket provides two forms of _parallelism_: futures and places. On a
platform that provides multiple processors, parallelism can improve the
run-time performance of a program.

See also Performance for information on sequential performance in
Racket. Racket also provides threads for concurrency, but threads do not
provide parallelism; see Concurrency and Synchronization for more
information.

### 20.1. Parallelism with Futures

The `racket/future` library provides support for performance improvement
through parallelism with _futures_ and the `future` and `touch`
functions. The level of parallelism available from those constructs,
however, is limited by several factors, and the current implementation
is best suited to numerical tasks. The caveats in Performance in
DrRacket also apply to futures; notably, the debugging instrumentation
currently defeats futures.

> Other functions, such as `thread`, support the creation of reliably
> concurrent tasks. However, threads never run truly in parallel, even if
> the hardware and operating system support parallelism.

As a starting example, the `any-double?` function below takes a list of
numbers and determines whether any number in the list has a double that
is also in the list:

```racket
(define (any-double? l)       
  (for/or ([i (in-list l)])   
    (for/or ([i2 (in-list l)])
      (= i2 (* 2 i)))))       
```

This function runs in quadratic time, so it can take a long time \(on
the order of a second\) on large lists like `l1` and `l2`:

```racket
(define l1 (for/list ([i (in-range 5000)])
             (+ (* 2 i) 1)))              
(define l2 (for/list ([i (in-range 5000)])
             (- (* 2 i) 1)))              
(or (any-double? l1)                      
    (any-double? l2))                     
```

The best way to speed up `any-double?`  is to use a different algorithm.
However, on a machine that offers at least two processing units, the
example above can run in about half the time using `future` and `touch`:

```racket
(let ([f (future (lambda () (any-double? l2)))])
  (or (any-double? l1)                          
      (touch f)))                               
```

The future `f` runs `(any-double? l2)` in parallel to `(any-double?
l1)`, and the result for `(any-double? l2)` becomes available about the
same time that it is demanded by `(touch f)`.

Futures run in parallel as long as they can do so safely, but the notion
of “future safe” is inherently tied to the implementation. The
distinction between “future safe” and “future unsafe” operations may be
far from apparent at the level of a Racket program. The remainder of
this section works through an example to illustrate this distinction and
to show how to use the future visualizer can help shed light on it.

Consider the following core of a Mandelbrot-set computation:

```racket
(define (mandelbrot iterations x y n)               
  (let ([ci (- (/ (* 2.0 y) n) 1.0)]                
        [cr (- (/ (* 2.0 x) n) 1.5)])               
    (let loop ([i 0] [zr 0.0] [zi 0.0])             
      (if (> i iterations)                          
          i                                         
          (let ([zrq (* zr zr)]                     
                [ziq (* zi zi)])                    
            (cond                                   
              [(> (+ zrq ziq) 4) i]                 
              [else (loop (add1 i)                  
                          (+ (- zrq ziq) cr)        
                          (+ (* 2 zr zi) ci))]))))))
```

The expressions `(mandelbrot 10000000 62 500 1000)` and `(mandelbrot
10000000 62 501 1000)` each take a while to produce an answer. Computing
them both, of course, takes twice as long:

```racket
(list (mandelbrot 10000000 62 500 1000) 
      (mandelbrot 10000000 62 501 1000))
```

Unfortunately, attempting to run the two computations in parallel with
`future` does not improve performance:

```racket
(let ([f (future (lambda () (mandelbrot 10000000 62 501 1000)))])
  (list (mandelbrot 10000000 62 500 1000)                        
        (touch f)))                                              
```

To see why, use the `future-visualizer`, like this:

```racket
(require future-visualizer)                                       
(visualize-futures                                                
 (let ([f (future (lambda () (mandelbrot 10000000 62 501 1000)))])
   (list (mandelbrot 10000000 62 500 1000)                        
         (touch f))))                                             
```

This opens a window showing a graphical view of a trace of the
computation. The upper-left portion of the window contains an execution
timeline:

`#<pict>`

Each horizontal row represents an OS-level thread, and the colored dots
represent important events in the execution of the program \(they are
color-coded to distinguish one event type from another\).  The
upper-left blue dot in the timeline represents the future’s creation.
The future executes for a brief period \(represented by a green bar in
the second line\) on thread 1, and then pauses to allow the runtime
thread to perform a future-unsafe operation.

In the Racket implementation, future-unsafe operations fall into one of
two categories. A _blocking_ operation halts the evaluation of the
future, and will not allow it to continue until it is touched.  After
the operation completes within `touch`, the remainder of the future’s
work will be evaluated sequentially by the runtime thread.  A
_synchronized_ operation also halts the future, but the runtime thread
may perform the operation at any time and, once completed, the future
may continue running in parallel.  Memory allocation and JIT compilation
are two common examples of synchronized operations.

In the timeline, we see an orange dot just to the right of the green bar
on thread 1 – this dot represents a synchronized operation \(memory
allocation\).  The first orange dot on thread 0 shows that the runtime
thread performed the allocation shortly after the future paused.  A
short time later, the future halts on a blocking operation \(the first
red dot\) and must wait until the `touch` for it to be evaluated
\(slightly after the 1049ms mark\).

When you move your mouse over an event, the visualizer shows you
detailed information about the event and draws arrows connecting all of
the events in the corresponding future. This image shows those
connections for our future.

`#<pict>`

The dotted orange line connects the first event in the future to the
future that created it, and the purple lines connect adjacent events
within the future.

The reason that we see no parallelism is that the `<` and `*` operations
in the lower portion of the loop in `mandelbrot` involve a mixture of
floating-point and fixed \(integer\) values.  Such mixtures typically
trigger a slow path in execution, and the general slow path will usually
be blocking.

Changing constants to be floating-points numbers in `mandelbrot`
addresses that first problem:

```racket
(define (mandelbrot iterations x y n)                 
  (let ([ci (- (/ (* 2.0 y) n) 1.0)]                  
        [cr (- (/ (* 2.0 x) n) 1.5)])                 
    (let loop ([i 0] [zr 0.0] [zi 0.0])               
      (if (> i iterations)                            
          i                                           
          (let ([zrq (* zr zr)]                       
                [ziq (* zi zi)])                      
            (cond                                     
              [(> (+ zrq ziq) 4.0) i]                 
              [else (loop (add1 i)                    
                          (+ (- zrq ziq) cr)          
                          (+ (* 2.0 zr zi) ci))]))))))
```

With that change, `mandelbrot` computations can run in parallel.
Nevertheless, we still see a special type of slow-path operation
limiting our parallelism \(orange dots\):

`#<pict>`

The problem is that most every arithmetic operation in this example
produces an inexact number whose storage must be allocated.  While some
allocation can safely be performed exclusively without the aid of the
runtime thread, especially frequent allocation requires synchronized
operations which defeat any performance improvement.

By using flonum-specific operations \(see Fixnum and Flonum
Optimizations\), we can re-write `mandelbrot` to use much less
allocation:

```racket
(define (mandelbrot iterations x y n)                           
  (let ([ci (fl- (fl/ (* 2.0 (->fl y)) (->fl n)) 1.0)]          
        [cr (fl- (fl/ (* 2.0 (->fl x)) (->fl n)) 1.5)])         
    (let loop ([i 0] [zr 0.0] [zi 0.0])                         
      (if (> i iterations)                                      
          i                                                     
          (let ([zrq (fl* zr zr)]                               
                [ziq (fl* zi zi)])                              
            (cond                                               
              [(fl> (fl+ zrq ziq) 4.0) i]                       
              [else (loop (add1 i)                              
                          (fl+ (fl- zrq ziq) cr)                
                          (fl+ (fl* 2.0 (fl* zr zi)) ci))]))))))
```

This conversion can speed `mandelbrot` by a factor of 8, even in
sequential mode, but avoiding allocation also allows `mandelbrot` to run
usefully faster in parallel. Executing this program yields the following
in the visualizer:

`#<pict>`

Notice that only one green bar is shown here because one of the
mandelbrot computations is not being evaluated by a future \(on the
runtime thread\).

As a general guideline, any operation that is inlined by the JIT
compiler runs safely in parallel, while other operations that are not
inlined \(including all operations if the JIT compiler is disabled\) are
considered unsafe. The `raco decompile` tool annotates operations that
can be inlined by the compiler \(see \[missing\]\), so the decompiler
can be used to help predict parallel performance.

### 20.2. Parallelism with Places

The `racket/place` library provides support for performance improvement
through parallelism with the `place` form. The `place` form creates a
_place_, which is effectively a new Racket instance that can run in
parallel to other places, including the initial place.  The full power
of the Racket language is available at each place, but places can
communicate only through message passing—using the `place-channel-put`
and `place-channel-get` functions on a limited set of values—which helps
ensure the safety and independence of parallel computations.

As a starting example, the racket program below uses a place to
determine whether any number in the list has a double that is also in
the list:

```racket
#lang racket                            
                                        
(provide main)                          
                                        
(define (any-double? l)                 
  (for/or ([i (in-list l)])             
    (for/or ([i2 (in-list l)])          
      (= i2 (* 2 i)))))                 
                                        
(define (main)                          
  (define p                             
    (place ch                           
      (define l (place-channel-get ch)) 
      (define l-double? (any-double? l))
      (place-channel-put ch l-double?)))
                                        
  (place-channel-put p (list 1 2 4 8))  
                                        
  (place-channel-get p))                
```

The identifier `ch` after `place` is bound to a _place channel_. The
remaining body expressions within the `place` form are evaluated in a
new place, and the body expressions use `ch` to communicate with the
place that spawned the new place.

In the body of the `place` form above, the new place receives a list of
numbers over `ch` and binds the list to `l`.  It then calls
`any-double?` on the list and binds the result to `l-double?`. The final
body expression sends the `l-double?` result back to the original place
over `ch`.

In DrRacket, after saving and running the above program, evaluate
`(main)` in the interactions window to create the new place. When using
places inside DrRacket, the module containg place code must be saved to
a file before it will execute.  Alternatively, save the program as
`"double.rkt"` and run from a command line with

  `racket -tm double.rkt`

where the `-t` flag tells `racket` to load the `double.rkt` module, the
`-m` flag calls the exported `main` function, and `-tm` combines the two
flags.

The `place` form has two subtle features. First, it lifts the `place`
body to an anonymous, module-level function.  This lifting means that
any binding referenced by the `place` body must be available in the
module’s top level. Second, the `place` form `dynamic-require`s the
enclosing module in a newly created place. As part of the
`dynamic-require`, the current module body is evaluated in the new
place.  The consequence of this second feature is that `place` should
not appear immediately in a module or in a function that is called in a
module’s top level; otherwise, invoking the module will invoke the same
module in a new place, and so on, triggering a cascade of place
creations that will soon exhaust memory.

```racket
#lang racket                                    
                                                
(provide main)                                  
                                                
; Don't do this!                                
(define p (place ch (place-channel-get ch)))    
                                                
(define (indirect-place-invocation)             
  (define p2 (place ch (place-channel-get ch))))
                                                
; Don't do this, either!                        
(indirect-place-invocation)                     
```

### 20.3. Distributed Places

The `racket/place/distributed` library provides support for distributed
programming.

The example bellow demonstrates how to launch a remote racket node
instance, launch remote places on the new remote node instance, and
start an event loop that monitors the remote node instance.

The example code can also be found in
`"racket/distributed/examples/named/master.rkt"`.

```racket
#lang racket/base                                      
(require racket/place/distributed                      
         racket/class                                  
         racket/place                                  
         racket/runtime-path                           
         "bank.rkt"                                    
         "tuple.rkt")                                  
(define-runtime-path bank-path "bank.rkt")             
(define-runtime-path tuple-path "tuple.rkt")           
                                                       
(provide main)                                         
                                                       
(define (main)                                         
  (define remote-node (spawn-remote-racket-node        
                        "localhost"                    
                        #:listen-port 6344))           
  (define tuple-place (supervise-place-at              
                        remote-node                    
                        #:named 'tuple-server          
                        tuple-path                     
                        'make-tuple-server))           
  (define bank-place  (supervise-place-at              
                        remote-node bank-path          
                        'make-bank))                   
                                                       
  (message-router                                      
    remote-node                                        
    (after-seconds 4                                   
      (displayln (bank-new-account bank-place 'user0)) 
      (displayln (bank-add bank-place 'user0 10))      
      (displayln (bank-removeM bank-place 'user0 5)))  
                                                       
    (after-seconds 2                                   
      (define c (connect-to-named-place remote-node    
                                        'tuple-server))
      (define d (connect-to-named-place remote-node    
                                        'tuple-server))
      (tuple-server-hello c)                           
      (tuple-server-hello d)                           
      (displayln (tuple-server-set c "user0" 100))     
      (displayln (tuple-server-set d "user2" 200))     
      (displayln (tuple-server-get c "user0"))         
      (displayln (tuple-server-get d "user2"))         
      (displayln (tuple-server-get d "user0"))         
      (displayln (tuple-server-get c "user2"))         
      )                                                
    (after-seconds 8                                   
      (node-send-exit remote-node))                    
    (after-seconds 10                                  
      (exit 0))))                                      
                                                       
```
Figure 1: examples/named/master.rkt

The `spawn-remote-racket-node` primitive connects to `"localhost"` and
starts a racloud node there that listens on port 6344 for further
instructions.  The handle to the new racloud node is assigned to the
`remote-node` variable. Localhost is used so that the example can be run
using only a single machine.  However localhost can be replaced by any
host with ssh publickey access and racket.  The `supervise-place-at`
creates a new place on the `remote-node`.  The new place will be
identified in the future by its name symbol `'tuple-server`.  A place
descriptor is expected to be returned by invoking `dynamic-place` with
the `tuple-path` module path and the `'make-tuple-server` symbol.

The code for the tuple-server place exists in the file `"tuple.rkt"`.
The `"tuple.rkt"` file contains the use of `define-named-remote-server`
form, which defines a RPC server suitiable for invocation by
`supervise-place-at`.

```racket
#lang racket/base                          
(require racket/match                      
         racket/place/define-remote-server)
                                           
(define-named-remote-server tuple-server   
  (define-state h (make-hash))             
  (define-rpc (set k v)                    
    (hash-set! h k v)                      
    v)                                     
  (define-rpc (get k)                      
    (hash-ref h k #f))                     
  (define-cast (hello)                     
    (printf "Hello from define-cast\n")    
    (flush-output)))                       
                                           
```
Figure 2: examples/named/tuple.rkt

The `define-named-remote-server` form takes an identifier and a list of
custom expressions as its arguments.  From the identifier a place-thunk
function is created by prepending the `make-` prefix. In this case
`make-tuple-server`.  The `make-tuple-server` identifier is the
`place-function-name` given to the `supervise-named-dynamic-place-at`
form above. The `define-state` custom form translates into a simple
`define` form, which is closed over by the `define-rpc` form.

The `define-rpc` form is expanded into two parts. The first part is the
client stubs that call the rpc functions. The client function name is
formed by concatenating the `define-named-remote-server` identifier,
`tuple-server`, with the RPC function name `set` to form
`tuple-server-set`. The RPC client functions take a destination argument
which is a `remote-connection%` descriptor and then the RPC function
arguments. The RPC client function sends the RPC function name, `set`,
and the RPC arguments to the destination by calling an internal function
`named-place-channel-put`. The RPC client then calls
`named-place-channel-get` to wait for the RPC response.

The second expansion part of `define-rpc` is the server implementation
of the RPC call.  The server is implemented by a match expression inside
the `make-tuple-server` function.  The match clause for
`tuple-server-set` matches on messages beginning with the `'set` symbol.
The server executes the RPC call with the communicated arguments and
sends the result back to the RPC client.

The `define-cast` form is similar to the `define-rpc` form except there
is no reply message from the server to client

```racket
(module tuple racket/base                              
  (require racket/place                                
           racket/match)                               
  (define/provide                                      
   (tuple-server-set dest k v)                         
   (named-place-channel-put dest (list 'set k v))      
   (named-place-channel-get dest))                     
  (define/provide                                      
   (tuple-server-get dest k)                           
   (named-place-channel-put dest (list 'get k))        
   (named-place-channel-get dest))                     
  (define/provide                                      
   (tuple-server-hello dest)                           
   (named-place-channel-put dest (list 'hello)))       
  (define/provide                                      
   (make-tuple-server ch)                              
    (let ()                                            
      (define h (make-hash))                           
      (let loop ()                                     
        (define msg (place-channel-get ch))            
        (define (log-to-parent-real                    
                  msg                                  
                  #:severity (severity 'info))         
          (place-channel-put                           
            ch                                         
            (log-message severity msg)))               
        (syntax-parameterize                           
         ((log-to-parent (make-rename-transformer      
                           #'log-to-parent-real)))     
         (match                                        
          msg                                          
          ((list (list 'set k v) src)                  
           (define result (let () (hash-set! h k v) v))
           (place-channel-put src result)              
           (loop))                                     
          ((list (list 'get k) src)                    
           (define result (let () (hash-ref h k #f)))  
           (place-channel-put src result)              
           (loop))                                     
          ((list (list 'hello) src)                    
           (define result                              
             (let ()                                   
               (printf "Hello from define-cast\n")     
               (flush-output)))                        
           (loop))))                                   
        loop))))                                       
```
Figure 3: Expansion of define-named-remote-server

## 21. Running and Creating Executables

While developing programs, many Racket programmers use the DrRacket
programming environment. To run a program without the development
environment, use `racket` \(for console-based programs\) or `gracket`
\(for GUI programs\). This chapter mainly explains how to run `racket`
and `gracket`.

    21.1 Running `racket` and `gracket`  
      21.1.1 Interactive Mode            
      21.1.2 Module Mode                 
      21.1.3 Load Mode                   
    21.2 Scripts                         
      21.2.1 Unix Scripts                
      21.2.2 Windows Batch Files         
    21.3 Creating Stand-Alone Executables

### 21.1. Running `racket` and `gracket`

The `gracket` executable is the same as `racket`, but with small
adjustments to behave as a GUI application rather than a console
application. For example, `gracket` by default runs in interactive mode
with a GUI window instead of a console prompt. GUI applications can be
run with plain `racket`, however.

Depending on command-line arguments, `racket` or `gracket` runs in
interactive mode, module mode, or load mode.

#### 21.1.1. Interactive Mode

When `racket` is run with no command-line arguments \(other than
confguration options, like `-j`\), then it starts a REPL with a `> `
prompt:

  `Welcome to Racket v7.1.0.6.`
  `>`                          

> For enhancing your REPL experience, see `xrepl`; for information on GNU
> Readline support, see `readline`.

To initialize the REPL’s environment, `racket` first requires the
`racket/init` module, which provides all of `racket`, and also installs
`pretty-print` for display results. Finally, `racket` loads the file
reported by `(find-system-path 'init-file)`, if it exists, before
starting the REPL.

If any command-line arguments are provided \(other than configuration
options\), add `-i` or `--repl` to re-enable the REPL. For example,

  `racket -e '(display "hi\n")' -i`

displays “hi” on start-up, but still presents a REPL.

If module-requiring flags appear before `-i`/`--repl`, they cancel the
automatic requiring of `racket/init`. This behavior can be used to
initialize the REPL’s environment with a different language. For
example,

  `racket -l racket/base -i`

starts a REPL using a much smaller initial language \(that loads much
faster\). Beware that most modules do not provide the basic syntax of
Racket, including function-call syntax and `require`. For example,

  `racket -l racket/date -i`

produces a REPL that fails for every expression, because `racket/date`
provides only a few functions, and not the `#%top-interaction` and
`#%app` bindings that are needed to evaluate top-level function calls in
the REPL.

If a module-requiring flag appears after `-i`/`--repl` instead of before
it, then the module is required after `racket/init` to augment the
initial environment. For example,

  `racket -i -l racket/date`

starts a useful REPL with `racket/date` available in addition to the
exports of `racket`.

#### 21.1.2. Module Mode

If a file argument is supplied to `racket` before any command-line
switch \(other than configuration options\), then the file is required
as a module, and \(unless `-i`/`--repl` is specified\), no REPL is
started. For example,

  `racket hello.rkt`

requires the `"hello.rkt"` module and then exits. Any argument after the
file name, flag or otherwise, is preserved as a command-line argument
for use by the required module via `current-command-line-arguments`.

If command-line flags are used, then the `-u` or `--require-script` flag
can be used to explicitly require a file as a module.  The `-t` or
`--require` flag is similar, except that additional command-line flags
are processed by `racket`, instead of preserved for the required module.
For example,

  `racket -t hello.rkt -t goodbye.rkt`

requires the `"hello.rkt"` module, then requires the `"goodbye.rkt"`
module, and then exits.

The `-l` or `--lib` flag is similar to `-t`/`--require`, but it requires
a module using a `lib` module path instead of a file path. For example,

  `racket -l raco`

is the same as running the `raco` executable with no arguments, since
the `raco` module is the executable’s main module.

Note that if you wanted to pass command-line flags to `raco` above, you
would need to protect the flags with a `--`, so that `racket` doesn’t
try to parse them itself:

  `racket -l raco -- --help`

#### 21.1.3. Load Mode

The `-f` or `--load` flag supports `load`ing top-level expressions in a
file directly, as opposed to expressions within a module file. This
evaluation is like starting a REPL and typing the expressions directly,
except that the results are not printed. For example,

  `racket -f hi.rkts`

`load`s `"hi.rkts"` and exits. Note that load mode is generally a bad
idea, for the reasons explained in A Note to Readers with Lisp/Scheme
Experience; using module mode is typically better.

The `-e` or `--eval` flag accepts an expression to evaluate directly.
Unlike file loading, the result of the expression is printed, as in a
REPL. For example,

  `racket -e '(current-seconds)'`

prints the number of seconds since January 1, 1970.

For file loading and expression evaluation, the top-level environment is
created in the same way for interactive mode: `racket/init` is required
unless another module is specified first. For example,

  `racket -l racket/base -e '(current-seconds)'`

likely runs faster, because it initializes the environment for
evaluation using the smaller `racket/base` language, instead of
`racket/init`.

### 21.2. Scripts

Racket files can be turned into executable scripts on Unix and Mac OS.
On Windows, a compatibility layer like Cygwin support the same kind of
scripts, or scripts can be implemented as batch files.

#### 21.2.1. Unix Scripts

In a Unix environment \(including Linux and Mac OS\), a Racket file can
be turned into an executable script using the shell’s `#!` convention.
The first two characters of the file must be `#!`; the next character
must be either a space or `/`, and the remainder of the first line must
be a command to execute the script. For some platforms, the total length
of the first line is restricted to 32 characters, and sometimes the
space is required.

> Use `#lang` `racket/base` instead of `#lang` `racket` to produce scripts
> with a faster startup time.

The simplest script format uses an absolute path to a `racket`
executable followed by a module declaration. For example, if `racket` is
installed in `"/usr/local/bin"`, then a file containing the following
text acts as a “hello world” script:

  `#! /usr/local/bin/racket`
  `#lang racket/base`       
  `"Hello, world!"`         

In particular, if the above is put into a file `"hello"` and the file is
made executable \(e.g., with `chmod a+x hello`\), then typing `./hello`
at the shell prompt produces the output `"Hello, world!"`.

The above script works because the operating system automatically puts
the path to the script as the argument to the program started by the
`#!` line, and because `racket` treats a single non-flag argument as a
file containing a module to run.

Instead of specifying a complete path to the `racket` executable, a
popular alternative is to require that `racket` is in the user’s command
path, and then “trampoline” using `/usr/bin/env`:

  `#! /usr/bin/env racket`
  `#lang racket/base`     
  `"Hello, world!"`       

In either case, command-line arguments to a script are available via
`current-command-line-arguments`:

  `#! /usr/bin/env racket`                   
  `#lang racket/base`                        
  `(printf "Given arguments: ~s\n"`          
          `(current-command-line-arguments))`

If the name of the script is needed, it is available via
`(find-system-path 'run-file)`, instead of via
`(current-command-line-arguments)`.

Usually, the best way to handle command-line arguments is to parse them
using the `command-line` form provided by `racket`. The `command-line`
form extracts command-line arguments from
`(current-command-line-arguments)` by default:

  `#! /usr/bin/env racket`                     
  `#lang racket`                               
                                               
  `(define verbose? (make-parameter #f))`      
                                               
  `(define greeting`                           
    `(command-line`                            
     `#:once-each`                             
     `[("-v") "Verbose mode" (verbose? #t)]`   
     `#:args`                                  
     `(str) str))`                             
                                               
  `(printf "~a~a\n"`                           
          `greeting`                           
          `(if (verbose?) " to you, too!" ""))`

Try running the above script with the `--help` flag to see what
command-line arguments are allowed by the script.

An even more general trampoline uses `/bin/sh` plus some lines that are
comments in one language and expressions in the other. This trampoline
is more complicated, but it provides more control over command-line
arguments to `racket`:

  `#! /bin/sh`                                                 
  `#|`                                                         
  `exec racket -e '(printf "Running...\n")' -u "$0" ${1+"$@"}` 
  `|#`                                                         
  `#lang racket/base`                                          
  `(printf "The above line of output had been produced via\n")`
  `(printf "a use of the `-e' flag.\n")`                       
  `(printf "Given arguments: ~s\n"`                            
          `(current-command-line-arguments))`                  

Note that `#!` starts a line comment in Racket, and `#|`...`|#` forms a
block comment. Meanwhile, `#` also starts a shell-script comment, while
`exec racket` aborts the shell script to start `racket`. That way, the
script file turns out to be valid input to both `/bin/sh` and `racket`.

#### 21.2.2. Windows Batch Files

A similar trick can be used to write Racket code in Windows `.bat` batch
files:

  `; @echo off`           
  `; Racket.exe "%~f0" %*`
  `; exit /b`             
  `#lang racket/base`     
  `"Hello, world!"`       

### 21.3. Creating Stand-Alone Executables

For information on creating and distributing executables, see
\[missing\] and \[missing\] in \[missing\].

## 22. More Libraries

This guide covers only the Racket language and libraries that are
documented in \[missing\]. The Racket distribution includes many
additional libraries.

### 22.1. Graphics and GUIs

Racket provides many libraries for graphics and graphical user
interfaces \(GUIs\):

* The `racket/draw` library provides basic drawing tools, including
  drawing contexts such as bitmaps and PostScript files.

  See \[missing\] for more information.

* The `racket/gui` library provides GUI widgets such as windows,
  buttons, checkboxes, and text fields. The library also includes a
  sophisticated and extensible text editor.

  See \[missing\] for more information.

* The `pict` library provides a more functional abstraction layer over
  `racket/draw`. This layer is especially useful for creating slide
  presentations with Slideshow, but it is also useful for creating
  images for Scribble documents or other drawing tasks. Pictures created
  with the `pict` library can be rendered to any drawing context.

  See \[missing\] for more information.

* The `2htdp/image` library is similar to `pict`. It is more streamlined
  for pedagogical use, but also slightly more specific to screen and
  bitmap drawing.

  See `2htdp/image` for more information.

* The `sgl` library provides OpenGL for 3-D graphics. The context for
  rendering OpenGL can be a window or bitmap created with `racket/gui`.

  See the SGL documentation for more information.

### 22.2. The Web Server

\[missing\] describes the Racket web server, which supports servlets
implemented in Racket.

### 22.3. Using Foreign Libraries

\[missing\] describes tools for using Racket to access libraries that
are normally used by C programs.

### 22.4. And More

[Racket Documentation](../index.html) lists documentation for many other
installed libraries. Run `raco docs` to find documentation for libraries
that are installed on your system and specific to your user account.

[The Racket package repository](https://pkgs.racket-lang.org/) offer
even more downloadable packages that are contributed by Racketeers.

The legacy [PLaneT](http://planet.racket-lang.org/) site offers
additional packages, although maintained packages have generally
migrated to the newer package repository.

## 23. Dialects of Racket and Scheme

We use “Racket” to refer to a specific dialect of the Lisp language, and
one that is based on the Scheme branch of the Lisp family. Despite
Racket’s similarity to Scheme, the `#lang` prefix on modules is a
particular feature of Racket, and programs that start with `#lang` are
unlikely to run in other implementations of Scheme. At the same time,
programs that do not start with `#lang` do not work with the default
mode of most Racket tools.

“Racket” is not, however, the only dialect of Lisp that is supported by
Racket tools. On the contrary, Racket tools are designed to support
multiple dialects of Lisp and even multiple languages, which allows the
Racket tool suite to serve multiple communities. Racket also gives
programmers and researchers the tools they need to explore and create
new languages.

    23.1 More Rackets
    23.2 Standards   
      23.2.1 R5RS    
      23.2.2 R6RS    
    23.3 Teaching    

### 23.1. More Rackets

“Racket” is more of an idea about programming languages than a language
in the usual sense. Macros can extend a base language \(as described in
Macros\), and alternate parsers can construct an entirely new language
from the ground up \(as described in Creating Languages\).

The `#lang` line that starts a Racket module declares the base language
of the module. By “Racket,” we usually mean `#lang` followed by the base
language `racket` or `racket/base` \(of which `racket` is an
extension\). The Racket distribution provides additional languages,
including the following:

* `typed/racket` — like `racket`, but statically typed; see \[missing\]

* `lazy` — like `racket/base`, but avoids evaluating an expression until
  its value is needed; see the Lazy Racket documentation.

* `frtime` — changes evaluation in an even more radical way to support
  reactive programming; see the FrTime documentation.

* `scribble/base` — a language, which looks more like Latex than Racket,
  for writing documentation; see \[missing\]

Each of these languages is used by starting module with the language
name after `#lang`. For example, this source of this document starts
with `#lang scribble/base`.

Furthermore, Racket users can define their own languages, as discussed
in Creating Languages. Typically, a language name maps to its
implementation through a module path by adding `/lang/reader`; for
example, the language name `scribble/base` is expanded to
`scribble/base/lang/reader`, which is the module that implements the
surface-syntax parser. Some language names act as language loaders; for
example, `#lang planet planet-path` downloads, installs, and uses a
language via PLaneT.

### 23.2. Standards

Standard dialects of Scheme include the ones defined by R5RS and R6RS.

#### 23.2.1. R5RS

“R5RS” stands for [The Revised5 Report on the Algorithmic Language
Scheme](../r5rs/r5rs-std/index.html), and it is currently the most
widely implemented Scheme standard.

Racket tools in their default modes do not conform to R5RS, mainly
because Racket tools generally expect modules, and R5RS does not define
a module system. Typical single-file R5RS programs can be converted to
Racket programs by prefixing them with `#lang r5rs`, but other Scheme
systems do not recognize `#lang r5rs`. The `plt-r5rs` executable \(see
\[missing\]\) more directly conforms to the R5RS standard.

Aside from the module system, the syntactic forms and functions of R5RS
and Racket differ. Only simple R5RS become Racket programs when prefixed
with `#lang racket`, and relatively few Racket programs become R5RS
programs when a `#lang` line is removed. Also, when mixing “R5RS
modules” with Racket modules, beware that R5RS pairs correspond to
Racket mutable pairs \(as constructed with `mcons`\).

See \[missing\] for more information about running R5RS programs with
Racket.

#### 23.2.2. R6RS

“R6RS” stands for [The Revised6 Report on the Algorithmic Language
Scheme](../r6rs/r6rs-std/index.html), which extends R5RS with a module
system that is similar to the Racket module system.

When an R6RS library or top-level program is prefixed with `#!r6rs`
\(which is valid R6RS syntax\), then it can also be used as a Racket
program. This works because `#!` in Racket is treated as a shorthand for
`#lang` followed by a space, so `#!r6rs` selects the `r6rs` module
language. As with R5RS, however, beware that the syntactic forms and
functions of R6RS differ from Racket, and R6RS pairs are mutable pairs.

See \[missing\] for more information about running R6RS programs with
Racket.

### 23.3. Teaching

The _[How to Design Programs](http://www.htdp.org)_ textbook relies on
pedagogic variants of Racket that smooth the introduction of programming
concepts for new programmers. See the _[How to Design
Programs](http://www.htdp.org)_ language documentation.

The _[How to Design Programs](http://www.htdp.org)_ languages are
typically not used with `#lang` prefixes, but are instead used within
DrRacket by selecting the language from the Choose Language... dialog.

## 24. Command-Line Tools and Your Editor of Choice

Although DrRacket is the easiest way for most people to start with
Racket, many Racketeers prefer command-line tools and other text
editors.  The Racket distribution includes several command-line tools,
and popular editors include or support packages to make them work well
with Racket.

    24.1 Command-Line Tools                       
      24.1.1 Compilation and Configuration: `raco`
      24.1.2 Interactive evaluation               
      24.1.3 Shell completion                     
    24.2 Emacs                                    
      24.2.1 Major Modes                          
      24.2.2 Minor Modes                          
      24.2.3 Packages specific to Evil Mode       
    24.3 Vim                                      
    24.4 Sublime Text                             

### 24.1. Command-Line Tools

Racket provides, as part of its standard distribution, a number of
command-line tools that can make racketeering more pleasant.

#### 24.1.1. Compilation and Configuration: `raco`

The `raco` \(short for “**Ra**cket **co**mmand”\) program provides a
command-line interface to many additional tools for compiling Racket
programs and maintaining a Racket installation.

* `raco make` compiles Racket source to bytecode.

  For example, if you have a program `"take-over-world.rkt"` and you’d
  like to compile it to bytecode, along with all of its dependencies, so
  that it loads more quickly, then run

    `raco make take-over-the-world.rkt`

  The bytecode file is written as `"take-over-the-world_rkt.zo"` in a
  `"compiled"` subdirectory; `".zo"` is the file suffix for a bytecode
  file.

* `raco setup` manages a Racket installation, including manually
  installed packages.

  For example, if you create your own library collection called
  `"take-over"`, and you’d like to build all bytecode and documentation
  for the collection, then run

    `raco setup take-over`

* `raco pkg` manages packages that can be installed through the Racket
  package manager.

  For example, to see the list of installed packages run:

    `raco pkg show`

  To install a new package named `<package-name>` run:

    `raco pkg install <package-name>`

  See \[missing\] for more details about package management.

For more information on `raco`, see \[missing\].

#### 24.1.2. Interactive evaluation

The Racket REPL provides everything you expect from a modern interactive
environment. For example, it provides an `,enter` command to have a REPL
that runs in the context of a given module, and an `,edit` command to
invoke your editor \(as specified by the `EDITOR` environment variable\)
on the file you entered. A `,drracket` command makes it easy to use your
favorite editor to write code, and still have DrRacket at hand to try
things out.

For more information, see \[missing\].

#### 24.1.3. Shell completion

Shell auto-completion for `bash` and `zsh` is available in
`"share/pkgs/shell-completion/racket-completion.bash"` and
`"share/pkgs/shell-completion/racket-completion.zsh"`, respectively. To
enable it, just run the appropriate file from your `.bashrc` or your
`.zshrc`.

The `"shell-completion"` collection is only available in the Racket Full
distribution. The completion scripts are also available
[online](https://github.com/racket/shell-completion).

### 24.2. Emacs

Emacs has long been a favorite among Lispers and Schemers, and is
popular among Racketeers as well.

#### 24.2.1. Major Modes

* [Racket mode](https://github.com/greghendershott/racket-mode) provides
  thorough syntax highlighting and DrRacket-style REPL and buffer
  execution support for Emacs.

  Racket mode can be installed via [MELPA](http://melpa.milkbox.net) or
  manually from the Github repository.

* [Quack](http://www.neilvandyke.org/quack/) is an extension of Emacs’s
  `scheme-mode` that provides enhanced support for Racket, including
  highlighting and indentation of Racket-specific forms, and
  documentation integration.

  Quack is included in the Debian and Ubuntu repositories as part of the
  `emacs-goodies-el` package. A Gentoo port is also available \(under
  the name `app-emacs/quack`\).

* [Geiser](http://www.nongnu.org/geiser/) provides a programming
  environment where the editor is tightly integrated with the Racket
  REPL. Programmers accustomed to environments such as Slime or Squeak
  should feel at home using Geiser. Geiser requires GNU Emacs 23.2 or
  better.

  Quack and Geiser can be used together, and complement each other
  nicely. More information is available in the [Geiser
  manual](http://www.nongnu.org/geiser/).

  Debian and Ubuntu packages for Geiser are available under the name
  `geiser`.

* Emacs ships with a major mode for Scheme, `scheme-mode`, that while
  not as featureful as the above options, works reasonably well for
  editing Racket code. However, this mode does not provide support for
  Racket-specific forms.

* No Racket program is complete without documentation. Scribble support
  for Emacs is available with Neil Van Dyke’s [Scribble
  Mode](http://www.neilvandyke.org/scribble-emacs/).

  In addition, `texinfo-mode` \(included with GNU Emacs\) and  plain
  text modes work well when editing Scribble  documents. The Racket
  major modes above are not really suited  to this task, given how
  different Scribble’s syntax is from  Racket’s.

#### 24.2.2. Minor Modes

* [Paredit](http://mumble.net/~campbell/emacs/paredit.el) is a minor
  mode for pseudo-structurally editing programs in Lisp-like languages.
  In addition to providing high-level S-expression editing commands, it
  prevents you from accidentally unbalancing parentheses.

  Debian and Ubuntu packages for Paredit are available under the name
  `paredit-el`.

* [Smartparens](https://github.com/Fuco1/smartparens) is a minor mode
  for editing s-expressions, keeping parentheses balanced, etc.  Similar
  to Paredit.

* Alex Shinn’s
  [scheme-complete](http://synthcode.com/wiki/scheme-complete) provides
  intelligent, context-sensitive code completion. It also integrates
  with Emacs’s `eldoc` mode to provide live documentation in the
  minibuffer.

  While this mode was designed for R5RS, it can still be useful for
  Racket development. The tool is unaware of large portions of the
  Racket standard library, and there may be some discrepancies in the
  live documentation in cases where Scheme and Racket have diverged.

* The
  [RainbowDelimiters](http://www.emacswiki.org/emacs/RainbowDelimiters)
  mode colors parentheses and other delimiters according to their
  nesting depth. Coloring by nesting depth makes it easier to know, at a
  glance, which parentheses match.

* [ParenFace](http://www.emacswiki.org/emacs/ParenFace) lets you choose
  in which face \(font, color, etc.\) parentheses should be displayed.
  Choosing an alternate face makes it possible to make “tone down”
  parentheses.

#### 24.2.3. Packages specific to Evil Mode

* [on-parens](https://github.com/willghatch/emacs-on-parens) is a
  wrapper for smartparens motions to work better with evil-mode’s normal
  state.

* [evil-surround](https://github.com/timcharper/evil-surround) provides
  commands to add, remove, and change parentheses and other delimiters.

* [evil-textobj-anyblock](https://github.com/noctuid/evil-textobj-anyblock)
  adds a text-object that matches the closest of any parenthesis or
  other delimiter pair.

### 24.3. Vim

Many distributions of Vim ship with support for Scheme, which will
mostly work for Racket. You can enable filetype detection of Racket
files as Scheme with the following:

  `if has("autocmd")`                                
    `au BufReadPost *.rkt,*.rktl set filetype=scheme`
  `endif`                                            

Alternatively, you can use the
[vim-racket](https://github.com/wlangstroth/vim-racket) plugin to enable
auto-detection, indentation, and syntax highlighting specifically for
Racket files. Using the plugin is the easiest method, but if you would
like to roll your own settings or override settings from the plugin, add
something like the following to your `".vimrc"` file:

  `if has("autocmd")`                                
    `au BufReadPost *.rkt,*.rktl set filetype=racket`
    `au filetype racket set lisp`                    
    `au filetype racket set autoindent`              
  `endif`                                            

However, if you take this path you may need to do more work when
installing plugins because many Lisp-related plugins and scripts for vim
are not aware of Racket. You can also set these conditional commands in
a `"scheme.vim"` or `"racket.vim"` file in the `"ftplugin"` subdirectory
of your vim folder.

Most installations of vim will automatically have useful defaults
enabled, but if your installation does not, you will want to set at
least the following in your `".vimrc"` file:

  `" Syntax highlighting`                      
  `syntax on`                                  
                                               
  `" These lines make vim load various plugins`
  `filetype on`                                
  `filetype indent on`                         
  `filetype plugin on`                         
                                               
  `" No tabs!`                                 
  `set expandtab`                              

Indentation

You can enable indentation for Racket by setting both the `lisp` and
`autoindent` options in Vim. However, the indentation is limited and not
as complete as what you can get in Emacs. You can also use Dorai
Sitaram’s [scmindent](https://github.com/ds26gte/scmindent) for better
indentation of Racket code. The instructions on how to use the indenter
are available on the website.

If you use the built-in indenter, you can customize it by setting how to
indent certain keywords. The vim-racket plugin mentioned above sets some
default keywords for you. You can add keywords yourself in your
`".vimrc"` file like this:

  `" By default vim will indent arguments after the function name`               
  `" but sometimes you want to only indent by 2 spaces similar to`               
  `" how DrRacket indents define. Set the `lispwords' variable to`               
  `" add function names that should have this type of indenting.`                
                                                                                 
  `set                                                                           
lispwords+=public-method,override-method,private-method,syntax-case,syntax-rules`
  `set lispwords+=..more..`                                                      

Highlighting

The [Rainbow
Parenthesis](http://www.vim.org/scripts/script.php?script_id=1230)
script for vim can be useful for more visible parenthesis matching.
Syntax highlighting for Scheme is shipped with vim on many platforms,
which will work for the most part with Racket. The vim-racket script
provides good default highlighting settings for you.

Structured Editing

The [Slimv](http://www.vim.org/scripts/script.php?script_id=2531) plugin
has a paredit mode that works like paredit in Emacs. However, the plugin
is not aware of Racket. You can either set vim to treat Racket as Scheme
files or you can modify the paredit script to load on `".rkt"` files.

Scribble

Vim support for writing scribble documents is provided by the
[scribble.vim](http://www.vim.org/scripts/script.php?script_id=3756)
plugin.

Miscellaneous

If you are installing many vim plugins \(not necessary specific to
Racket\), we recommend using a plugin that will make loading other
plugins easier.
[Pathogen](http://www.vim.org/scripts/script.php?script_id=2332) is one
plugin that does this; using it, you can install new plugins by
extracting them to subdirectories in the `"bundle"` folder of your Vim
installation.

### 24.4. Sublime Text

The [Racket package](https://sublime.wbond.net/packages/Racket) provides
support for syntax highlighting and building for Sublime Text.

## Bibliography

\[Goldberg04\] David Goldberg, Robert Bruce Findler, and Matthew Flatt, “Super and                                                   
               Inner—Together at Last!,” Object-Oriented Programming, Languages,                                                     
               Systems, and Applications, 2004.                                                                                      
               [`http://www.cs.utah.edu/plt/publications/oopsla04-gff.pdf`](http://www.cs.utah.edu/plt/publications/oopsla04-gff.pdf)
\[Flatt02\]    Matthew Flatt, “Composable and Compilable Macros: You Want it When?,”                                                 
               International Conference on Functional Programming, 2002.                                                             
\[Flatt06\]    Matthew Flatt, Robert Bruce Findler, and Matthias Felleisen, “Scheme                                                  
               with Classes, Mixins, and Traits \(invited tutorial\),” Asian Symposium                                               
               on Programming Languages and Systems, 2006.                                                                           
               [`http://www.cs.utah.edu/plt/publications/aplas06-fff.pdf`](http://www.cs.utah.edu/plt/publications/aplas06-fff.pdf)  
\[Mitchell02\] Richard Mitchell and Jim McKim, _Design by Contract, by Example_. 2002.                                               
\[Sitaram05\]  Dorai Sitaram, “pregexp: Portable Regular Expressions for Scheme and                                                  
               Common Lisp.” 2002.                                                                                                   
               [`http://www.ccs.neu.edu/home/dorai/pregexp/`](http://www.ccs.neu.edu/home/dorai/pregexp/)                            

## Index

[A](#alpha:A) [B](#alpha:B) [C](#alpha:C) [D](#alpha:D) [E](#alpha:E)
[F](#alpha:F) [G](#alpha:G) [H](#alpha:H) [I](#alpha:I) [J](#alpha:J)
[K](#alpha:K) [L](#alpha:L) [M](#alpha:M) [N](#alpha:N) [O](#alpha:O)
[P](#alpha:P) [Q](#alpha:Q) [R](#alpha:R) [S](#alpha:S) [T](#alpha:T)
[U](#alpha:U) [V](#alpha:V) [W](#alpha:W) X Y Z                      
                                                                     
`#!`                                                                 
`.bat`                                                               
.zo                                                                  
_3m_                                                                 
A Customer-Manager Component                                         
A Dictionary                                                         
A Note to Readers with Lisp/Scheme Experience                        
A Parameteric \(Simple\) Stack                                       
A Queue                                                              
Abbreviating `quote` with `'`                                        
_aborts_                                                             
Abstract Contracts using `#:exists` and `#:∃`                        
_accessor_                                                           
Adding Collections                                                   
Adding Contracts to Signatures                                       
Adding Contracts to Units                                            
Additional Examples                                                  
Alternation                                                          
An Aside on Indenting Code                                           
An Extended Example                                                  
And More                                                             
Anonymous Functions with `lambda`                                    
`any` and `any/c`                                                    
Argument and Result Dependencies                                     
Arity-Sensitive Functions: `case-lambda`                             
_arms_                                                               
_assertions_                                                         
Assignment and Redefinition                                          
Assignment: `set!`                                                   
_attached_                                                           
_available_                                                          
_backreference_                                                      
Backreferences                                                       
Backtracking                                                         
_backtracking_                                                       
Basic Assertions                                                     
benchmarking                                                         
_blocking_                                                           
Booleans                                                             
_box_                                                                
Boxes                                                                
_bracketed character class_                                          
Breaking an Iteration                                                
Buffered Asynchronous Channels                                       
Building New Contracts                                               
Building Your Own Synchronization Patterns                           
Built-In Datatypes                                                   
_byte_                                                               
_byte string_                                                        
Bytes and Byte Strings                                               
Bytes, Characters, and Encodings                                     
_call-by-reference_                                                  
_CGC_                                                                
Chaining Tests: `cond`                                               
Channels                                                             
_character_                                                          
_character class_                                                    
Characters                                                           
Characters and Character Classes                                     
Checking Properties of Data Structures                               
Checking State Changes                                               
Class Contracts                                                      
Classes and Objects                                                  
_cloister_                                                           
Cloisters                                                            
_closures_                                                           
_Clustering_                                                         
Clusters                                                             
_code inspectors_                                                    
_collection_                                                         
Combining Tests: `and` and `or`                                      
Command-Line Tools                                                   
Command-Line Tools and Your Editor of Choice                         
comments                                                             
Compilation and Configuration: `raco`                                
Compile and Run-Time Phases                                          
Compile-Time Instantiation                                           
_complex_                                                            
_components_                                                         
_composable continuations_                                           
_concurrency_                                                        
Concurrency and Synchronization                                      
Conditionals                                                         
Conditionals with `if`, `and`, `or`, and `cond`                      
_conservative garbage collector_                                     
_constructor_                                                        
_constructor guard_                                                  
_continuation_                                                       
Continuations                                                        
Contract boundaries and `define/contract`                            
_contract combinator_                                                
Contract Messages with “???”                                         
Contract Struct Properties                                           
Contract Violations                                                  
Contracts                                                            
Contracts and Boundaries                                             
Contracts and `eq?`                                                  
Contracts for `case-lambda`                                          
Contracts for Units                                                  
Contracts on Functions in General                                    
Contracts on Higher-order Functions                                  
Contracts on Structures                                              
Contracts: A Thorough Example                                        
Controlling the Scope of External Names                              
Copying and Update                                                   
Creating and Installing Namespaces                                   
Creating Executables                                                 
Creating Languages                                                   
Creating Stand-Alone Executables                                     
_CS_                                                                 
_current continuation_                                               
_current namespace_                                                  
Curried Function Shorthand                                           
Datatypes and Serialization                                          
Declaration versus Instantiation                                     
Declaring a Rest Argument                                            
Declaring Keyword Arguments                                          
Declaring Optional Arguments                                         
Default Ports                                                        
_default prompt tag_                                                 
`define-syntax` and `syntax-rules`                                   
`define-syntax-rule`                                                 
Defining new `#lang` Languages                                       
Defining Recursive Contracts                                         
Definitions                                                          
Definitions and Interactions                                         
_definitions area_                                                   
Definitions: `define`                                                
_delimited continuations_                                            
Designating a `#lang` Language                                       
destructing bind                                                     
Dialects of Racket and Scheme                                        
_disarm_                                                             
Dissecting a contract error message                                  
Distributed Places                                                   
_domain_                                                             
_dye pack_                                                           
Dynamic Binding: `parameterize`                                      
Effects After: `begin0`                                              
Effects Before: `begin`                                              
Effects If...: `when` and `unless`                                   
Emacs                                                                
`eval`                                                               
Evaluation Order and Arity                                           
_exception_                                                          
Exceptions                                                           
Exceptions and Control                                               
Exists Contracts and Predicates                                      
_expander_                                                           
_expands_                                                            
Experimenting with Contracts and Modules                             
Experimenting with Nested Contract Boundaries                        
Exports: `provide`                                                   
Expressions and Definitions                                          
Extended Example: Call-by-Reference Functions                        
External Class Contracts                                             
Final, Augment, and Inner                                            
First-Class Units                                                    
Fixed but Statically Unknown Arities                                 
_fixnum_                                                             
Fixnum and Flonum Optimizations                                      
_flat named contracts_                                               
_flonum_                                                             
`for` and `for*`                                                     
`for/and` and `for/or`                                               
`for/first` and `for/last`                                           
`for/fold` and `for*/fold`                                           
`for/list` and `for*/list`                                           
`for/vector` and `for*/vector`                                       
Foreign Pointers                                                     
Function Calls \(Procedure Applications\)                            
Function Calls \(Procedure Applications\)                            
Function Calls, Again                                                
Function Shorthand                                                   
Function-Call Optimizations                                          
_functional update_                                                  
Functions \(Procedures\): `lambda`                                   
_futures_                                                            
General Macro Transformers                                           
General Phase Levels                                                 
_generational garbage collector_                                     
Gotchas                                                              
Graphics and GUIs                                                    
_greedy_                                                             
Guarantees for a Specific Value                                      
Guarantees for All Values                                            
Guidelines for Using Assignment                                      
_hash table_                                                         
Hash Tables                                                          
I/O Patterns                                                         
_identifier macro_                                                   
Identifier Macros                                                    
_identifier syntax object_                                           
Identifiers                                                          
Identifiers and Binding                                              
_implicit begin_                                                     
Implicit Form Bindings                                               
Imports: `require`                                                   
_incremental garbage-collection_                                     
_index pairs_                                                        
Inherit and Super in Traits                                          
Initialization Arguments                                             
Input and Output                                                     
Installing a Language                                                
_instantiated_                                                       
_instantiates_                                                       
_instantiation_                                                      
_integer_                                                            
Interacting with Racket                                              
Interactive evaluation                                               
Interactive Mode                                                     
Interfaces                                                           
Internal and External Names                                          
Internal Class Contracts                                             
Internal Definitions                                                 
_invoked_                                                            
Invoking Units                                                       
Iteration Performance                                                
Iterations and Comprehensions                                        
_JIT_                                                                
_just-in-time_                                                       
_keyword_                                                            
Keyword Arguments                                                    
Keyword Arguments                                                    
Keywords                                                             
Lazy Visits via Available Modules                                    
`letrec` Performance                                                 
Lexical Scope                                                        
Library Collections                                                  
_link_                                                               
Linking Units                                                        
_list_                                                               
List Iteration from Scratch                                          
Lists and Racket Syntax                                              
Lists, Iteration, and Recursion                                      
Load Mode                                                            
Local Binding                                                        
Local Binding with `define`, `let`, and `let*`                       
Local Scopes                                                         
Lookahead                                                            
Lookbehind                                                           
Looking Ahead and Behind                                             
_macro_                                                              
_macro pattern variables_                                            
_macro transformer_                                                  
Macro Transformer Procedures                                         
_macro-generating macro_                                             
Macro-Generating Macros                                              
Macros                                                               
Main and Test Submodules                                             
`main` submodule                                                     
_major collections_                                                  
Major Modes                                                          
Manipulating Namespaces                                              
Matching Regexp Patterns                                             
Matching Sequences                                                   
Memory Management                                                    
_meta-compile phase level_                                           
_metacharacters_                                                     
_metasequences_                                                      
Methods                                                              
_minor collections_                                                  
Minor Modes                                                          
_mixin_                                                              
Mixing Patterns and Expressions: `syntax-case`                       
Mixing `set!` and `contract-out`                                     
Mixins                                                               
Mixins and Interfaces                                                
Module Basics                                                        
Module Instantiations and Visits                                     
_module language_                                                    
Module Languages                                                     
Module Mode                                                          
_module path_                                                        
Module Paths                                                         
Module Syntax                                                        
Module-Handling Configuration                                        
Modules                                                              
Modules and Macros                                                   
Modules and Performance                                              
More Libraries                                                       
More Rackets                                                         
More Structure Type Options                                          
_multi-line mode_                                                    
Multiple Result Values                                               
Multiple Values and `define-values`                                  
Multiple Values: `let-values`, `let*-values`, `letrec-values`        
Multiple Values: `set!-values`                                       
Multiple-Valued Sequences                                            
_mutable pair_                                                       
Mutation and Performance                                             
_mutator_                                                            
Named `let`                                                          
_namespace_                                                          
Namespaces                                                           
Namespaces and Modules                                               
_non-capturing_                                                      
Non-capturing Clusters                                               
_non-greedy_                                                         
Notation                                                             
_number_                                                             
Numbers                                                              
_opaque_                                                             
Opaque versus Transparent Structure Types                            
Optional Arguments                                                   
Optional Keyword Arguments                                           
Organizing Modules                                                   
_package_                                                            
Packages and Collections                                             
Packages specific to Evil Mode                                       
_pair_                                                               
Pairs and Lists                                                      
Pairs, Lists, and Racket Syntax                                      
Parallel Binding: `let`                                              
Parallelism                                                          
_parallelism_                                                        
Parallelism with Futures                                             
Parallelism with Places                                              
_parameter_                                                          
Parameterized Mixins                                                 
Pattern Matching                                                     
_pattern variables_                                                  
_pattern-based macro_                                                
Pattern-Based Macros                                                 
Performance                                                          
Performance in DrRacket                                              
_phase_                                                              
_phase level_                                                        
_phase level -1_                                                     
_phase level 2_                                                      
Phases and Bindings                                                  
Phases and Modules                                                   
_place_                                                              
_place channel_                                                      
_port_                                                               
_POSIX character class_                                              
POSIX character classes                                              
Predefined List Loops                                                
_predicate_                                                          
_prefab_                                                             
Prefab Structure Types                                               
Programmer-Defined Datatypes                                         
_prompt_                                                             
_prompt tag_                                                         
Prompts and Aborts                                                   
_property_                                                           
_protected_                                                          
Protected Exports                                                    
protected method                                                     
Quantifiers                                                          
_quantifiers_                                                        
Quasiquoting: `quasiquote` and `‘`                                   
Quoting Pairs and Symbols with `quote`                               
Quoting: `quote` and `'`                                             
R5RS                                                                 
R6RS                                                                 
Racket Essentials                                                    
`racket/exists`                                                      
_range_                                                              
_rational_                                                           
Reachability and Garbage Collection                                  
_reader_                                                             
Reader Extensions                                                    
Reading and Writing Racket Data                                      
_readtable_                                                          
Readtables                                                           
_real_                                                               
Recursion versus Iteration                                           
Recursive Binding: `letrec`                                          
Reducing Garbage Collection Pauses                                   
Reflection and Dynamic Evaluation                                    
_regexp_                                                             
Regular Expression Performance                                       
Regular Expressions                                                  
_REPL_                                                               
_rest argument_                                                      
Rest Arguments                                                       
Rolling Your Own Contracts                                           
_run-time configuration_                                             
Running and Creating Executables                                     
Running `racket` and `gracket`                                       
S-expression                                                         
Scripting Evaluation and Using `load`                                
Scripts                                                              
Semaphores                                                           
Sequence Constructors                                                
Sequencing                                                           
Sequential Binding: `let*`                                           
_serialization_                                                      
`set!` Transformers                                                  
_shadows_                                                            
Sharing Data and Code Across Namespaces                              
Shell completion                                                     
_signatures_                                                         
Signatures and Units                                                 
Simple Branching: `if`                                               
Simple Contracts on Functions                                        
Simple Definitions and Expressions                                   
Simple Dispatch: `case`                                              
Simple Structure Types: `struct`                                     
Simple Values                                                        
Some Frequently Used Character Classes                               
Source Locations                                                     
Source-Handling Configuration                                        
speed                                                                
Standards                                                            
_string_                                                             
Strings \(Unicode\)                                                  
Structure Comparisons                                                
Structure Subtypes                                                   
_structure type descriptor_                                          
Structure Type Generativity                                          
Styles of `->`                                                       
_subcluster_                                                         
Sublime Text                                                         
_submatch_                                                           
_submodule_                                                          
Submodules                                                           
_subpattern_                                                         
_symbol_                                                             
Symbols                                                              
Synchronizable Events and `sync`                                     
_synchronized_                                                       
Syntax Objects                                                       
_syntax objects_                                                     
Syntax Taints                                                        
_tail position_                                                      
Tail Recursion                                                       
_tainted_                                                            
Tainting Modes                                                       
Taints and Code Inspectors                                           
Teaching                                                             
_template_                                                           
_template phase level_                                               
_text string_                                                        
The `#lang` Shorthand                                                
The `apply` Function                                                 
The Bytecode and Just-in-Time \(JIT\) Compilers                      
The `mixin` Form                                                     
The `module` Form                                                    
The Racket Guide                                                     
The `trait` Form                                                     
The Web Server                                                       
Thread Mailboxes                                                     
Threads                                                              
_threads_                                                            
Traits                                                               
Traits as Sets of Mixins                                             
_transformer_                                                        
_transformer binding_                                                
_transparent_                                                        
Unchecked, Unsafe Operations                                         
`unit` versus `module`                                               
_Units_                                                              
Units \(Components\)                                                 
Unix Scripts                                                         
Using `#lang reader`                                                 
Using `#lang s-exp`                                                  
Using `#lang s-exp syntax/module-reader`                             
Using `define/contract` and `->`                                     
Using Foreign Libraries                                              
Varieties of Ports                                                   
_vector_                                                             
Vectors                                                              
Vim                                                                  
_visit_                                                              
Visiting Modules                                                     
Void and Undefined                                                   
Weak Boxes and Testing                                               
Welcome to Racket                                                    
Whole-`module` Signatures and Units                                  
Windows Batch Files                                                  
With all the Bells and Whistles                                      
`with-syntax` and `generate-temporaries`                             
Writing Regexp Patterns