typesetting/quad/qtest/mds/welcome.md

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 [missing] 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 Racket commands 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 [missing]. 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).

missing

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. 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:

> 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:

> "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:

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

2. Definitions and Interactions

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

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

> (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:

#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.

> (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.

3. Creating Executables

If your file or definitions area in DrRacket contains

#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 [missing] 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.

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

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

into "extract.rktl" and run racket with

> (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.