typesetting/quad/qtest/mds/dialects.md

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.

1 More Rackets
              
2 Standards   
  2.1 R5RS    
  2.2 R6RS    
              
3 Teaching    

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

missing

language from the ground up as described in \[missing\].

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 [missing]. 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.

2. Standards

Standard dialects of Scheme include the ones defined by R5RS and R6RS.

2.1. R5RS

“R5RS” stands for The Revised5 Report on the Algorithmic Language Scheme, 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

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.

2.2. R6RS

“R6RS” stands for The Revised6 Report on the Algorithmic Language Scheme, 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.

3. Teaching

The How to Design Programs textbook relies on pedagogic variants of Racket that smooth the introduction of programming concepts for new programmers. See the How to Design Programs language documentation.

The How to Design Programs languages are typically not used with #lang prefixes, but are instead used within DrRacket by selecting the language from the Choose Language... dialog.