Racket is a general-purpose,

multi-paradigm programming language Programming languages can be grouped by the number and types of Programming paradigm, paradigms supported. Paradigm summaries A concise reference for the programming paradigms listed in this article. * Concurrent programming language, Concurrent ...

. The Racket language is a modern dialect of

Lisp Lisp (historically LISP, an abbreviation of "list processing") is a family of programming languages with a long history and a distinctive, fully parenthesized Polish notation#Explanation, prefix notation. Originally specified in the late 1950s, ...

and a descendant of Scheme. It is designed as a platform for

programming language design Programming languages are typically created by designing a form of representation of a computer program, and writing an implementation for the developed concept, usually an interpreter or compiler. Interpreters are designed to read programs, usu ...

and implementation. In addition to the core Racket language, ''Racket'' is also used to refer to the family of programming languages and set of tools supporting development on and with Racket. Racket is also used for scripting,

computer science Computer science is the study of computation, information, and automation. Computer science spans Theoretical computer science, theoretical disciplines (such as algorithms, theory of computation, and information theory) to Applied science, ...

education, and research. The Racket platform provides an implementation of the Racket language (including a

runtime system In computer programming, a runtime system or runtime environment is a sub-system that exists in the computer where a program is created, as well as in the computers where the program is intended to be run. The name comes from the compile time ...

, libraries, and

compiler In computing, a compiler is a computer program that Translator (computing), translates computer code written in one programming language (the ''source'' language) into another language (the ''target'' language). The name "compiler" is primaril ...

supporting several compilation modes: machine code, machine-independent, interpreted, and JIT) along with the DrRacket

integrated development environment An integrated development environment (IDE) is a Application software, software application that provides comprehensive facilities for software development. An IDE normally consists of at least a source-code editor, build automation tools, an ...

(IDE) written in Racket. Racket is used by the ProgramByDesign outreach program, which aims to turn

into "an indispensable part of the

liberal arts Liberal arts education () is a traditional academic course in Western higher education. ''Liberal arts'' takes the term ''skill, art'' in the sense of a learned skill rather than specifically the fine arts. ''Liberal arts education'' can refe ...

curriculum In education, a curriculum (; : curriculums or curricula ) is the totality of student experiences that occur in an educational process. The term often refers specifically to a planned sequence of instruction, or to a view of the student's experi ...

". The core Racket language is known for its extensive macro system which enables creating embedded and domain-specific languages, language constructs such as classes or modules, and separate dialects of Racket with different

semantics Semantics is the study of linguistic Meaning (philosophy), meaning. It examines what meaning is, how words get their meaning, and how the meaning of a complex expression depends on its parts. Part of this process involves the distinction betwee ...

. The platform distribution is

free and open-source software Free and open-source software (FOSS) is software available under a license that grants users the right to use, modify, and distribute the software modified or not to everyone free of charge. FOSS is an inclusive umbrella term encompassing free ...

distributed under the Apache 2.0 and

MIT license The MIT License is a permissive software license originating at the Massachusetts Institute of Technology (MIT) in the late 1980s. As a permissive license, it puts very few restrictions on reuse and therefore has high license compatibility. Unl ...

s. Extensions and packages written by the community may be uploaded to Racket's package catalog.

History

Development

Matthias Felleisen founded PLT Inc. in the mid 1990s, first as a research group, soon after as a project dedicated to producing pedagogic materials for novice programmers (lectures, exercises/projects, software). In January 1995, the group decided to develop a pedagogic programming environment based on Scheme. Matthew Flatt cobbled together MrEd, the original

virtual machine In computing, a virtual machine (VM) is the virtualization or emulator, emulation of a computer system. Virtual machines are based on computer architectures and provide the functionality of a physical computer. Their implementations may involve ...

for Racket, from libscheme,

wxWidgets wxWidgets (formerly wxWindows) is a widget toolkit and tools library for creating graphical user interfaces (GUIs) for cross-platform applications. wxWidgets enables a program's GUI code to compile and run on several computer platforms with no s ...

, and a few other free systems. In the years that followed, a team including Flatt, Robby Findler, Shriram Krishnamurthi, Cormac Flanagan, and many others produced DrScheme, a programming environment for novice Scheme programmers and a research environment for gradual typing. The main development language that DrScheme supported was named PLT Scheme. In parallel, the team began conducting workshops for high school teachers, training them in program design and functional programming. Field tests with these teachers and their students provided essential clues for directing the development. Over the following years, PLT added teaching languages, an algebraic stepper, a transparent

read–eval–print loop A read–eval–print loop (REPL), also termed an interactive toplevel or language shell, is a simple interactive computer programming environment that takes single user inputs, executes them, and returns the result to the user; a program written ...

, a constructor-based printer, and many other innovations to DrScheme, producing an application-quality pedagogic program development environment. By 2001, the core team (Felleisen, Findler, Flatt, Krishnamurthi) had also written and published their first textbook, '' How to Design Programs'', based on their teaching philosophy. ''The Racket Manifesto'' details the principles driving the development of Racket, presents the evaluation framework behind the design process, and details opportunities for future improvements.

Version history

The first generation of PLT Scheme revisions introduced features for programming in the large with both modules and classes. Version 42 introduced units – a first-class module system – to complement classes for large scale development. The class system gained features (e.g.

Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...

-style interfaces) and also lost several features (e.g. multiple inheritance) throughout these versions. The language evolved throughout a number of successive versions, and gaining milestone popularity in Version 53, leading to extensive work and the following Version 100, which would be equivalent to a "1.0" release in current popular version systems. The next major revision was named Version 200, which introduced a new default module system that cooperates with macros. In particular, the module system ensures that run-time and compile-time computation are separated to support a "tower of languages". Unlike units, these modules are not first-class objects. Version 300 introduced

Unicode Unicode or ''The Unicode Standard'' or TUS is a character encoding standard maintained by the Unicode Consortium designed to support the use of text in all of the world's writing systems that can be digitized. Version 16.0 defines 154,998 Char ...

support, foreign library support, and refinements to the class system. Later on, the 300 series improved the

performance A performance is an act or process of staging or presenting a play, concert, or other form of entertainment. It is also defined as the action or process of carrying out or accomplishing an action, task, or function. Performance has evolved glo ...

of the language runtime with an addition of a JIT compiler and a switch to a default generational garbage collection. By the next major release, the project had switched to a more conventional sequence-based version numbering. Version 4.0 introduced the #lang shorthand to specify the language that a module is written in. Further, the revision introduced immutable pairs and lists, support for fine-grained parallelism, and a statically-typed dialect. On 7 June 2010, PLT Scheme was renamed Racket. The renaming coincided with the release of Version 5.0. Subsequently, the

graphical user interface A graphical user interface, or GUI, is a form of user interface that allows user (computing), users to human–computer interaction, interact with electronic devices through Graphics, graphical icon (computing), icons and visual indicators such ...

(GUI) backend was rewritten in Racket from C++ in Version 5.1 using native UI toolkits on all platforms. Version 5.2 included a background syntax checking tool, a new plotting library, a database library, and a new extended REPL. Version 5.3 included a new submodule feature for optionally loaded modules, new

optimization Mathematical optimization (alternatively spelled ''optimisation'') or mathematical programming is the selection of a best element, with regard to some criteria, from some set of available alternatives. It is generally divided into two subfiel ...

tools, a

JSON JSON (JavaScript Object Notation, pronounced or ) is an open standard file format and electronic data interchange, data interchange format that uses Human-readable medium and data, human-readable text to store and transmit data objects consi ...

library, and other features. Version 5.3.1 introduced major improvements to DrRacket: the background syntax checker was turned on by default and a new documentation preview tool was added. In version 6.0, Racket released its second-generation package management system. As part of this development, the principal DrRacket and Racket repository was reorganized and split into a large set of small packages, making it possible to install a ''minimal racket'' and to install only those packages needed. Version 7 of Racket was released with a new macro expander written in Racket as part the preparations for supporting moving to the Chez Scheme runtime system and supporting multiple runtime systems. On 19 November 2019, Racket 7.5 was released. The license of Racket 7.5 was less restrictive. They use now either the Apache 2.0 license or the MIT license. On 2021 February 13, Racket 8.0 was released. Racket 8.0 marks the first release where Racket with the Chez Scheme runtime system, known as Racket CS, is the default implementation. Racket CS is faster, easier to maintain and develop, backward-compatible with existing Racket programs, and has better parallel garbage collection.

Features

Racket's core language includes macros, modules, lexical closures, tail calls, delimited continuations, parameters (fluid variables), software contracts, green threads and OS threads, and more. The language also comes with primitives, such as eventspaces and custodians, which control resource management and enables the language to act like an

operating system An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

for loading and managing other programs. Further extensions to the language are created with the powerful macro system, which together with the module system and custom parsers can control all aspects of a language. Most language constructs in Racket are implemented as macros in the base language. These include a mixin class system, a component (or module) system as expressive as opaque ascription in the ML module system, and

pattern matching In computer science, pattern matching is the act of checking a given sequence of tokens for the presence of the constituents of some pattern. In contrast to pattern recognition, the match usually must be exact: "either it will or will not be a ...

. Further, the language features the first contract system for a higher-order programming language. Racket's contract system is inspired by the Design by Contract work for Eiffel and extends it to work for higher-order values such as

first-class function In computer science, a programming language is said to have first-class functions if it treats function (programming), functions as first-class citizens. This means the language supports passing functions as arguments to other functions, returning ...

s, objects,

reference A reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a ''nam ...

cells, and so on. For example, an object that is checked by a contract can be ensured to make contract checks when its methods are eventually invoked. Racket includes both

bytecode Bytecode (also called portable code or p-code) is a form of instruction set designed for efficient execution by a software interpreter. Unlike human-readable source code, bytecodes are compact numeric codes, constants, and references (normal ...

and JIT (JIT) compilers. The bytecode compiler produces an internal bytecode format run by the Racket

, and the JIT compiler translates bytecode to machine code at runtime. Since 2004, the language has also shipped with PLaneT, a package manager that is integrated into the module system so that third-party libraries can be transparently imported and used. Also, PLaneT has a built-in versioning policy to prevent

dependency hell Dependency hell is a colloquial term for the frustration of some software users who have installed software packages which have dependencies on specific versions of other software packages. The dependency issue arises when several packages ha ...

. At the end of 2014, much of Racket's code was moved into a new packaging system separate from the main code base. This new packaging system is serviced by a client program named ''raco''. The new package system provides fewer features than PLaneT; a blog post by Jay McCarthy on the Racket blog explains the rationale for the change and how to duplicate the older system.

Integrated language extensibility and macros

The features that most clearly distinguish Racket from other languages in the Lisp family are its integrated language

extensibility Extensibility is a software engineering and systems design principle that provides for future growth. Extensibility is a measure of the ability to extend a system and the level of effort required to implement the extension. Extensions can be t ...

features that support building new domain-specific and general-purpose languages. Racket's extensibility features are built into the module system to allow context-sensitive and module-level control over syntax. For example, the #%app syntactic form can be overridden to change the semantics of

function application In mathematics, function application is the act of applying a function to an argument from its domain so as to obtain the corresponding value from its range. In this sense, function application can be thought of as the opposite of function abs ...

. Similarly, the #%module-begin form allows arbitrary static analysis of the entire module. Since any module can be used as a language, via the #lang notation, this effectively means that virtually any aspect of the language can be programmed and controlled. The module-level extensibility features are combined with a Scheme-like hygienic macro system, which provides more features than Lisp's s-expression manipulation system, Scheme 84's hygienic extend-syntax macros, or R5RS's syntax-rules. Indeed, it is fair to say that the macro system is a carefully tuned

application programming interface An application programming interface (API) is a connection between computers or between computer programs. It is a type of software Interface (computing), interface, offering a service to other pieces of software. A document or standard that des ...

(API) for

extensions. Using this compiler API, programmers can add features and entire domain-specific languages in a manner that makes them completely indistinguishable from built-in language constructs. The ''macro'' system in Racket has been used to construct entire language dialects. This includes Typed Racket, which is a gradually typed dialect of Racket that eases the migration from untyped to typed code, Lazy Racket—a dialect with

lazy evaluation In programming language theory, lazy evaluation, or call-by-need, is an evaluation strategy which delays the evaluation of an Expression (computer science), expression until its value is needed (non-strict evaluation) and which avoids repeated eva ...

, and Hackett, which combines Haskell and Racket. The pedagogical programming language Pyret was originally implemented in Racket. Other dialects include FrTime ( functional reactive programming), Scribble (documentation language), Slideshow (

presentation A presentation conveys information from a speaker to an audience. Presentations are typically demonstrations, introduction, lecture, or speech meant to inform, persuade, inspire, motivate, build goodwill, or present a new idea/product. Presenta ...

language), and several languages for education. Racket's core distribution provides libraries to aid the development of programming languages. Such languages are not restricted to

s-expression In computer programming, an S-expression (or symbolic expression, abbreviated as sexpr or sexp) is an expression in a like-named notation for nested List (computing), list (Tree (data structure), tree-structured) data. S-expressions were invented ...

based syntax. In addition to conventional readtable-based syntax extensions, the directive #lang enables the invocation of arbitrary parsers, which can be implemented using the parser tools library. See Racket logic programming for an example of such a language.

Programming environment

The language platform provides a self-hosted IDE named DrRacket, a continuation-based

web server A web server is computer software and underlying Computer hardware, hardware that accepts requests via Hypertext Transfer Protocol, HTTP (the network protocol created to distribute web content) or its secure variant HTTPS. A user agent, co ...

, a

, and other tools. As a viable scripting tool with libraries like common

scripting language In computing, a script is a relatively short and simple set of instructions that typically automation, automate an otherwise manual process. The act of writing a script is called scripting. A scripting language or script language is a programming ...

s, it can be used for scripting the Unix shell. It can parse command-line arguments and execute external tools.

DrRacket IDE

DrRacket (formerly DrScheme) is widely used among introductory computer science courses that teach Scheme or Racket and is lauded for its simplicity and appeal to beginner programmers. The IDE was originally built for use with the TeachScheme! project (now ProgramByDesign), an outreach effort by

Northeastern University Northeastern University (NU or NEU) is a private university, private research university with its main campus in Boston, Massachusetts, United States. It was founded by the Boston Young Men's Christian Association in 1898 as an all-male instit ...

and a number of affiliated universities for attracting high school students to computer science courses at the college level. The editor provides highlighting for syntax and run-time errors, parenthesis matching, a

debugger A debugger is a computer program used to test and debug other programs (the "target" programs). Common features of debuggers include the ability to run or halt the target program using breakpoints, step through code line by line, and display ...

and an algebraic stepper. Its student-friendly features include support for multiple "language levels" (Beginning Student, Intermediate Student and so on). It also has integrated library support, and sophisticated

analysis Analysis (: analyses) is the process of breaking a complex topic or substance into smaller parts in order to gain a better understanding of it. The technique has been applied in the study of mathematics and logic since before Aristotle (38 ...

tools for advanced programmers. Further, module-oriented programming is supported with the module browser, a contour view, integrated testing and coverage measurements, and refactoring support. It provides integrated, context-sensitive access to an extensive hyper-linked help system named "Help Desk". DrRacket is available for

Windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...

macOS macOS, previously OS X and originally Mac OS X, is a Unix, Unix-based operating system developed and marketed by Apple Inc., Apple since 2001. It is the current operating system for Apple's Mac (computer), Mac computers. With ...

Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...

, and

Linux Linux ( ) is a family of open source Unix-like operating systems based on the Linux kernel, an kernel (operating system), operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically package manager, pac ...

with the

X Window System The X Window System (X11, or simply X) is a windowing system for bitmap displays, common on Unix-like operating systems. X originated as part of Project Athena at Massachusetts Institute of Technology (MIT) in 1984. The X protocol has been at ...

and programs behave similarly on all these platforms.

Code examples

Here is a trivial

"Hello, World!" program A "Hello, World!" program is usually a simple computer program that emits (or displays) to the screen (often the Console application, console) a message similar to "Hello, World!". A small piece of code in most general-purpose programming languag ...

: #lang racket "Hello, World!" Running this program produces the output: : Here is a slightly less trivial program: Sierpinski Racket example

#lang racket (require 2htdp/image) (let sierpinski ( 8 (if (zero? n) (triangle 2 'solid 'red) (let ( (sierpinski (- n 1)) (freeze (above t (beside t t)))))) This program, taken from the Racket website, draws a Sierpinski triangle, nested to depth 8. Using the #lang directive, a source file can be written in different dialects of Racket. Here is an example of the factorial program in Typed Racket, a

statically typed In computer programming, a type system is a logical system comprising a set of rules that assigns a property called a ''type'' (for example, integer, floating point, string) to every '' term'' (a word, phrase, or other set of symbols). Usu ...

dialect of Racket: #lang typed/racket (: fact (Integer -> Integer)) (define (fact n) (if (zero? n) 1 (* n (fact (- n 1)))))

Implementations

Racket currently has two implementations. Both support Linux, Windows and MacOS on a variety of architectures and are supported as at version 8.8 (2023). The default implementation uses the Chez Scheme incremental compiler and runtime. The alternate implementation generates platform-independent bytecode and uses Just-in-time compilation to generate machine code as it is loaded. In addition, there are experimental implementations: * RacketScript is an experimental Racket to JavaScript (ES6) compiler. It allows programmers to use both JavaScript’s and Racket’s ecosystem and aims to make this interoperability as smooth as possible. * Pycket is a Racket implementation generated using the RPython framework.

Applications and practical use

Apart from having a basis in

programming language theory Programming language theory (PLT) is a branch of computer science that deals with the design, implementation, analysis, characterization, and classification of formal languages known as programming languages. Programming language theory is clos ...

, Racket was designed as a general-purpose language for production systems. Thus, the Racket distribution features an extensive library that covers systems and network programming, web development, a uniform interface to the underlying operating system, a dynamic foreign function interface, several flavours of

regular expressions A regular expression (shortened as regex or regexp), sometimes referred to as rational expression, is a sequence of character (computing), characters that specifies a pattern matching, match pattern in string (computer science), text. Usually ...

, lexer/parser generators,

logic programming Logic programming is a programming, database and knowledge representation paradigm based on formal logic. A logic program is a set of sentences in logical form, representing knowledge about some problem domain. Computation is performed by applyin ...

, and a complete GUI framework. Racket has several features useful for a commercial language, among them an ability to compile standalone executables under Windows, macOS, and Unix, a profiler and

included in the

(IDE), and a

unit testing Unit testing, component or module testing, is a form of software testing by which isolated source code is tested to validate expected behavior. Unit testing describes tests that are run at the unit-level to contrast testing at the Integration ...

framework. Racket has been used for commercial projects and web applications. A notable example is the Hacker News website, which runs on Arc, which is developed in Racket. Naughty Dog has used it as a scripting language in several video games. Racket is used to teach students algebra through game design in the Bootstrap program.

References

External links