Scheme

Scheme is a minimalist, multi-paradigm programming language in the Lisp family. It emphasizes a small, clean core and the use of first-class procedures, enabling expressive higher-order programming. Scheme was developed in the 1970s by Guy L. Steele Jr. and Gerald Jay Sussman at the MIT Artificial Intelligence Laboratory, with the aim of exploring fundamental programming-language concepts and providing a practical Lisp dialect for teaching and research.

Core features of Scheme include lexical scoping, first-class and first-class continuations, and tail call optimization. Its syntax is based on S-expressions and prefix notation, which promotes homoiconicity—the idea that code and data share the same structure. Scheme also supports a powerful macro system that is designed to be hygienic, commonly implemented through constructs such as define-syntax and syntax-rules. Procedures in Scheme are first-class values and can be passed as arguments, returned from functions, and stored in data structures. The language also provides facilities for structured programming and a minimalist standard library.

Scheme has undergone several standards, beginning with R3RS and continuing through R5RS, R6RS, and R7RS. The standards define core language features and, in later editions, separate the specifications into small and large dialects. Because of the minimal core, many implementations provide extensions or libraries that vary across environments.

Implementations and influence: Scheme has many implementations, including MIT Scheme, Chez Scheme, Gambit, Guile, Scheme48, and Racket (originally PLT Scheme), the latter being a prominent successor that broadens Scheme beyond the original scope. Scheme has influenced many other languages and remains widely used in education and research to teach concepts such as recursion, lexical scope, macros, and functional programming.