Scheme is a minimalist dialect of the Lisp programming language, designed to facilitate a functional programming style while promoting a simple and clean syntax. It is characterized by its use of parentheses, a powerful macro system, and a strong emphasis on recursion and first-class procedures. The language encourages a functional programming paradigm and supports multiple programming techniques, including functional, imperative, and logic programming.
Scheme was created in the 1970s by Gerald Jay Sussman and Guy L. Steele Jr. at the Massachusetts Institute of Technology (MIT). It was developed as an attempt to simplify and improve upon the original Lisp language, which had grown complex over time. The motivation behind Scheme was to create a language that would be easier to implement and teach, while still powerful enough to express complex ideas.
Over the years, Scheme has undergone various revisions and standardization efforts. The most notable of these standards are the RnRS (Revisedn Reports on the Algorithmic Language Scheme), which formalized the language and its features. The Scheme community has since continued to develop the language, leading to newer standards like R6RS and R7RS, which introduced new features and improvements.
As of now, Scheme remains popular in academic environments, particularly in computer science education, due to its elegance and clean syntax. Its influence can be seen in many modern programming languages and paradigms. Scheme is often associated with the functional programming community and has inspired languages such as Clojure and Racket, which built upon its principles and extended its capabilities.
Scheme uses a fully parenthetical syntax where code is written in prefix notation; for instance, an addition operation is written as (+ 1 2).
Functions in Scheme are first-class citizens, meaning they can be passed as arguments or returned from other functions. Example:
(define (make-adder x)
(lambda (y) (+ x y)))
(define add5 (make-adder 5))
(add5 10) ; returns 15
Scheme supports tail call optimization, allowing functions to reuse the current stack frame for recursive calls that occur in tail position, preventing stack overflow.
Scheme allows the use of continuations, enabling programmers to capture the current state of computation and manipulate control flow:
(call-with-current-continuation
(lambda (exit)
(exit 'done)))
Scheme has a powerful macro system that allows programmers to create syntactic extensions. A simple macro can be defined as follows:
(define-syntax my-if
(syntax-rules ()
((_ test then else)
(if test then else))))
Scheme supports data abstraction through structures like lists, pairs, and higher-order functions, enabling efficient manipulation of data.
Scheme employs lexical scoping, where the scope of a variable is determined by its physical location in the source code. This leads to predictable behavior regarding variable bindings.
Scheme is dynamically typed, allowing variables to hold values of any type without the need for explicit type declarations.
As a Lisp dialect, Scheme is optimized for list processing, making it easy to perform operations on lists:
(define my-list (list 1 2 3 4))
(car my-list) ; returns 1
(cdr my-list) ; returns (2 3 4)
Scheme functions can be defined recursively with tail position to optimize memory usage and performance. For example:
(define (factorial n)
(define (fact-helper n acc)
(if (zero? n)
acc
(fact-helper (- n 1) (* n acc))))
(fact-helper n 1))
Scheme applications can generally be built using the chosen implementation's runtime environment. For example, in Racket, you can use the command-line tool racket to run .rkt files or compile them into executables.
Several compilers and interpreters are available for Scheme, including:
Scheme is widely used in academia for teaching programming principles and as a tool for research in computer science. It has applications in artificial intelligence, language design, and scripting, as well as in various domains requiring symbolic computation or complex data manipulation.
Both languages support a functional programming style, but Python emphasizes readability and simplicity, while Scheme focuses on powerful and concise expressions. Scheme's parentheses-heavy syntax may be seen as a barrier for beginners, whereas Python’s indentation-based syntax is generally more accessible.
JavaScript also supports functional programming and has first-class functions. However, JavaScript has a prototype-based object system, while Scheme relies on functional paradigms. Scheme's macro system offers capabilities not present in JavaScript.
C is a low-level, statically typed language designed for system programming, while Scheme is high-level and dynamically typed. C focuses on performance and hardware-level manipulation, while Scheme emphasizes abstraction and theoretical aspects of computation.
Both languages are rooted in functional programming. Haskell uses a more rigid type system and supports lazy evaluation, whereas Scheme's dynamic typing and eager evaluation allow for more flexible programming at the cost of some performance.
Ruby is an object-oriented programming language that mixes functional programming features, whereas Scheme is purely functional. Ruby's syntax is more verbose, while Scheme's simplicity comes from its minimalist design.
The translation of Scheme code to other languages can be facilitated by tools that understand both Scheme syntax and the target language. Some useful source-to-source code translation tools include:
Chicken Scheme provides a compiler that can translate Scheme code into C, making it possible to leverage C libraries and create efficient executables.
Racket, a derivative of Scheme, has built-in capabilities for transforming and compiling code into JavaScript, making it easier to deploy web applications.
Gambit Scheme can compile Scheme code into C and native executables, providing efficient translation options for performance-critical applications.