View source for Scheme (programming language)

{{Short description|Dialect of Lisp}}
{{Infobox programming language
| name = Scheme
| logo = Lambda lc.svg
| logo size = 121px
| logo class = skin-invert
| paradigms = [[Multi-paradigm programming language|Multi-paradigm]]: [[Functional programming|functional]], [[Imperative programming|imperative]], [[Metaprogramming|meta]]
| family = [[Lisp (programming language)|Lisp]]
| year = {{Start date and age|1975}}
| designers = [[Guy L. Steele]]<br/>[[Gerald Jay Sussman]]
| latest release version = R7RS
| latest release date = {{Start date and age|2013}}
| latest test version = 
| latest test date = 
| typing = [[Dynamic typing|Dynamic]], [[latent typing|latent]], [[strong typing|strong]]
| scope = [[Scope (computer science)#Lexical scoping and dynamic scoping|Lexical]]
| operating system = 
| license = 
| website = {{URL|https://www.scheme.org/}}
| file_ext = .scm, .ss
| implementations = Many<br/>(see [[:Category:Scheme (programming language) implementations|Scheme implementations]])
| influenced by = [[ALGOL]], [[Lisp (programming language)|Lisp]], [[MDL (programming language)|MDL]]
| influenced = [[Clojure]], [[Common Lisp]], [[Dylan (programming language)|Dylan]], [[EuLisp]], [[Haskell]], [[Hop (software)|Hop]], [[JavaScript]], [[Julia (programming language)|Julia]], [[Lua (programming language)|Lua]], [[MultiLisp]], [[Python (programming language)|Python]], [[Q (programming language)|Q]], [[R (programming language)|R]], [[Racket (programming language)|Racket]], [[Ruby (programming language)|Ruby]], [[Rust (programming language)|Rust]],<ref>{{Cite web |url=https://doc.rust-lang.org/reference/influences.html |title=Influences - The Rust Reference |website=The Rust Reference |access-date=2023-04-18}}</ref> [[S (programming language)|S]], [[Scala (programming language)|Scala]], [[T (programming language)|T]]
| wikibooks = Scheme
}}

'''Scheme''' is a [[programming language dialect|dialect]] of the [[Lisp (programming language)|Lisp]] family of [[programming language]]s. Scheme was created during the 1970s at the [[MIT Computer Science and Artificial Intelligence Laboratory]] (MIT CSAIL) and released by its developers, [[Guy L. Steele]] and [[Gerald Jay Sussman]], via a series of memos now known as the [[Lambda Papers]]. It was the first dialect of Lisp to choose [[Scope (computer science)#Lexical scoping and dynamic scoping|lexical scope]] and the first to require implementations to perform [[tail-call optimization]], giving stronger support for functional programming and associated techniques such as recursive algorithms. It was also one of the first programming languages to support [[First-class object|first-class]] [[continuation]]s. It had a significant influence on the effort that led to the development of [[Common Lisp]].<ref name="cl_steele">Common LISP: The Language, 2nd Ed., Guy L. Steele Jr. Digital Press; 1981. {{ISBN|978-1-55558-041-4}}. "Common Lisp is a new dialect of Lisp, a successor to MacLisp, influenced strongly by ZetaLisp and to some extent by Scheme and InterLisp."</ref>

The Scheme language is standardized in the official [[Institute of Electrical and Electronics Engineers]] (IEEE) standard<ref name="ieee1178">1178-1990 (Reaff 2008) IEEE Standard for the Scheme Programming Language. IEEE part number STDPD14209, [https://web.archive.org/web/20100706171338/http://standards.ieee.org/board/rev/308minutes.html unanimously reaffirmed] at a meeting of the IEEE-SA Standards Board Standards Review Committee (RevCom), March 26, 2008 (item 6.3 on minutes), reaffirmation minutes accessed October 2009. This document is available from IEEE for purchase only, and not online at time of writing: 2009.</ref> <!--
 -->and a ''de facto'' standard called the ''Revised{{padlsup|n}} Report on the Algorithmic Language Scheme'' (R''n''RS). A widely implemented standard is R5RS (1998).<ref name="r5rs">{{Cite journal |last1=Richard Kelsey |last2=William Clinger |last3=Jonathan Rees |last4=Rozas |first4=G.J. |last5=Adams Iv |first5=N.I. |last6=Friedman |first6=D.P. |last7=Kohlbecker |first7=E. |last8=Steele Jr. |first8=G.L. |last9=Bartley |first9=D.H. |display-authors=3 |date=August 1998 |title=Revised<sup>5</sup> Report on the Algorithmic Language Scheme |url=http://www.schemers.org/Documents/Standards/R5RS/ |journal=Higher-Order and Symbolic Computation |volume=11 |issue=1 |pages=7–105 |doi=10.1023/A:1010051815785 |access-date=2012-08-09 |s2cid=14069423|url-access=subscription }}</ref> <!--
 -->The most recently ratified standard of Scheme<!--
 --> is "R7RS-small" (2013).<ref name="r7rs">{{Cite web |date=2013-07-06 |title=R7RS final available |url=http://trac.sacrideo.us/wg/raw-attachment/wiki/WikiStart/r7rs.pdf}}</ref> The more expansive and modular R6RS was ratified in 2007.<ref name="r6rs">{{Cite web |last1=Sperber |first1=Michael |last2=Dybvig |first2=R. Kent |last3=Flatt |first3=Matthew |last4=Van Straaten |first4=Anton |display-authors=etal |date=August 2007 |title=Revised<sup>6</sup> Report on the Algorithmic Language Scheme (R6RS) |url=http://www.r6rs.org |access-date=2011-09-13 |publisher=Scheme Steering Committee}}</ref> Both trace their descent from R5RS; the timeline below reflects the chronological order of ratification.<!--
 -->

== History ==
{{Main|History of the Scheme programming language}}

===Origins===
Scheme started in the 1970s as an attempt to understand [[Carl Hewitt]]'s [[Actor model]], for which purpose Steele and Sussman wrote a "tiny Lisp interpreter" using [[Maclisp]] and then "added mechanisms for creating actors and sending messages".<ref name="revisited">{{Cite journal |last1=Sussman |first1=Gerald Jay |last2=Steele |first2=Guy L. |date=1 December 1998 |title=The First Report on Scheme Revisited |journal=Higher-Order and Symbolic Computation |volume=11 |issue=4 |pages=399–404 |doi=10.1023/A:1010079421970 |s2cid=7704398}}</ref>  Scheme was originally called "Schemer", in the tradition of other [[Lisp (programming language)|Lisp]]-derived languages such as [[Planner (programming language)|Planner]] or ''Conniver''. The current name resulted from the authors' use of the [[Incompatible Timesharing System|ITS operating system]], which limited filenames to two components of at most six characters each. Currently, "Schemer" is commonly used to refer to a Scheme programmer.

===R6RS===
A new language standardization process began at the 2003 Scheme workshop, with the goal of producing an R6RS standard in 2006. This process broke with the earlier R''n''RS approach of unanimity.

R6RS features a standard module system, allowing a split between the core language and [[Library (computing)|libraries]]. Several drafts of the R6RS specification were released, the final version being R5.97RS. A successful vote resulted in ratifying the new standard, announced on August 28, 2007.<ref name="r6rs"/>

Currently the newest releases of various Scheme implementations<ref name="rs6s_Implementations">{{Cite web |title=R6RS Implementations |url=http://www.r6rs.org/implementations.html |access-date=2017-11-24 |publisher=r6rs.org}}</ref> support the R6RS standard. There is a portable reference implementation of the proposed implicitly phased libraries for R6RS, called psyntax, which loads and bootstraps itself properly on various older Scheme implementations.<ref name="psyntax">{{Cite web |last=Abdulaziz Ghuloum |date=2007-10-27 |title=R6RS Libraries and syntax-case system (psyntax) |url=https://www.cs.indiana.edu/~aghuloum/r6rs-libraries/ |access-date=2009-10-20 |publisher=Ikarus Scheme}}</ref>

A feature of R6RS is the record-type descriptor (RTD). When an RTD is created and used, the record type representation can show the memory layout. It also calculated object field bit mask and mutable Scheme object field bit masks, and helped the garbage collector know what to do with the fields without traversing the whole fields list that are saved in the RTD. RTD allows users to expand the basic RTD to create a new record system.<ref>{{Cite journal |last1=Keep |first1=Andrew W. |last2=Dybvig |first2=R. Kent |date=November 2014 |title=A run-time representation of scheme record types |journal=Journal of Functional Programming |volume=24 |issue=6 |pages=675–716 |doi=10.1017/S0956796814000203 |s2cid=40001845|doi-access=free}}</ref>

R6RS introduces numerous significant changes to the language.<ref name="r6rs_Language_changes">{{Cite web |date=2007-09-26 |title=Revised^6 Report on the Algorithmic Language Scheme, Appendix E: language changes |url=http://www.r6rs.org/final/html/r6rs/r6rs-Z-H-19.html#node_chap_E |access-date=2009-10-20 |publisher=Scheme Steering Committee}}</ref>  The source code is now specified in [[Unicode]], and a large subset of Unicode characters may now appear in Scheme symbols and [[identifier]]s, and there are other minor changes to the lexical rules. Character data is also now specified in Unicode. Many standard procedures have been moved to the new standard libraries, which themselves form a large expansion of the standard, containing procedures and syntactic forms that were formerly not part of the standard. A new module system has been introduced, and systems for exception handling are now standardized. Syntax-rules has been replaced with a more expressive syntactic abstraction facility (syntax-case) which allows the use of all of Scheme at macro expansion time. Compliant implementations are now ''required'' to support Scheme's full [[Numerical tower|numeric tower]], and the semantics of numbers have been expanded, mainly in the direction of support for the [[IEEE 754-1985|IEEE 754]] standard for floating point numerical representation.

===R7RS===
The R6RS standard has caused controversy because some see it as a departure from the minimalist philosophy.<ref name="r6rs_electorate">{{Cite web |year=2007 |title=R6RS Electorate |url=http://www.r6rs.org/ratification/electorate.html |access-date=2012-08-09 |publisher=Scheme Steering Committee}}</ref><ref name="r6rs_feeley">{{Cite web |last=Marc Feeley (compilation) |date=2007-10-26 |title=Implementors' intentions concerning R6RS |url=http://lists.r6rs.org/pipermail/r6rs-discuss/2007-October/003351.html |access-date=2012-08-09 |publisher=Scheme Steering Committee, r6rs-discuss mailing list |archive-date=2008-08-20 |archive-url=https://web.archive.org/web/20080820040409/http://lists.r6rs.org/pipermail/r6rs-discuss/2007-October/003351.html |url-status=dead }}</ref> In August 2009, the Scheme Steering Committee, which oversees the standardization process, announced its intention to recommend splitting Scheme into two languages: a large modern programming language for programmers; and a small version, a subset of the large version retaining the minimalism praised by educators and casual implementors.<ref name="position_paper">{{Cite web |last=Will Clinger, Marc Feeley, Chris Hanson, Jonathan Rees and Olin Shivers |date=2009-08-20 |title=Position Statement ''(draft)'' |url=http://scheme-reports.org/2009/position-statement.html |access-date=2012-08-09 |publisher=Scheme Steering Committee}}</ref> Two working groups were created to work on these two new versions of Scheme. The Scheme Reports Process site has links to the working groups' charters, public discussions and issue tracking system.

The ninth draft of R7RS (small language) was made available on April 15, 2013.<ref name="r7rs-draft-9">{{Cite web |date=2013-04-15 |title=R7RS 9th draft available |url=http://trac.sacrideo.us/wg/raw-attachment/wiki/WikiStart/r7rs-draft-9.pdf}}</ref> A vote ratifying this draft closed on May 20, 2013,<ref name="r7rs_clinger">{{Cite web |last=Will Clinger |date=2013-05-10 |title=extension of voting period |url=http://lists.scheme-reports.org/pipermail/scheme-reports/2013-May/003401.html |url-status=dead |archive-url=https://web.archive.org/web/20130721162308/http://lists.scheme-reports.org/pipermail/scheme-reports/2013-May/003401.html |archive-date=2013-07-21 |access-date=2013-07-07 |publisher=Scheme Language Steering Committee, scheme-reports mailing list}}</ref> and the final report has been available since August 6, 2013,<!-- quote= the Scheme Steering Committee decided in August 2009 to divide the standard into two separate but compatible languages – a "small" language, suitable for educators, researchers, and users of embedded languages, focused on R5RS compatibility, and a "large" language focused on the practical needs of mainstream software development, intended to become a replacement for R6RS. The present report --> describing "the 'small' language of that effort: therefore it cannot be considered in isolation as the successor to R6RS".<ref name="r7rs"/>

{{Lisp}}

== Distinguishing features ==
{{See also|Lisp (programming language)}}

Scheme is primarily a [[functional programming]] language. It shares many characteristics with other members of the Lisp programming language family. Scheme's very simple syntax is based on [[s-expression]]s, parenthesized lists in which a prefix operator is followed by its arguments. Scheme programs thus consist of sequences of nested lists. Lists are also the main data structure in Scheme, leading to a close equivalence between source code and data formats ([[homoiconicity]]). Scheme programs can easily create and evaluate pieces of Scheme code dynamically.

The reliance on lists as data structures is shared by all Lisp dialects. Scheme inherits a rich set of [[List (computing)|list-processing]] primitives such as [[Cons|<code>cons</code>]], [[CAR and CDR|<code>car</code> and <code>cdr</code>]] from its Lisp progenitors. Scheme uses strictly but [[Type system|dynamically typed variables]] and supports [[first-class function|first class procedures]]. Thus, procedures can be assigned as values to variables or passed as arguments to procedures.

This section concentrates mainly on innovative features of the language, including those features that distinguish Scheme from other Lisps. Unless stated otherwise, descriptions of features relate to the R5RS standard. In examples provided in this section, the notation "===> result" is used to indicate the result of evaluating the expression on the immediately preceding line. This is the same convention used in R5RS.

===Minimalism===
{{Main|Minimalism (computing)}}
Scheme is a very simple language, much easier to implement than many other languages of comparable [[Expressive power (computer science)|expressive power]].<ref name="easy_to_implement_scheme48">The [[Scheme 48]] implementation is so-named because the interpreter was written by Richard Kelsey and Jonathan Rees in 48 hours (August 6th{{spndash}}7th, 1986. See {{Cite web |last1=Richard Kelsey |last2=Jonathan Rees |last3=Mike Sperber |date=2008-01-10 |title=The Incomplete Scheme 48 Reference Manual for release 1.8 |url=http://s48.org/1.8/manual/manual.html |access-date=2012-08-09 |publisher=Jonathan Rees, s48.org}}</ref>  This ease is attributable to the use of [[lambda calculus]] to derive much of the syntax of the language from more primitive forms. For instance of the 23 s-expression-based syntactic constructs defined in the R5RS Scheme standard, 14 are classed as derived or library forms, which can be written as macros involving more fundamental forms, principally lambda. As R5RS (§3.1) says: "The most fundamental of the variable binding constructs is the lambda expression, because all other variable binding constructs can be explained in terms of lambda expressions."<ref name="r5rs"/>

: '''Fundamental forms''': define, lambda, quote, if, define-syntax, let-syntax, letrec-syntax, syntax-rules, set!
: '''Derived forms''': do, let, let*, letrec, cond, case, and, or, begin, named let, delay, unquote, unquote-splicing, quasiquote

Example: a macro to implement <code>let</code> as an expression using <code>lambda</code> to perform the variable bindings.

<syntaxhighlight lang="Scheme">
(define-syntax let
  (syntax-rules ()
    ((let ((var expr) ...) body ...)
      ((lambda (var ...) body ...) expr ...))))
</syntaxhighlight>

Thus using <code>let</code> as defined above a Scheme implementation would rewrite "<code>(let ((a 1)(b 2)) (+ b a))</code>" as "<code>((lambda (a b) (+ b a)) 1 2)</code>", which reduces implementation's task to that of coding procedure instantiations.

In 1998, Sussman and Steele remarked that the minimalism of Scheme was not a conscious design goal, but rather the unintended outcome of the design process. "We were actually trying to build something complicated and discovered, serendipitously, that we had accidentally designed something that met all our goals but was much simpler than we had intended....we realized that the lambda calculus—a small, simple formalism—could serve as the core of a powerful and expressive programming language."<ref name="revisited"/>

===Lexical scope===
{{See also|Scope (programming)}}
Like most modern programming languages and unlike earlier Lisps such as [[Maclisp]], Scheme is lexically scoped: all possible variable bindings in a program unit can be analyzed by reading the text of the program unit without consideration of the contexts in which it may be called. This contrasts with dynamic scoping which was characteristic of early Lisp dialects, because of the processing costs associated with the primitive textual substitution methods used to implement lexical scoping algorithms in compilers and interpreters of the day. In those Lisps, it was perfectly possible for a reference to a [[free variable]] inside a procedure to refer to quite distinct bindings external to the procedure, depending on the context of the call.

The impetus to incorporate lexical scoping, which was an unusual scoping model in the early 1970s, into their new version of Lisp, came from Sussman's studies of [[ALGOL]]. He suggested that [[Block (programming)|ALGOL-like lexical scoping mechanisms]] would help to realize their initial goal of implementing [[Carl Hewitt#Actor model|Hewitt's Actor model]] in Lisp.<ref name="revisited"/>

The key insights on how to introduce lexical scoping into a Lisp dialect were popularized in Sussman and Steele's 1975 Lambda Paper, "Scheme: An Interpreter for Extended Lambda Calculus",<ref name="lambda_paper_1">{{Cite journal |last1=Gerald Jay Sussman |last2=Guy Lewis Steele Jr. |name-list-style=amp |date=December 1975 |title=Scheme: An Interpreter for Extended Lambda Calculus |url=https://dspace.mit.edu/bitstream/handle/1721.1/5794/AIM-349.pdf |journal=AI Memos |publisher=[[MIT Computer Science and Artificial Intelligence Laboratory|MIT AI Lab]] |volume=AIM-349 |access-date=23 December 2021 |hdl=1721.1/5794}}</ref> where they adopted the concept of the [[Closure (computer science)|lexical closure]] (on page 21), which had been described in an [[AI Memo]] in 1970 by [[Joel Moses]], who attributed the idea to [[Peter J. Landin]].<!--
 --><ref name="Moses">{{Citation |last=Joel Moses |title=The Function of FUNCTION in LISP, or Why the FUNARG Problem Should Be Called the Environment Problem |date=June 1970 |id=[[AI Memo]] 199 |quote=A useful metaphor for the difference between FUNCTION and QUOTE in LISP is to think of QUOTE as a porous or an open covering of the function since free variables escape to the current environment. FUNCTION acts as a closed or nonporous covering (hence the term "closure" used by Landin). Thus we talk of "open" Lambda expressions (functions in LISP are usually Lambda expressions) and "closed" Lambda expressions. [...] My interest in the environment problem began while Landin, who had a deep understanding of the problem, visited MIT during 1966-67. I then realized the correspondence between the FUNARG lists which are the results of the evaluation of "closed" Lambda expressions in [[LISP 1.5|LISP]] and [[ISWIM]]'s Lambda Closures. |author-link=Joel Moses |hdl=1721.1/5854}}</ref>

===Lambda calculus===
{{See also|Lambda calculus}}
[[Alonzo Church]]'s mathematical notation, the lambda calculus, has inspired Lisp's use of "lambda" as a keyword for introducing a procedure, as well as influencing the development of [[functional programming]] techniques involving the use of [[higher-order function]]s in Lisp. But early Lisps were not suitable expressions of the lambda calculus because of their treatment of [[Free variables and bound variables|free variables]].<ref name="revisited"/>

A formal lambda system has axioms and a complete calculation rule. It is helpful for the analysis using mathematical logic and tools. In this system, calculation can be seen as a directional deduction. The syntax of lambda calculus follows the recursive expressions from x, y, z, ...,parentheses, spaces, the period and the symbol λ.<ref>{{Cite journal |last=van Tonder |first=André |date=1 January 2004 |title=A Lambda Calculus for Quantum Computation |journal=SIAM Journal on Computing |volume=33 |issue=5 |pages=1109–1135 |arxiv=quant-ph/0307150 |doi=10.1137/S0097539703432165 |s2cid=613571}}</ref> The function of lambda calculation includes: First, serve as a starting point of powerful mathematical logic. Second, it can reduce the requirement of programmers to consider the implementation details, because it can be used to imitate machine evaluation. Finally, the lambda calculation created a substantial meta-theory.<ref>{{Cite journal |last1=Niehren |first1=J. |last2=Schwinghammer |first2=J. |last3=Smolka |first3=G. |date=November 2006 |title=A concurrent lambda calculus with futures |url=https://hal.inria.fr/inria-00090434/file/0.pdf |journal=Theoretical Computer Science |volume=364 |issue=3 |pages=338–356 |doi=10.1016/j.tcs.2006.08.016}}</ref>

The introduction of lexical scope resolved the problem by making an equivalence between some forms of lambda notation and their practical expression in a working programming language. Sussman and Steele showed that the new language could be used to elegantly derive all the imperative and declarative semantics of other programming languages including ALGOL and [[Fortran]], and the dynamic scope of other Lisps, by using lambda expressions not as simple procedure instantiations but as "control structures and environment modifiers".<ref name="lambda_paper_2">{{Cite journal |last1=Gerald Jay Sussman |last2=Guy Lewis Steele Jr. |name-list-style=amp |date=March 1976 |title=Lambda: The Ultimate Imperative |url=https://dspace.mit.edu/handle/1721.1/5790 |format=postscript or PDF |journal=AI Memos |publisher=[[MIT Computer Science and Artificial Intelligence Laboratory|MIT AI Lab]] |volume=AIM-353 |hdl=1721.1/5790 }} {{Cite web |title=Lambda: The Ultimate Imperative (transcription) |url=https://research.scheme.org/lambda-papers/lambda-papers-ltu-imperative.html |website=scheme.org}}</ref> They introduced [[continuation-passing style]] along with their first description of Scheme in the first of the Lambda Papers, and in subsequent papers, they proceeded to demonstrate the raw power of this practical use of lambda calculus.

===Block structure===
Scheme inherits its block structure from earlier block structured languages, particularly [[ALGOL]]. In Scheme, blocks are implemented by three ''binding constructs'': [[Let expression|<code>let</code>]], <code>let*</code> and <code>letrec</code>. For instance, the following construct creates a [[Block (programming)|block]] in which a symbol called <code>var</code> is bound to the number 10:

<syntaxhighlight lang="Scheme">
(define var "goose")
;; Any reference to var here will be bound to "goose"
(let ((var 10))
  ;; statements go here. Any reference to var here will be bound to 10.
  )
;; Any reference to var here will be bound to "goose"
</syntaxhighlight>

Blocks can be [[Nesting (computing)|nested]] to create arbitrarily complex block structures according to the need of the programmer. The use of block structuring to create local bindings alleviates the risk of [[Naming collision|namespace collision]] that can otherwise occur.

One variant of <code>let</code>, <code>let*</code>, permits bindings to refer to variables defined earlier in the same construct, thus:

<syntaxhighlight lang="Scheme">
(let* ((var1 10)
       (var2 (+ var1 12)))
  ;; But the definition of var1 could not refer to var2
  )

</syntaxhighlight>
The other variant, <code>letrec</code>, is designed to enable [[mutual recursion|mutually recursive]] procedures to be bound to one another.

<syntaxhighlight lang="Scheme">
;; Calculation of Hofstadter's male and female sequences as a list of pairs

(define (hofstadter-male-female n)
  (letrec ((female (lambda (n)
		     (if (= n 0)
			 1
			 (- n (male (female (- n 1)))))))
	   (male (lambda (n)
		   (if (= n 0)
		       0
		       (- n (female (male (- n 1))))))))
    (let loop ((i 0))
      (if (> i n)
	  '()
	  (cons (cons (female i)
		      (male i))
		(loop (+ i 1)))))))

(hofstadter-male-female 8)

===> ((1 . 0) (1 . 0) (2 . 1) (2 . 2) (3 . 2) (3 . 3) (4 . 4) (5 . 4) (5 . 5))
</syntaxhighlight>
(See [[Hofstadter sequence#Hofstadter Female and Male sequences|Hofstadter's male and female sequences]] for the definitions used in this example.)

All procedures bound in a single <code>letrec</code> may refer to one another by name, as well as to values of variables defined earlier in the same <code>letrec</code>, but they may not refer to ''values'' defined later in the same <code>letrec</code>.

A variant of <code>let</code>, the "named let" form, has an identifier after the <code>let</code> keyword. This binds the let variables to the argument of a procedure whose name is the given identifier and whose body is the body of the let form. The body may be repeated as desired by calling the procedure. The named let is widely used to implement iteration.

Example: a simple counter

<syntaxhighlight lang="Scheme">
(let loop ((n 1))
  (if (> n 10)
      '()
      (cons n
	    (loop (+ n 1)))))

===> (1 2 3 4 5 6 7 8 9 10)
</syntaxhighlight>

Like any procedure in Scheme, the procedure created in the named let is a first-class object.

===Proper tail recursion===
{{Further|Tail recursion}}
Scheme has an iteration construct, <code>do</code>, but it is more [[Programming idiom|idiomatic]] in Scheme to use [[tail recursion]] to express [[iteration]]. Standard-conforming Scheme implementations are required to optimize tail calls so as to support an unbounded number of active tail calls (R5RS sec. 3.5)<ref name="r5rs"/>—a property the Scheme report describes as ''proper tail recursion''—making it safe for Scheme programmers to write iterative algorithms using recursive structures, which are sometimes more intuitive. Tail recursive procedures and the ''named <code>let</code>'' form provide support for iteration using tail recursion.

<syntaxhighlight lang="Scheme">
;; Building a list of squares from 0 to 9:
;; Note: loop is simply an arbitrary symbol used as a label. Any symbol will do.

(define (list-of-squares n)
  (let loop ((i n) (res '()))
    (if (< i 0)
        res
        (loop (- i 1) (cons (* i i) res)))))

(list-of-squares 9)
===> (0 1 4 9 16 25 36 49 64 81)
</syntaxhighlight>

===First-class continuations===
{{Main|Continuation}}
Continuations in Scheme are [[first-class object]]s. Scheme provides the procedure <code>[[call-with-current-continuation]]</code> (also known as <code>call/cc</code>) to capture the current continuation by packing it up as an escape procedure bound to a formal argument in a procedure provided by the programmer. (R5RS sec. 6.4)<ref name="r5rs"/> First-class continuations enable the programmer to create non-local [[Control flow|control constructs]] such as [[iterator]]s, [[coroutine]]s, and [[backtracking]].

Continuations can be used to emulate the behavior of [[return statement]]s in imperative programming languages. The following function <code>find-first</code>, given function <code>func</code> and list <code>lst</code>, returns the first element <code>x</code> in <code>lst</code> such that <code>(func x)</code> returns true.

<syntaxhighlight lang="scheme">
(define (find-first func lst)
  (call-with-current-continuation
   (lambda (return-immediately)
     (for-each (lambda (x)
		 (if (func x)
		     (return-immediately x)))
	  lst)
     #f)))

(find-first integer? '(1/2 3/4 5.6 7 8/9 10 11))
===> 7
(find-first zero? '(1 2 3 4))
===> #f
</syntaxhighlight>

The following example, a traditional programmer's puzzle, shows that Scheme can handle continuations as first-class objects, binding them to variables and passing them as arguments to procedures.

<syntaxhighlight lang="scheme">
(let* ((yin
         ((lambda (cc) (display "@") cc) (call-with-current-continuation (lambda (c) c))))
       (yang
         ((lambda (cc) (display "*") cc) (call-with-current-continuation (lambda (c) c)))))
    (yin yang))
</syntaxhighlight>
When executed this code displays a counting sequence: <code>@*@**@***@****@*****@******@*******@********...</code>
<!-- Bear with me, I'm writing a clear English explanation of how this works, but it isn't easy. I'll add it when it's done. -->

===Shared namespace for procedures and variables===
In contrast to Common Lisp, all data and procedures in Scheme share a common namespace, whereas in Common Lisp [[Common Lisp#The function namespace|functions and data have separate namespaces]] making it possible for a function and a variable to have the same name, and requiring special notation for referring to a function as a value. This is sometimes known as the "[[Lisp-1 vs. Lisp-2]]" distinction, referring to the unified namespace of Scheme and the separate namespaces of Common Lisp.<ref>{{Cite news |last1=Gabriel |first1=Richard P. |author-link=Richard P. Gabriel |last2=Pitman |first2=Kent |author-link2=Kent Pitman |year=1988 |title=Technical Issues of Separation in Function Cells and Value Cells |volume=1 |pages=81–101 |work=LISP and Symbolic Computation |issue=1 |publication-date=June 1988 |url=http://www.nhplace.com/kent/Papers/Technical-Issues.html |access-date=2012-08-09 |doi=10.1007/BF01806178}}</ref>

In Scheme, the same primitives that are used to manipulate and bind data can be used to bind procedures. There is no equivalent of Common Lisp's <code>defun</code> and <code>#'</code> primitives.

<syntaxhighlight lang="Scheme">
;; Variable bound to a number:
(define f 10)
f
===> 10
;; Mutation (altering the bound value)
(set! f (+ f f 6))
f
===> 26
;; Assigning a procedure to the same variable:
(set! f (lambda (n) (+ n 12)))
(f 6)
===> 18
;; Assigning the result of an expression to the same variable:
(set! f (f 1))
f
===> 13
;; functional programming:
(apply + '(1 2 3 4 5 6))
===> 21
(set! f (lambda (n) (+ n 100)))
(map f '(1 2 3))
===> (101 102 103)
</syntaxhighlight>

==Implementation standards==
This subsection documents design decisions that have been taken over the years which have given Scheme a particular character, but are not the direct outcomes of the original design.

===Numerical tower===
{{Main|Numerical tower}}
Scheme specifies a comparatively full set of numerical datatypes including [[complex number|complex]] and [[rational number|rational]] types, which is known in Scheme as the numerical tower (R5RS sec. 6.2<ref name="r5rs"/>). The standard treats these as abstractions, and does not commit the implementor to any particular internal representations.

Numbers may have the quality of exactness. An exact number can only be produced by a sequence of exact operations involving other exact numbers—inexactness is thus contagious. The standard specifies that any two implementations must produce equivalent results for all operations resulting in exact numbers.

The R5RS standard specifies procedures <code>exact->inexact</code> and <code>inexact->exact</code> which can be used to change the exactness of a number. <code>inexact->exact</code> produces "the exact number that is numerically closest to the argument". <code>exact->inexact</code> produces  "the inexact number that is numerically closest to the argument". The R6RS standard omits these procedures from the main report, but specifies them as R5RS compatibility procedures in the standard library (rnrs r5rs (6)).

In the R5RS standard, Scheme implementations are not required to implement the whole numerical tower, but they must implement "a coherent subset consistent with both the purposes of the implementation and the spirit of the Scheme language" (R5RS sec. 6.2.3).<ref name="r5rs"/>  The new R6RS standard does require implementation of the whole tower, and "exact integer objects and exact rational number objects of practically unlimited size and precision, and to implement certain procedures...so they always return exact results when given exact arguments" (R6RS sec. 3.4, sec. 11.7.1).<ref name="r6rs"/>

Example 1: exact arithmetic in an implementation that supports exact 
rational complex numbers.
<syntaxhighlight lang="Scheme">
;; Sum of three rational real numbers and two rational complex numbers
(define x (+ 1/3 1/4 -1/5 -1/3i 405/50+2/3i))
x
===> 509/60+1/3i
;; Check for exactness.
(exact? x)
===> #t
</syntaxhighlight>

Example 2: Same arithmetic in an implementation that supports neither exact 
rational numbers nor complex numbers but does accept real numbers in rational notation.
<syntaxhighlight lang="Scheme">
;; Sum of four rational real numbers
(define xr (+ 1/3 1/4 -1/5 405/50))
;; Sum of two rational real numbers
(define xi (+ -1/3 2/3))
xr
===> 8.48333333333333
xi
===> 0.333333333333333
;; Check for exactness.
(exact? xr)
===> #f
(exact? xi)
===> #f
</syntaxhighlight>

Both implementations conform to the R5RS standard but the second does not conform to R6RS because it does not implement the full numerical tower.

===Delayed evaluation===
{{See also|Lazy evaluation}}
Scheme supports delayed evaluation through the <code>delay</code> form and the procedure <code>force</code>.

<syntaxhighlight lang="Scheme">(define a 10)
(define eval-aplus2 (delay (+ a 2)))
(set! a 20)
(force eval-aplus2)
===> 22
(define eval-aplus50 (delay (+ a 50)))
(let ((a 8))
  (force eval-aplus50))
===> 70
(set! a 100)
(force eval-aplus2)
===> 22
</syntaxhighlight>
The lexical context of the original definition of the promise is preserved, and its value is also preserved after the first use of <code>force</code>. The promise is only ever evaluated once.

These primitives, which produce or handle values known as [[Futures and promises|promises]], can be used to implement advanced [[lazy evaluation]] constructs such as [[stream (computing)|stream]]s.<ref name="srfi-41">{{Cite web |last=Philip L. Bewig |date=2008-01-24 |title=SRFI 41: Streams |url=http://srfi.schemers.org/srfi-41/srfi-41.html |access-date=2012-08-09 |publisher=The SRFI Editors, schemers.org}}</ref>

In the R6RS standard, these are no longer primitives, but instead, are provided as part of the R5RS compatibility library (rnrs r5rs (6)).

In R5RS, a suggested implementation of <code>delay</code> and <code>force</code> is given, implementing the promise as a procedure with no arguments (a [[thunk]]) and using [[memoization]] to ensure that it is only ever evaluated once, irrespective of the number of times <code>force</code> is called (R5RS sec. 6.4).<ref name="r5rs"/>

SRFI 41 enables the expression of both finite and infinite sequences with extraordinary economy. For example, this is a definition of the [[Fibonacci sequence]] using the functions defined in SRFI 41:<ref name="srfi-41"/>

<syntaxhighlight lang="Scheme">
;; Define the Fibonacci sequence:
(define fibs
  (stream-cons 0
    (stream-cons 1
      (stream-map +
        fibs
        (stream-cdr fibs)))))
;; Compute the hundredth number in the sequence:
(stream-ref fibs 99)
===>  218922995834555169026
</syntaxhighlight>

===Order of evaluation of procedure arguments===
Most Lisps specify an order of evaluation for procedure arguments. Scheme does not. Order of evaluation—including the order in which the expression in the operator position is evaluated—may be chosen by an implementation on a call-by-call basis, and the only constraint is that "the effect of any concurrent evaluation of the operator and operand expressions is constrained to be consistent with some sequential order of evaluation." (R5RS sec. 4.1.3)<ref name="r5rs"/>
<syntaxhighlight lang="Scheme">
(let ((ev (lambda(n) (display "Evaluating ")
                     (display (if (procedure? n) "procedure" n))
                     (newline) n)))
  ((ev +) (ev 1) (ev 2)))
===> 3
</syntaxhighlight>
<syntaxhighlight lang="output">
Evaluating 1
Evaluating 2
Evaluating procedure
</syntaxhighlight>
ev is a procedure that describes the argument passed to it, then returns the value of the argument. In contrast with other Lisps, the appearance of an expression in the operator position (the first item) of a Scheme expression is quite legal, as long as the result of the expression in the operator position is a procedure.

In calling the procedure "{{mono|+}}" to add 1 and 2, the expressions {{mono|(ev +), (ev 1)}} and {{mono|(ev 2)}} may be evaluated in any order, as long as the effect is not as if they were evaluated in parallel. Thus the following three lines may be displayed in any order by standard Scheme when the above example code is executed, although the text of one line may not be interleaved with another because that would violate the sequential evaluation constraint.

===Hygienic macros===
{{Main|Hygienic macro}}
In the R5RS standard and in later reports, the syntax of Scheme can easily be extended via the macro system. The R5RS standard introduced a powerful hygienic macro system that allows the programmer to add new syntactic constructs to the language using a simple [[pattern matching]] sublanguage (R5RS sec 4.3).<ref name="r5rs"/>  Prior to this, the hygienic macro system had been relegated to an appendix of the R4RS standard, as a "high level" system alongside a "low level" macro system, both of which were treated as extensions to Scheme rather than an essential part of the language.<ref name="r4rs">{{Cite journal |year=1991 |title=Revised<sup>4</sup> Report on the Algorithmic Language Scheme |url=http://www.cs.indiana.edu/scheme-repository/R4RS/r4rs_toc.html |journal=ACM Lisp Pointers |volume=4 |issue=3 |pages=1–55 |access-date=2012-08-09 |editor=William Clinger and Jonathan Rees}}</ref>

Implementations of the hygienic macro system, also called <code>syntax-rules</code>, are required to respect the lexical scoping of the rest of the language. This is assured by special naming and scoping rules for macro expansion and avoids common programming errors that can occur in the macro systems of other programming languages. R6RS specifies a more sophisticated transformation system, <code>syntax-case</code>, which has been available as a language extension to R5RS Scheme for some time.

<syntaxhighlight lang="Scheme">
;; Define a macro to implement a variant of "if" with a multi-expression
;; true branch and no false branch.
(define-syntax when
  (syntax-rules ()
    ((when pred exp exps ...)
      (if pred (begin exp exps ...)))))
</syntaxhighlight>

Invocations of macros and procedures bear a close resemblance—both are s-expressions—but they are treated differently. When the compiler encounters an s-expression in the program, it first checks to see if the symbol is defined as a syntactic keyword within the current lexical scope. If so, it then attempts to expand the macro, treating the items in the tail of the s-expression as arguments without compiling code to evaluate them, and this process is repeated recursively until no macro invocations remain. If it is not a syntactic keyword, the compiler compiles code to evaluate the arguments in the tail of the s-expression and then to evaluate the variable represented by the symbol at the head of the s-expression and call it as a procedure with the evaluated tail expressions passed as arguments to it.

Most Scheme implementations also provide additional macro systems. Among popular ones are [[Hygienic macro#syntactic closures|syntactic closures]], [[Hygienic macro#explicit renaming|explicit renaming macros]] and <code>define-macro</code>, a non-hygienic macro system similar to <code>defmacro</code> system provided in [[Common Lisp]].

The inability to specify whether or not a macro is hygienic is one of the shortcomings of the macro system. Alternative models for expansion such as scope sets provide a potential solution.<ref>{{Cite book |last=Flatt |first=Matthew |title=Proceedings of the 43rd Annual ACM SIGPLAN-SIGACT Symposium on Principles of Programming Languages |year=2016 |isbn=978-1-4503-3549-2 |pages=705–717 |chapter=Binding as sets of scopes |doi=10.1145/2837614.2837620 |s2cid=15401805}}</ref>

===Environments and eval===
Prior to R5RS, Scheme had no standard equivalent of the <code>eval</code> procedure which is ubiquitous in other Lisps, although the first Lambda Paper had described <code>evaluate</code> as "similar to the LISP function EVAL"<ref name="lambda_paper_1"/> and the first Revised Report in 1978 replaced this with <code>enclose</code>, which took two arguments. The second, third and fourth revised reports omitted any equivalent of <code>eval</code>.

The reason for this confusion is that in Scheme with its lexical scoping the result of evaluating an expression depends on where it is evaluated. For instance, it is not clear whether the result of evaluating the following expression should be 5 or 6:<ref name="rees_1992">Jonathan Rees, [http://mumble.net/~jar/pubs/scheme-of-things/june-92-meeting.ps The Scheme of Things The June 1992 Meeting] {{Webarchive|url=https://web.archive.org/web/20110716071317/http://mumble.net/~jar/pubs/scheme-of-things/june-92-meeting.ps |date=2011-07-16}} (postscript), in Lisp Pointers, V(4), October–December 1992. 
Retrieved 2012-08-09</ref>
<syntaxhighlight lang="Scheme">
(let ((name '+))
  (let ((+ *))
    (evaluate (list name 2 3))))
</syntaxhighlight>

If it is evaluated in the outer environment, where <code>name</code> is defined, the result is the sum of the operands. If it is evaluated in the inner environment, where the symbol "+" has been bound to the value of the procedure "*", the result is the product of the two operands.

R5RS resolves this confusion by specifying three procedures that return environments and providing a procedure <code>eval</code> that takes an s-expression and an environment and evaluates the expression in the environment provided. (R5RS sec. 6.5)<ref name="r5rs"/>  R6RS extends this by providing a procedure called <code>environment</code> by which the programmer can specify exactly which objects to import into the evaluation environment.

With modern scheme (usually compatible with R5RS) to evaluate this expression, one needs to define a function <code>evaluate</code> which can look like this:

<syntaxhighlight lang="Scheme">
(define (evaluate expr)
   (eval expr (interaction-environment)))
</syntaxhighlight>

<code>interaction-environment</code> is the interpreter's global environment.

===Treatment of non-Boolean values in Boolean expressions===
In most dialects of Lisp including Common Lisp, by convention the value <code>NIL</code> evaluates to the value false in a Boolean expression. In Scheme, since the IEEE standard in 1991,<ref name="ieee1178"/> all values except <code>#f</code>, including <code>NIL</code>'s equivalent in Scheme which is written as <code>'()</code>, evaluate to the value true in a Boolean expression. (R5RS sec. 6.3.1)<ref name="r5rs"/>

Where the constant representing the Boolean value of true is <code>T</code> in most Lisps, in Scheme it is <code>#t</code>.

===Disjointness of primitive datatypes===
In Scheme the primitive datatypes are disjoint. Only one of the following predicates can be true of any Scheme object: <code>boolean?</code>, <code>pair?</code>, <code>symbol?</code>, <code>number?</code>, <code>char?</code>, <code>string?</code>, <code>vector?</code>, <code>port?</code>, <code>procedure?</code>. (R5RS sec 3.2)<ref name="r5rs"/>

Within the numerical datatype, by contrast, the numerical values overlap. For example, an integer value satisfies all of the <code>integer?</code>, <code>rational?</code>, <code>real?</code>, <code>complex?</code> and <code>number?</code> predicates at the same time. (R5RS sec 6.2)<ref name="r5rs"/>

===Equivalence predicates===
{{See also|relational operator}}
Scheme has three different types of equivalence between arbitrary objects denoted by three different ''equivalence predicates'', relational operators for testing equality, <code>eq?</code>, <code>eqv?</code> and <code>equal?</code>:
*  <code>eq?</code> evaluates to <code>#f</code> unless its parameters represent the same data object in memory;
* <code>eqv?</code> is generally the same as <code>eq?</code> but treats primitive objects (e.g. characters and numbers) specially so that numbers that represent the same value are <code>eqv?</code> even if they do not refer to the same object;
* <code>equal?</code> compares data structures such as lists, vectors and strings to determine if they have congruent structure and <code>eqv?</code> contents.(R5RS sec. 6.1)<ref name="r5rs"/>

Type dependent equivalence operations also exist in Scheme: <code>string=?</code> and <code>string-ci=?</code> compare two strings (the latter performs a case-independent comparison); <code>char=?</code> and <code>char-ci=?</code> compare characters; <code>=</code> compares numbers.<ref name="r5rs"/>

===Comments===
{{See also|Comment (computer programming)}}
Up to the R5RS standard, the standard comment in Scheme was a semicolon, which makes the rest of the line invisible to Scheme. Numerous implementations have supported alternative conventions permitting comments to extend for more than a single line, and the R6RS standard permits two of them: an entire [[s-expression]] may be turned into a comment (or "commented out") by preceding it with <code><nowiki>#;</nowiki></code> (introduced in SRFI 62<ref>{{Cite web |last=Taylor Campbell |date=2005-07-21 |title=SRFI 62: S-expression comments |url=http://srfi.schemers.org/srfi-62/srfi-62.html |access-date=2012-08-09 |publisher=The SRFI Editors, schemers.org}}</ref>) and a multiline comment or "block comment" may be produced by surrounding text with <code><nowiki>#</nowiki>|</code> and <code>|#</code>.

===Input/output===
Scheme's input and output is based on the ''port'' datatype. (R5RS sec 6.6)<ref name="r5rs"/>  R5RS defines two default ports, accessible with the procedures <code>current-input-port</code> and <code>current-output-port</code>, which correspond to the Unix notions of [[Standard streams|standard input and standard output]]. Most implementations also provide <code>current-error-port</code>. [[Redirection (computing)|Redirection]] of input and standard output is supported in the standard, by standard procedures such as <code>with-input-from-file</code> and <code>with-output-to-file</code>. Most implementations provide string ports with similar redirection capabilities, enabling many normal input-output operations to be performed on string buffers instead of files, using procedures described in SRFI 6.<ref name="srfi-6">{{Cite web |last=William D Clinger |date=1999-07-01 |title=SRFI 6: Basic String Ports |url=http://srfi.schemers.org/srfi-6/srfi-6.html |access-date=2012-08-09 |publisher=The SRFI Editors, schemers.org}}</ref> The R6RS standard specifies much more sophisticated and capable port procedures and many new types of port.

The following examples are written in strict R5RS Scheme.

Example 1: With output defaulting to (current-output-port):
<syntaxhighlight lang="Scheme">
(let ((hello0 (lambda() (display "Hello world") (newline))))
  (hello0))
</syntaxhighlight>

Example 2: As 1, but using optional port argument to output procedures
<syntaxhighlight lang="Scheme">
(let ((hello1 (lambda (p) (display "Hello world" p) (newline p))))
  (hello1 (current-output-port)))
</syntaxhighlight>

Example 3: As 1, but output is redirected to a newly created file
<syntaxhighlight lang="Scheme">
;; NB: with-output-to-file is an optional procedure in R5RS
(let ((hello0 (lambda () (display "Hello world") (newline))))
  (with-output-to-file "helloworldoutputfile" hello0))
</syntaxhighlight>

Example 4: As 2, but with explicit file open and port close to send output to file
<syntaxhighlight lang="Scheme">
(let ((hello1 (lambda (p) (display "Hello world" p) (newline p)))
      (output-port (open-output-file "helloworldoutputfile")))
  (hello1 output-port)
  (close-output-port output-port))
</syntaxhighlight>

Example 5: As 2, but with using call-with-output-file to send output to a file.
<syntaxhighlight lang="Scheme">
(let ((hello1 (lambda (p) (display "Hello world" p) (newline p))))
  (call-with-output-file "helloworldoutputfile" hello1))
</syntaxhighlight>

Similar procedures are provided for input. R5RS Scheme provides the predicates <code>input-port?</code> and <code>output-port?</code>. For character input and output, <code>write-char</code>, <code>read-char</code>, <code>peek-char</code> and <code>char-ready?</code> are provided. For writing and reading Scheme expressions, Scheme provides <code>read</code> and <code>write</code>. On a read operation, the result returned is the end-of-file object if the input port has reached the end of the file, and this can be tested using the predicate <code>eof-object?</code>.

With the standard, SRFI 28 also defines a basic formatting procedure resembling Common Lisp's <code>format</code> function, after which it is named.<ref name="srfi-28">{{Cite web |last=Scott G. Miller |date=2002-06-25 |title=SRFI 28: Basic Format Strings |url=http://srfi.schemers.org/srfi-28/srfi-28.html |publisher=The SRFI Editors, schemers.org |access-date=2012-08-09}}</ref>

===Redefinition of standard procedures===
In Scheme, procedures are bound to variables. At R5RS the language standard formally mandated that programs may change the variable bindings of built-in procedures, effectively redefining them. (R5RS "Language changes")<ref name="r5rs"/>  For example, <code>+</code> can be extended to accept strings as well as numbers by redefining it:

<syntaxhighlight lang="Scheme">
(set! +
      (let ((original+ +))
        (lambda args
          (apply (if (or (null? args) (string? (car args)))
                     string-append
                     original+)
                 args))))
(+ 1 2 3)
===> 6
(+ "1" "2" "3")
===> "123"
</syntaxhighlight>

In R6RS every binding, including the standard ones, belongs to some library, and all exported bindings are immutable. (R6RS sec 7.1)<ref name="r6rs"/>  Because of this, redefinition of standard procedures by mutation is forbidden. Instead, it is possible to import a different procedure under the name of a standard one, which in effect is similar to redefinition.

===Nomenclature and naming conventions===
In Standard Scheme, procedures that convert from one datatype to another contain the character string "->" in their name, predicates end with a "?", and procedures that change the value of already-allocated data end with a "!". These conventions are often followed by Scheme programmers.

In formal contexts such as Scheme standards, the word "procedure" is used in preference to "function" to refer to a lambda expression or primitive procedure. In normal usage, the words "procedure" and "function" are used interchangeably. Procedure application is sometimes referred to formally as ''combination''.

As in other Lisps, the term "[[thunk]]" is used in Scheme to refer to a procedure with no arguments. The term "proper tail recursion" refers to the property of all Scheme implementations, that they perform tail-call optimization so as to support an indefinite number of active [[tail call]]s.

The form of the titles of the standards documents since R3RS, "Revised<sup>n</sup> Report on the Algorithmic Language Scheme", is a reference to the title of the [[ALGOL|ALGOL 60]] standard document, "Revised Report on the Algorithmic Language Algol 60," The Summary page of R3RS is closely modeled on the Summary page of the ALGOL 60 Report.<!--
 --><ref name="algol_report">{{Cite journal |last1=J.W. Backus |last2=F.L. Bauer |last3=J.Green |last4=C. Katz |last5=J. McCarthy P. Naur |display-authors=etal |date=January–April 1960 |title=Revised Report on the Algorithmic Language Algol 60 |url=http://www.masswerk.at/algol60/report.htm |journal=Numerische Mathematik, Communications of the ACM, and Journal of the British Computer Society |access-date=2012-08-09}}</ref><ref name="r3rs">{{Cite journal |date=December 1986 |editor2-last=William Clinger |title=Revised(3) Report on the Algorithmic Language Scheme (Dedicated to the Memory of ALGOL 60) |url=http://groups.csail.mit.edu/mac/ftpdir/scheme-reports/r3rs-html/r3rs_toc.html |journal=ACM SIGPLAN Notices |volume=21 |issue=12 |pages=37–79 |citeseerx=10.1.1.29.3015 |doi=10.1145/15042.15043 |access-date=2012-08-09 |editor=Jonathan Rees |hdl=1721.1/6424 |s2cid=43884422}}</ref>

== Review of standard forms and procedures ==
The language is formally defined in the standards R5RS (1998)<ref name="r5rs"/>, R6RS (2007)<ref name="r6rs"/>, and R7RS (2013)<ref name="r7rs" />. They describe standard "forms": keywords and accompanying syntax, which provide the control structure of the language, and standard procedures which perform common tasks.

===Standard forms===
This table describes the standard forms in Scheme. Some forms appear in more than one row because they cannot easily be classified into a single function in the language.

Forms marked "L" in this table are classed as derived "library" forms in the standard and are often implemented as macros using more fundamental forms in practice, making the task of implementation much easier than in other languages.

{| class="wikitable"
|+ Standard forms in the language R5RS Scheme
! Purpose                !!   Forms
|-
| Definition             || {{mono|define}}
|-
| Binding constructs     || {{mono|lambda, do (L), let (L), let* (L), letrec (L)}}
|-
| Conditional evaluation || {{mono|if, cond (L), case (L), and (L), or (L)}}
|-
| Sequential evaluation  || {{mono|begin (*)}}
|-
| Iteration              || {{mono|lambda, do (L), named let (L)}}
|-
| Syntactic extension    || {{mono|define-syntax, let-syntax, letrec-syntax, syntax-rules (R5RS), syntax-case (R6RS)}}
|-
| Quoting                || {{mono|quote('), unquote(,), quasiquote(`), unquote-splicing(,@)}}
|-
| Assignment             || {{mono|set!}}
|-
| Delayed evaluation     || {{mono|delay (L)}}
|}

While <code>begin</code> is defined as a library syntax in R5RS, the expander must know about it to achieve the splicing function. In R6RS it is no longer a library syntax.

===Standard procedures===
The following two tables describe the standard procedures in R5RS Scheme. R6RS is far more extensive and a summary of this type would not be practical.

Some procedures appear in more than one row because they cannot easily be classified into a single function in the language.
{| class="wikitable"
|+ Standard procedures in the language R5RS Scheme
! Purpose !! Procedures
|-
| Construction    || {{mono|vector, make-vector, make-string, list}}
|-
| Equivalence predicates || {{mono|1=eq?, eqv?, equal?, string=?, string-ci=?, char=?, char-ci=?}}
|-
| [[Type conversion]] || {{mono|vector->list, list->vector, number->string, string->number, symbol->string, string->symbol, char->integer, integer->char, string->list, list->string}}
|-
| Numbers         || {{mono|''See separate table''}}
|-
| Strings         || {{mono|1=string?, make-string, string, string-length, string-ref, string-set!, string=?, string-ci=?, string<? string-ci<?, string<=? string-ci<=?, string>? string-ci>?, string>=? string-ci>=?, substring, string-append, string->list, list->string, string-copy, string-fill!}}
|-
| Characters      || {{mono|1=char?, char=?, char-ci=?, char<? char-ci<?, char<=? char-ci<=?, char>? char-ci>?, char>=? char-ci>=?, char-alphabetic?, char-numeric?, char-whitespace?, char-upper-case?, char-lower-case?, char->integer, integer->char, char-upcase, char-downcase}}
|-
| Vectors         || {{mono|make-vector, vector, vector?, vector-length, vector-ref, vector-set!, vector->list, list->vector, vector-fill!}}
|-
| Symbols         || {{mono|symbol->string, string->symbol, symbol?}}
|-
| Pairs and lists || {{mono|pair?, cons, car, cdr, set-car!, set-cdr!, null?, list?, list, length, append, reverse, list-tail, list-ref, memq. memv. member, assq, assv, assoc, list->vector, vector->list, list->string, string->list}}
|-
| Identity predicates || {{mono|boolean?, pair?, symbol?, number?, char?, string?, vector?, port?, procedure?}}
|-
| Continuations   || {{mono|call-with-current-continuation (call/cc), values, call-with-values, dynamic-wind}}
|-
| Environments    || {{mono|eval, scheme-report-environment, null-environment, interaction-environment (optional)}}
|-
| Input/output    || {{mono|display, newline, read, write, read-char, write-char, peek-char, char-ready?, eof-object? open-input-file, open-output-file, close-input-port, close-output-port, input-port?, output-port?, current-input-port, current-output-port, call-with-input-file, call-with-output-file, with-input-from-file(optional), with-output-to-file(optional)}}
|-
| System interface || {{mono|load (optional), transcript-on (optional), transcript-off (optional)}}
|-
| Delayed evaluation || {{mono|force}}
|-
| Functional programming || {{mono|procedure?, apply, map, for-each}}
|-
| Booleans        || {{mono|boolean? not}}
|}

String and character procedures that contain "-ci" in their names perform case-independent comparisons between their arguments: upper case and lower case versions of the same character are taken to be equal.

{| class="wikitable"
|+ Standard numeric procedures in the language R5RS Scheme
! Purpose !! Procedures
|-
| Basic arithmetic operators || {{mono|+, -, *, /, abs, quotient, remainder, modulo, gcd, lcm, expt, sqrt}}
|-
| Rational numbers || {{mono|numerator, denominator, rational?, rationalize}}
|-
| Approximation    || {{mono|floor, ceiling, truncate, round}}
|-
| Exactness        || {{mono|inexact->exact, exact->inexact, exact?, inexact?}}
|-
| Inequalities     || {{mono|1=<, <= , >, >=, =}}
|-
| Miscellaneous predicates || {{mono|zero?, negative?, positive? odd? even?}}
|-
| Maximum and minimum || {{mono|max, min}}
|-
| Trigonometry     || {{mono|sin, cos, tan, asin, acos, atan}}
|-
| Exponentials     || {{mono|exp, log}}
|-
| Complex numbers  || {{mono|make-rectangular, make-polar, real-part, imag-part, magnitude, angle, complex?}}
|-
| Input-output     || {{mono|number->string, string->number}}
|-
| Type predicates  || {{mono|integer?, rational?, real?, complex?, number?}}
|}

Implementations of - and / that take more than two arguments are defined but left optional at R5RS.

== Scheme Requests for Implementation ==
{{Main|Scheme Requests for Implementation}}
Because of Scheme's minimalism, many common procedures and syntactic forms are not defined by the standard. In order to keep the core language small but facilitate standardization of extensions, the Scheme community has a "Scheme Request for Implementation" (SRFI) process by which extension libraries are defined through careful discussion of extension proposals. This promotes code portability. Many of the SRFIs are supported by all or most Scheme implementations.

SRFIs with fairly wide support in different implementations include:<ref name="srfi_support">{{Cite web |date=2009-08-30 |title=Scheme Systems Supporting SRFIs |url=http://srfi.schemers.org/srfi-implementers.html |access-date=2012-08-09 |publisher=The SRFI Editors, schemers.org |archive-date=2021-06-20 |archive-url=https://web.archive.org/web/20210620154132/https://srfi.schemers.org/srfi-implementers.html |url-status=dead }}</ref>

* 0: feature-based conditional expansion construct
* 1: list library
* 4: homogeneous numeric vector datatypes
* 6: basic string ports
* 8: receive, binding to multiple values
* 9: defining record types
* 13: string library
* 14: character-set library
* 16: syntax for procedures of variable [[arity]]
* 17: generalized set!
* 18: Multithreading support
* 19: time data types and procedures
* 25: multi-dimensional array primitives
* 26: notation for specializing parameters without [[currying]]
* 27: sources of random bits
* 28: basic format strings
* 29: [[Internationalization and localization|localization]]
* 30: nested multi-line comments
* 31: a special form for recursive evaluation
* 37: args-fold: a program argument processor
* 39: parameter objects
* 41: [[Stream (computing)|stream]]s
* 42: [[List comprehension|eager comprehensions]]
* 43: vector library
* 45: primitives for expressing iterative lazy algorithms
* 60: integers as bits
* 61: a more general cond clause
* 66: octet vectors
* 67: compare procedures

== Implementations ==
{{Main category|Scheme (programming language) implementations}}

The elegant, minimalist design has made Scheme a popular target for language designers, hobbyists, and educators, and because of its small size, that of a typical [[Interpreter (computing)|interpreter]], it is also a popular choice for [[embedded system]]s and [[scripting language|scripting]]. This has resulted in scores of implementations,<ref name="scheme_implementations">75 known implementations of Scheme are listed by {{Cite web |date=2009-06-25 |title=scheme-faq-standards |url=http://community.schemewiki.org/?scheme-faq-standards#implementations |access-date=2009-10-20 |publisher=Community Scheme Wiki}}</ref> most of which differ from each other so much that porting programs from one implementation to another is quite difficult, and the small size of the standard language means that writing a useful program of any great complexity in standard, portable Scheme is almost impossible.<ref name="position_paper"/> The R6RS standard specifies a much broader language, in an attempt to broaden its appeal to programmers.

Almost all implementations provide a traditional Lisp-style [[read–eval–print loop]] for development and debugging. Many also [[Compiler|compile]] Scheme programs to executable binary. Support for embedding Scheme code in programs written in other languages is also common, as the relative simplicity of Scheme implementations makes it a popular choice for adding scripting capabilities to larger systems developed in languages such as [[C (programming language)|C]]. The [[Gambit (Scheme implementation)|Gambit]], [[Chicken (Scheme implementation)|Chicken]], and [[Bigloo]] Scheme interpreters compile Scheme to C, which makes embedding far easier. Further, Bigloo's compiler can be configured to generate [[bytecode]] for the [[Java virtual machine]] (JVM), and has an experimental bytecode generator for [[.NET]].

Some implementations support added features. For example, [[Kawa (Scheme implementation)|Kawa]] and [[JScheme]] provide integration with [[Java (programming language)|Java]] classes, and the Scheme to C compilers often make it easy to use external libraries written in C, up to allowing the embedding of C code in the Scheme source code. Another example is Pvts, which offers a set of visual tools that support Scheme learning.

== Usage ==
Scheme is widely used by several<ref name="schemers_inc">{{Cite web |last=Ed Martin |date=2009-07-20 |title=List of Scheme-using schools |url=http://www.schemers.com/schools.html |access-date=2009-10-20 |publisher=Schemers Inc. |archive-date=2009-03-30 |archive-url=https://web.archive.org/web/20090330005354/http://www.schemers.com/schools.html |url-status=dead }}</ref> schools; in particular, several introductory [[computer science]] courses use Scheme in conjunction with the textbook ''[[Structure and Interpretation of Computer Programs]]'' (SICP).<ref name="sicp_adopters">{{Cite web |date=1999-01-26 |title=List of SICP-using schools |url=http://mitpress.mit.edu/sicp/adopt-list.html |access-date=2009-10-20 |publisher=MIT Press |archive-date=2008-06-15 |archive-url=https://web.archive.org/web/20080615212702/http://mitpress.mit.edu/sicp/adopt-list.html |url-status=dead }}</ref> For the past 12 years, [[Racket (programming language)|PLT]] has run the [[ProgramByDesign]] (formerly TeachScheme!) project, which has exposed close to 600 high school teachers and thousands of high school students to rudimentary Scheme programming. [[MIT]]'s old introductory programming class 6.001 was taught in Scheme,<ref name="6.001">{{Cite web |last=Eric Grimson |author-link=Eric Grimson |date=Spring 2005 |title=6.001 Structure and Interpretation of Computer Programs |url=http://ocw.mit.edu/OcwWeb/Electrical-Engineering-and-Computer-Science/6-001Spring-2005/CourseHome/index.htm |access-date=2009-10-20 |publisher=MIT Open Courseware}}</ref> Although 6.001 has been replaced by more modern courses, SICP continues to be taught at MIT.<ref name="6.001_zombies">{{Cite web |last1=Alex Vandiver |last2=Nelson Elhage |display-authors=etal |date=January 2009 |title=6.184 - Zombies drink caffeinated 6.001 |url=http://web.mit.edu/alexmv/6.001/ |access-date=2009-10-20 |publisher=MIT CSAIL |archive-date=2009-08-28 |archive-url=https://web.archive.org/web/20090828031234/http://web.mit.edu/alexmv/6.001/ |url-status=dead }}</ref> Likewise, the introductory class at [[UC Berkeley]], CS 61A, was until 2011 taught entirely in Scheme, save minor diversions into [[Logo (programming language)|Logo]] to demonstrate dynamic scope. Today, like MIT, Berkeley has replaced the syllabus with a more modern version that is primarily taught in [[Python (programming language)|Python 3]], but the current syllabus is still based on the old curriculum, and parts of the class are still taught in Scheme.<ref name="61A">{{Cite web |last=John DeNero |date=Fall 2019 |title=Computer Science 61A, Berkeley |url=https://cs61a.org/articles/about.html |access-date=2019-12-17 |publisher=Department of Electrical Engineering and Computer Sciences, Berkeley |archive-date=2020-11-11 |archive-url=https://web.archive.org/web/20201111175626/https://cs61a.org/articles/about.html |url-status=dead }}</ref>

The textbook ''[[How to Design Programs]]'' is used by some institutes of higher education for their introductory computer science courses. [[Worcester Polytechnic Institute]] uses Scheme exclusively for its introductory course Introduction to Program Design (CS1101).<ref name="wpi">[http://web.cs.wpi.edu/~cs1101/a05/details.html#software CS 1101: Introduction to Program Design (A05): course software], [[Worcester Polytechnic Institute]]</ref> [[Rose-Hulman Institute of Technology]] uses Scheme in its more advanced Programming Language Concepts course.<ref name="rhit">{{Cite web |title=CSSE 304: Programming Language Concepts |url=https://www.rose-hulman.edu/Users/faculty/young/CS-Classes/csse304/syllabus.html |publisher=[[Rose-Hulman Institute of Technology]]}}</ref> [[Brandeis University]]'s core course, Structure and Interpretations of Computer Programs (COSI121b), is also taught exclusively in Scheme by theoretical computer scientist [[Harry Mairson]].<ref name="brandeis">{{Cite web |title=Spring 2021 CS121b Syllabus |url=https://moodle2.brandeis.edu/syllabus/public/1202dcdfe19c0860e226f6447f66875c.pdf |publisher=[[Brandeis University]]}}</ref> [[Indiana University]]'s introductory class, C211, is taught entirely in Scheme. A self-paced version of the course, CS 61AS, continues to use Scheme.<ref>{{cite web |url=https://berkeley-cs61as.github.io/ |title=Home |website=berkeley-cs61as.github.io}}</ref> The introductory computer science courses at [[Yale]] and [[Grinnell College]] are also taught in Scheme.<ref name="yale_cs201">{{Cite web |last=Dana Angluin |date=Fall 2009 |title=Introduction to Computer Science (CPSC 201) |url=http://zoo.cs.yale.edu/classes/cs201/ |access-date=2009-10-20 |publisher=The Zoo, Yale University Computer Science Department}}</ref> 
The former introductory computer science course at the University of Minnesota - Twin Cities, CSCI 1901, also used Scheme as its primary language, followed by a course that introduced students to the Java language;<ref name="umn">[http://www-users.itlabs.umn.edu/classes/Spring-2010/csci1901/ Structure of Computer Programming I] {{Webarchive|url=https://web.archive.org/web/20100619111110/http://www-users.itlabs.umn.edu/classes/Spring-2010/csci1901/ |date=2010-06-19}}, Computer Science Department, University of Minnesota, Spring 2010 (accessed 2010-01-30).</ref> however, following the example of MIT, the department replaced 1901 with the Python-based CSCI 1133,<ref name="umn2">[https://www.cs.umn.edu/academics/undergraduate/curriculum/required CSci Required Class Course Descriptions and Other Information] {{Webarchive|url=https://web.archive.org/web/20191025145152/https://www.cs.umn.edu/academics/undergraduate/curriculum/required |date=2019-10-25}}, Computer Science Department, University of Minnesota (accessed 2019-10-25)</ref> while functional programming is covered in detail in the third-semester course CSCI 2041.<ref name="umn3">[https://www.csdy.umn.edu/~shield/csecc/minutes/2013-4-23/csci2041.html CSCI 2041—New Course] CSE Curriculum Committee, University of Minnesota (accessed 2019-10-25)</ref>

Scheme is/was also used for the following:

* The [[Document Style Semantics and Specification Language]] (DSSSL), which provides a method of specifying [[SGML]] [[Style sheet (web development)|stylesheet]]s, uses a Scheme subset.<ref name="dsssl">{{Cite web |last=Robin Cover |date=2002-02-25 |title=DSSSL - Document Style Semantics and Specification Language. ISO/IEC 10179:1996 |url=http://xml.coverpages.org/dsssl.html |access-date=2012-08-09 |publisher=Cover Pages}}</ref>
* The well-known [[Open-source software|open source]] [[raster graphics editor]] [[GIMP]] uses [[TinyScheme]] as a [[scripting language]].<ref name="gimp">"''The major scripting language for the GIMP that has been attached to it today is Scheme.''" From {{Cite web |last=Dov Grobgeld |year=2002 |title=The GIMP Basic Scheme Tutorial |url=http://www.gimp.org/tutorials/Basic_Scheme/ |access-date=2012-08-09 |publisher=The GIMP Team}}</ref>
* [[GNU Guile|Guile]] has been adopted by the [[GNU]] project as its official scripting language, and that implementation of Scheme is embedded in such applications as [[GNU LilyPond]] and [[GnuCash]] as a scripting language for extensions. Likewise, Guile used to be the scripting language for the [[desktop environment]] [[GNOME]],<ref name="archive_gnomefaq">{{Cite web |last1=Todd Graham Lewis |last2=David Zoll |last3=Julian Missig |year=2002 |title=GNOME FAQ from Internet Archive |url=http://www.gnome.org/gnomefaq/html/x930.html |archive-url=https://web.archive.org/web/20000522010523/http://www.gnome.org/gnomefaq/html/x930.html |archive-date=2000-05-22 |access-date=2012-08-09 |publisher=The Gnome Team, gnome.org}}</ref> and GNOME still has a project that provides Guile bindings to its library stack.<ref name="live_gnome">{{Cite web |title=guile-gnome |url=https://www.gnu.org/software/guile-gnome/ |access-date=2012-08-09 |publisher=Free Software Foundation}}</ref> There is a project to incorporate Guile into [[GNU Emacs]], GNU's flagship program, replacing the current [[Emacs Lisp]] interpreter.<ref name="guile-emacs">{{Cite web |title=Guile in Emacs |url=https://emacsconf.org/2024/talks/guile/ |access-date=2025-12-05 |publisher=EmacsConf}}</ref>
* [[Extension Language Kit|Elk Scheme]] is used by [[Synopsys]] as a scripting language for its [[Technology CAD|technology CAD (TCAD)]] tools.<ref name="tcad">{{Cite web |last=Laurence Brevard |date=2006-11-09 |title=Synopsys MAP-in<sup>SM</sup> Program Update: EDA Interoperability Developers' Forum |url=http://www.synopsys.com/community/interoperability/documents/devforum_pres/2006nov/milkywaysession_mapin_overview.pdf |access-date=2012-08-09 |publisher=Synopsis Inc}}</ref>
* Shiro Kawai, senior programmer on the movie ''[[Final Fantasy: The Spirits Within]]'', used Scheme as a scripting language for managing the real-time rendering engine.<ref name="shiro_kawai">{{Cite journal |last=Kawai |first=Shiro |date=October 2002 |title=Gluing Things Together - Scheme in the Real-time CG Content Production |url=http://practical-scheme.net/docs/ILC2002.html |journal=Proceedings of the First International Lisp Conference, San Francisco |pages=342–348 |access-date=2012-08-09}}</ref>
* [[Google App Inventor]] for [[Android (operating system)|Android]] uses Scheme, where [[Kawa (Scheme implementation)|Kawa]] is used to compile the Scheme code down to bytecodes for the [[Java virtual machine]] running on Android devices.<ref name="android">{{Cite web |last1=Bill Magnuson |last2=Hal Abelson |last3=Mark Friedman |name-list-style=amp |date=2009-08-11 |title=Under the Hood of App Inventor for Android |url=http://googleresearch.blogspot.com/2009/08/under-hood-of-app-inventor-for-android.html |access-date=2012-08-09 |publisher=Google Inc, Official Google Research blog}}</ref>

== See also ==
{{Portal|Computer programming}}
*''[[Essentials of Programming Languages]]'', textbook using Scheme as foundation

== References ==
{{Reflist|30em}}

== Further reading ==
* [https://web.archive.org/web/20220313232321/ftp://ftp.cs.utexas.edu/pub/garbage/cs345/schintro-v14/schintro_toc.html An Introduction to Scheme and its Implementation] ([http://icem-www.folkwang-hochschule.de/~finnendahl/cm_kurse/doc/schintro/schintro_toc.html a mirror])
* {{Cite web |last=Christopher T. Haynes |date=1999-06-22 |title=The Scheme Programming Language Standardization Experience |url=http://acm.org/tsc/sstd.html}}
* {{Cite web |last=[[Guy L. Steele Jr.]], [[Richard P. Gabriel]] |title=The Evolution of Lisp |url=http://www.dreamsongs.org/Files/HOPL2-Uncut.pdf|archive-url=https://web.archive.org/web/20160611055324/http://www.dreamsongs.org/Files/HOPL2-Uncut.pdf |archive-date=2016-06-11 }}
* {{cite wikisource |author1=Gerald Jay Sussman|author1-link=Gerald Sussman |author2=Guy Lewis Steele Jr.|author2-link=Guy Steele|name-list-style=amp |title=Scheme: An Interpreter for Extended Lambda Calculus |publisher=[[MIT Computer Science and Artificial Intelligence Laboratory|MIT Artificial Intelligence Lab]] |volume=AI Memo 349 |date=December 1975|citeseerx=10.1.1.128.80}}

== External links ==
*[https://scheme.org/ scheme.org] provides links to many Scheme resources, including the specifications
*{{Wikibooks inline|Scheme Programming}}
*[https://lips.js.org/docs/category/introduction-to-scheme Introduction to Scheme]
*{{Wikibooks inline|Write Yourself a Scheme in 48 Hours}}
*{{Commons category-inline|Scheme (programming language)}}
*[https://discu.eu/weekly/lisp/ Scheme Weekly]
*[https://lips.js.org/#bookmark Bookmarklet that add Interactive Scheme REPL to any website]

{{Lisp programming language}}
{{Authority control}}

<!--Eponymous categories:-->
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{{DEFAULTSORT:Scheme (Programming Language)}}
[[Category:Scheme (programming language)| ]]
[[Category:Academic programming languages]]
[[Category:Articles with example Scheme (programming language) code]]
[[Category:Dynamically typed programming languages]]
[[Category:Extensible syntax programming languages]]
[[Category:Lisp programming language family]]
[[Category:Multi-paradigm programming languages]]
[[Category:Programming languages created in 1975]]