Closure (computer programming): Difference between revisions

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{{Distinguish|text=the programming language [[Clojure]]}}

{{Use dmy dates|date=August 2020}}

In [[programming language]]s, a '''closure''', also '''lexical closure''' or '''function closure''', is meant a functiontechnique alongfor withimplementing a[[lexically setscoped]] of[[name valuesbinding]] associatedin a language with its[[first-class parametersfunction]]s. [[Operational semantics|Operationally]], a closure is a [[Record (computer science)|record]] storing a [[Function (computer science)|function]]{{efn|The function may be stored as a [[Reference (computer science)|reference]] to a function, such as a [[function pointer]].}} together with itsan contextenvironment.<ref>Sussman (iand Steele.e "Scheme: An interpreter for extended lambda calculus". "... a data structure containing a lambda expression, theand associationan environment to be used when that lambda expression is applied to arguments." ([[s:Page:Scheme - An interpreter for extended lambda calculus.djvu/22|Wikisource]])</ref> The environment is a mapping associating each [[free variable]] of itsthe parametersfunction (variables that are used locally, but defined in an enclosing scope) with the [[Value (computer science)|value]] or [[Reference (computer science)|reference]] to which the name was bound when the closure was created.{{efn|These names usually refer to values, mutable variables, or referencesfunctions, forbut them)can also be other entities such as constants, types, classes, or labels.}} Unlike a plain function, a closure allows the function to access those ''captured variables'' through the closure's copies of their values or references, even when the function is invoked outside their scope.

== History and etymology ==

The concept of closures wasinwas developed in the 1960s for the mechanical evaluation of expressions in the [[λ-calculus|λ-calcul(us]] and was f)iforfirst eachfully ofimplemented itain parameters.1970 Aas clsupporta lexicaoycatchlanguage the environmentfeature in which a scopedthe [[first-class function|f]]<nowiki/>is called, regardless the invocation scopePAL (as,programming bringinglanguage)|PAL theprogramming contextlanguage]] along,to thesupport invocationlexically isscoped thus self[[first-contained,class i.e. ''closed'')function]]s.<ref name=dat2012>{{cite conference |author-link=David A. Turner |first=David A. |last=Turner |year=2012 |url=http://www.cs.kent.ac.uk/people/staff/dat/tfp12/tfp12.pdf |title=Some History of Functional Programming Languages |book-title=International Symposium on Trends in Functional Programming |pages=1–20 See 12 §2, note 8 for the claim about M-expressions |publisher=Springer |doi=10.1007/978-3-642-40447-4_1 |isbn=978-3-642-40447-4 |series=Lecture Notes in Computer Science |volume=7829}}</ref>

[[Peter Landin]] defined the term ''closure'' in 1964 as having an ''environment part'' and a ''control part'' as used by his [[SECD machine]] for evaluating expressions.<ref name=landin>

{{cite journal |last=Landin |first=P.J. |author-link=Peter Landin |title=The mechanical evaluation of expressions |journal=The Computer Journal |volume=6 |issue=4 |date=January 1964 |pages=308–320 |doi=10.1093/comjnl/6.4.308 |url=https://academic.oup.com/comjnl/article-pdf/6/4/308/1067901/6-4-308.pdf }}</ref> [[Joel Moses]] credits Landin with introducing the term ''closure'' to refer to a [[Anonymous function|lambda expression]] with open bindings (free variables) that have been closed by (or bound in) the lexical environment, resulting in a ''closed expression'', or closure.<ref>

{{cite journal |last=Moses |first=Joel |author-link=Joel Moses |date=June 1970 |title=The Function of FUNCTION in LISP, or Why the FUNARG Problem Should Be Called the Environment Problem |journal=ACM SIGSAM Bulletin |issue=15 |pages=13–27 |doi=10.1145/1093410.1093411 |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.|hdl=1721.1/5854|s2cid=17514262 }}</ref><ref>{{cite book |last=Wikström |first=Åke |year=1987 |title=Functional Programming using Standard ML |publisher=Prentice Hall |isbn=0-13-331968-7 |quote=The reason it is called a "closure" is that an expression containing free variables is called an "open" expression, and by associating to it the bindings of its free variables, you close it.}}</ref> This use was subsequently adopted by [[Gerald Jay Sussman|Sussman]] and [[Guy L. Steele Jr.|Steele]] when they defined [[Scheme (programming language)|Scheme]] in 1975,<ref>{{cite report |last1=Sussman |first1=Gerald Jay |author1-link=Gerald Jay Sussman |last2=Steele |first2=Guy L. Jr. |author2-link=Guy L. Steele Jr. |date=December 1975 |title=Scheme: An Interpreter for the Extended Lambda Calculus |id=[[AI Memo]] 349}}</ref> a lexically scoped variant of [[Lisp (programming language)|Lisp]], and became widespread.