Closure (computer programming): Difference between revisions

Revision as of 15:34, 9 July 2023

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[[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 report |last=Landin |first=P. J. |author-link=Peter Landin |year=1964 |title=The mechanical evaluation of expressions}}</ref> [[Joel Moses]] credits Landin with introducing the term ''closure'' to refer to a [[Anonymous function|lambda expression]] whichwith open bindings (free variables) that have been closed by (or bound in) the lexical environment, resulting in a ''closed expression'', or closure.<ref>

{{cite report |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 |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}}</ref><ref>{{cite book |last=Wikström |first=Åke |year=1987 |title=Functional Programming using Standard ML |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.