Closure (computer programming): Difference between revisions

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{{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}}</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.

Sussman and [[Harold Abelson|Abelson]] also use the term ''closure'' in the 1980s with a second, unrelated meaning: the property of an operator that adds data to a [[data structure]] to also be able to add nested data structures. This use of the term comes from [[Closure (mathematics)|mathematics use]], rather than the prior use in computer science. The authors consider this overlap in terminology to be "unfortunate."<ref>{{cite book |last1=Abelson |first1=Harold |author1-link=Harold Abelson |last2=Sussman |first2=Gerald Jay |author2-link=Gerald Jay Sussman |last3=Sussman |first3=Julie |author3-link=Julie Sussman |date=1996 |title=Structure and Interpretation of Computer Programs |url=https://mitpress.mit.edu/sites/default/files/sicp/full-text/book/book.html |publisher=MIT Press |pages=98–99 |isbn={{Format ISBN|0262510871}}0-262-51087-1}}</ref>

== Anonymous functions ==