Editing Scheme (programming language) (section)

===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>