Editing Closure (computer programming) (section)

=== Other uses ===
Closures have many uses:
* Because closures delay evaluation—i.e., they do not "do" anything until they are called—they can be used to define control structures.  For example, all of [[Smalltalk]]'s standard control structures, including branches (if/then/else) and loops (while and for), are defined using objects whose methods accept closures.  Users can easily define their own control structures also.
* In languages which implement assignment, multiple functions can be produced that close over the same environment, enabling them to communicate privately by altering that environment. In Scheme:

<syntaxhighlight lang="scheme">
(define foo #f)
(define bar #f)

(let ((secret-message "none"))
  (set! foo (lambda (msg) (set! secret-message msg)))
  (set! bar (lambda () secret-message)))

(display (bar)) ; prints "none"
(newline)
(foo "meet me by the docks at midnight")
(display (bar)) ; prints "meet me by the docks at midnight"
</syntaxhighlight>

* Closures can be used to implement [[Object-oriented programming|object]] systems.<ref>{{cite web |url=http://okmij.org/ftp/Scheme/oop-in-fp.txt |title=Re: FP, OO and relations. Does anyone trump the others? |date=29 December 1999 |access-date=2008-12-23 |archive-url=https://web.archive.org/web/20081226055307/http://okmij.org/ftp/Scheme/oop-in-fp.txt |archive-date=26 December 2008 |url-status=dead}}</ref>

Note: Some speakers call any data structure that binds a [[Scope (programming)#Lexical scoping|lexical]] environment a closure, but the term usually refers specifically to functions.