Headline

A reference to a function with a referencing environment

Illustration

As an example, take Language:Perl's anonymous subroutines, which act as closures over all lexical variables in their scope:

#!/usr/bin/perl
use strict;
use warnings;

sub make_anonymous_sub {
    my $greeting = shift;   # lexical, block-scoped variable
    return sub {
        my $who = shift;
        print "$greeting $who!\n" # $greeting refers to above lexical variable
    };
    # $greeting goes out of scope here, but is still referenced by the returned
    # anonymous subroutine. The subroutine "remembers" its environment.
}

my $hello = make_anonymous_sub('Hello');
my $greet = make_anonymous_sub('Greetings');

$hello->('World'  ); # prints "Hello World!"
$greet->('Terrans'); # prints "Greetings Terrans!"

Citation

(http://en.wikipedia.org/wiki/Closure_(computer_science), 4 June 2013)

In computer science, a closure (also lexical closure or function closure) is a function or reference to a function together with a referencing environment—a table storing a reference to each of the non-local variables (also called free variables) of that function. ...

The concept of closures was developed in the 1960s and was first fully implemented in 1975 ... as a language feature in the Scheme programming language to support lexically scoped first-class functions.

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