FAQ 7.13 What's a closure?
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From: PerlFAQ Server on 24 May 2010 00:00 This is an excerpt from the latest version perlfaq7.pod, which comes with the standard Perl distribution. These postings aim to reduce the number of repeated questions as well as allow the community to review and update the answers. The latest version of the complete perlfaq is at http://faq.perl.org . -------------------------------------------------------------------- 7.13: What's a closure? Closures are documented in perlref. *Closure* is a computer science term with a precise but hard-to-explain meaning. Usually, closures are implemented in Perl as anonymous subroutines with lasting references to lexical variables outside their own scopes. These lexicals magically refer to the variables that were around when the subroutine was defined (deep binding). Closures are most often used in programming languages where you can have the return value of a function be itself a function, as you can in Perl. Note that some languages provide anonymous functions but are not capable of providing proper closures: the Python language, for example. For more information on closures, check out any textbook on functional programming. Scheme is a language that not only supports but encourages closures. Here's a classic non-closure function-generating function: sub add_function_generator { return sub { shift() + shift() }; } $add_sub = add_function_generator(); $sum = $add_sub->(4,5); # $sum is 9 now. The anonymous subroutine returned by add_function_generator() isn't technically a closure because it refers to no lexicals outside its own scope. Using a closure gives you a *function template* with some customization slots left out to be filled later. Contrast this with the following make_adder() function, in which the returned anonymous function contains a reference to a lexical variable outside the scope of that function itself. Such a reference requires that Perl return a proper closure, thus locking in for all time the value that the lexical had when the function was created. sub make_adder { my $addpiece = shift; return sub { shift() + $addpiece }; } $f1 = make_adder(20); $f2 = make_adder(555); Now "&$f1($n)" is always 20 plus whatever $n you pass in, whereas "&$f2($n)" is always 555 plus whatever $n you pass in. The $addpiece in the closure sticks around. Closures are often used for less esoteric purposes. For example, when you want to pass in a bit of code into a function: my $line; timeout( 30, sub { $line = <STDIN> } ); If the code to execute had been passed in as a string, '$line = <STDIN>', there would have been no way for the hypothetical timeout() function to access the lexical variable $line back in its caller's scope. Another use for a closure is to make a variable *private* to a named subroutine, e.g. a counter that gets initialized at creation time of the sub and can only be modified from within the sub. This is sometimes used with a BEGIN block in package files to make sure a variable doesn't get meddled with during the lifetime of the package: BEGIN { my $id = 0; sub next_id { ++$id } } This is discussed in more detail in perlsub, see the entry on *Persistent Private Variables*. -------------------------------------------------------------------- The perlfaq-workers, a group of volunteers, maintain the perlfaq. They are not necessarily experts in every domain where Perl might show up, so please include as much information as possible and relevant in any corrections. The perlfaq-workers also don't have access to every operating system or platform, so please include relevant details for corrections to examples that do not work on particular platforms. Working code is greatly appreciated. If you'd like to help maintain the perlfaq, see the details in perlfaq.pod. |