[svn:perlfaq] r7993 - perlfaq/trunk

Author: rafael
Date: Mon Oct 30 03:43:35 2006
New Revision: 7993

Modified:
   perlfaq/trunk/perlfaq7.pod

Log:
* What's a closure ?
	Some clarifications and addtions from the perl5-porters mailing list:
	Subject: Re: Closures with named subs
	From: Christian Winter <[email protected]>
	Date: Sun, 29 Oct 2006 21:31:57 +0100
	Message-ID: <[email protected]>


Modified: perlfaq/trunk/perlfaq7.pod
==============================================================================
--- perlfaq/trunk/perlfaq7.pod	(original)
+++ perlfaq/trunk/perlfaq7.pod	Mon Oct 30 03:43:35 2006
@@ -213,20 +213,21 @@
 Closures are documented in L<perlref>.
 
 I<Closure> is a computer science term with a precise but
-hard-to-explain meaning. 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 make sense in any programming language 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
+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 function-generating function:
+Here's a classic non-closure function-generating function:
 
     sub add_function_generator {
       return sub { shift() + shift() };
@@ -235,10 +236,10 @@
     $add_sub = add_function_generator();
     $sum = $add_sub->(4,5);                # $sum is 9 now.
 
-The closure works as a I<function template> with some customization
-slots left out to be filled later.  The anonymous subroutine returned
-by add_function_generator() isn't technically a closure because it
-refers to no lexicals outside its own scope.
+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 I<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
@@ -269,6 +270,21 @@
 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 I<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 L<perlsub>, see the entry on
+I<Persistent Private Variables>.
+
 =head2 What is variable suicide and how can I prevent it?
 
 This problem was fixed in perl 5.004_05, so preventing it means upgrading