Re: (Ovid) Re: Sub Definitions Within Subs: Best Way to Exploit

Actually, it doesn't need to be anonymous or a code ref to be a closure; it just has to refer to lexical variables defined outside of its scope. This is a closure:

my $foo;
sub bar {
    print $foo;
}
[download]

Any further changes to the value of $foo in the future will not change the private value that bar() now has.

Comment on Re: (Ovid) Re: Sub Definitions Within Subs: Best Way to Exploit Download Code

Replies are listed 'Best First'.
Re: Re: (Ovid) Re: Sub Definitions Within Subs: Best Way to Exploit by htoug (Deacon) on Oct 11, 2001 at 09:15 UTC
It does need to be inside another sub. The code has to be something like: `sub baz { my $foo=shift; sub bar { print $foo; } } baz("abc"); bar; baz("123"); bar;` [download] which will print `abcabc`. The change to the `$foo` variable in the second call does not affect subroutine bar. It has a 'closed', localised and personal `$foo` variable, that cannot be changed by outside influences - only by bar itself. Thus the name 'closure'.	[reply] [d/l] [select]
Definition of a Closure by perrin (Chancellor) on Oct 11, 2001 at 18:09 UTC
Your explanation makes sense, but it isn't correct. Based on the number of times I've had this conversation, I'd say there is massive misunderstanding around the word "closure" in the Perl community. Here's a definition of closure from Damian Conway: "In Perl, a closure is just a subroutine that refers to one or more lexical variables declared outside the subroutine itself" - OO perl, p 56. I grabbed that quote from a post on the Perl6 list, but I've used that as my standard definition of closure ever since the book came out. It's really the only one I've seen that makes sense. Note that merlyn also agrees with me, as shown here in this post to the mod_perl list.	[reply]
Re: Definition of a Closure by htoug (Deacon) on Oct 12, 2001 at 12:33 UTC
Quite true, but you should also include the last sentence of the section: "That's all there is to a closure: a subroutine that preserves any lexical variables it's using, even after they become invisible everywhere else." OO-Perl pg.57 Closures are only interesting (for some definition of interesting) when the lexical is invisible.	[reply]