DigitalKitty,
Before explaining the code, let me try and explain what a closure is the way I understand it.

A closure is a way to get at data that has gone out of scope. Because the reference count has not reached 0, it persists but is only accessible through the closure. 

#!/usr/bin/perl -w
use strict;

sub shoppingList {
    my $item = shift();
    return sub {
        my $otherItem = shift();
        print "I need to buy a $item and a $otherItem.\n";
    };
}

my $itemInBasket = shoppingList( "sweater" );
my $newItemInBasket = shoppingList( "lipstick" );

&$itemInBasket( "pair of shoes" );
&$newItemInBasket( "purse" );

[download]
Ok - lets take the seemingly simple sub shopping list:
It creates a lexical variable called $item and then returns a sub. The magic comes in that $item should disapear, but it doesn't since the returned sub keeps the reference count from reaching 0. You still use the lexical $item from elsewhere in the program, but you can still get at it through the closure.

Both $ItemInBasket and $newItemInBasket are now code references. They are the referant of the returned anonymous sub, which in turn remembers $item.

When they are de-referenced (I prefer the -> notation), they "remember" the $item and shift the argument list to get $otheritem.

References and closures are the things Perl OO is made of and that is the next logical step.

If you would like more information - let me know. I am sure there will be far better answers than mine anyway. I should point out that you assume the subs will be called with at least one argument. I know this is only for learning purposes, but coding for unexpected input will save you a great deal of troubleshooting time in the long run.
Cheers - L~R

In reply to Re: Help with the concept of closures. by Limbic~Region
in thread Help with the concept of closures. by DigitalKitty

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