There are practical uses for call-backs. One use is to
provide some user of a Perl module a way to modify the
behavior of that module. Here is a very simple example:
use strict;
package Foo;
use Carp;
sub new {
my ($class,$msg) = @_;
my $self = {msg => $msg};
return bless $self,$class;
}
sub print_me {
my ($self,$cb) = @_;
croak "not a code ref" unless ref $cb eq 'CODE';
return $cb->($self->{'msg'});
}
package main;
my $foo = Foo->new('hello world');
print $foo->print_me(sub {ucfirst shift}), "\n";
Now the user can specify how they want the message
printed. Like i said, a very simple example, but i am
sure you can imagine this module doing a lot more
behind the scenes. In these cases, providing a call-back
instead of providing a list of choices can make a module
more flexible, and keep you from maintaining that list
of choices.
UPDATE - drats. Just when i though i knew what a closure
was .... perrin pointed out that this is indeed, not
a closure ... a good example for a anonymous subs maybe,
but not a closure. I stand corrected and apologetic to
the 13 people that voted this node up.
jeffa
L-LL-L--L-LL-L--L-LL-L--
-R--R-RR-R--R-RR-R--R-RR
F--F--F--F--F--F--F--F--
(the triplet paradiddle)
