Hi guys,

I was just reading about closures on perl.com and read that Tom Christiansen said that closures can be used to achieve data hiding in OO perl. I thought about it and came up with an example to do that:

A module written without closures:

package Dinner;
use strict;
use warnings; 

my $self = {};

sub new {
    my ($class) = @_;
    bless $self, $class;
    return $self;
}

sub washhands {
    my ($obj) = @_;
    $obj->{'handsclean'} = 1;
    return 1;
}

sub eatfood {
    my ($obj) = @_;
    if (exists $obj->{'handsclean'}) {
        print "\nYou washed your hands -eat all you want\n";
    } else {
        print "\nYou filthy animal!! Go wash your hands\n";
    }
    return 1;
}

1;
[download]

Its corresponding use:

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

my $dirtyeater = Dinner->new();
# I don't want to wash hands, I will just set the flag directly
# $dirtyeater->washhands;
$dirtyeater->{'handsclean'} = 1;
$dirtyeater->eatfood;
[download]

A Data::Dumper dump of $dirtyeater gives

$VAR1 = bless( {
                 'handsclean' => 1
               }, 'Dinner' );
[download]

$dirtyeater can eat food if he knows what to change in the objects data structure. I know that if you are playing with the object's ds you do it at your own risk but I want to prevent the end user from changing any object ds specific variables. So I write another module called Dinnerclosure.pm:

package Dinnerclosure;
use strict;
use warnings; 

sub new {
    my ($class) = @_;
    my $self = {};
    my $selfmirage = {};
    $selfmirage->{'washhands'} = sub { 
                                   washhands($self); 
                                 };
    $selfmirage->{'eatfood'} = sub { 
                                   eatfood($self); 
                               };
    bless $selfmirage, $class;
    return $selfmirage;
}

sub washhands {
    my ($obj) = @_;
    $obj->{'handsclean'} = 1;
    return 1;
}

sub eatfood {
    my ($obj) = @_;
    if (exists $obj->{'handsclean'}) {
        print "\nYou washed your hands -eat all you want\n";
    } else {
        print "\nYou filthy animal!! Go wash your hands\n";
    }
    return 1;
}

1;
[download]

The testcode:

#!/usr/bin/perl -w
use strict;
use Dinnerclosure;
my $cleaneater = Dinnerclosure->new();
# comment next line and uncomment the one after it, but it won't wash 
+hands
$cleaneater->{'washhands'}->();
#$cleaneater->{'handsclean'} = 1;
$cleaneater->{'eatfood'}->();
print Dumper $cleaneater;
[download]

$dirtyeater cannot change the 'cleanhands' flag. If he does, he will be changing the $selfmirage hashref which does not decide cleanliness, $self does. And $dirtyeater cannot access $self because the object is:

$VAR1 = bless( {
                 'eatfood' => sub { "DUMMY" },
                 'washhands' => sub { "DUMMY" }
               }, 'Dinnerclosure' );
[download]

Advantages:

1. You have to wash hands before eating. :) - to generalize - you can only call subs to manipulate data.There is no other choice.
2. $self and $selfmirage are lexically scoped within the new sub. They don't even have to be declared at top of package before any subs in case of Dinner.pm - better lexical scoping
3. The end user does not see any object innards in the ds

WYDSIWYDG - What you don't see is what you don't get :)

-- Saurabh

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