Let's say that I want to write a Mixin module that adds the method frobnicate() to a given class Foo:

package Foo;
use Mixin qw(frobnicate);

sub new {
  bless {}, shift;
}

package main;

my $foo = Foo->new();
$foo->frobnicate();
[download]

The Mixin.pm implementation would then look something like this:

package Mixin;
use base 'Exporter';
our @EXPORT_OK = qw(frobnicate);

sub frobnicate { print "frobnicating!\n"; }
[download]

Now this works ok until I want to add another internal method to the mixin and call it in the same way:

package Mixin;
use base 'Exporter';
our @EXPORT_OK = qw(frobnicate);

sub frobnicate { 
    my($self) = @_;

    $self->frobnerize(); 
}

sub frobnerize {
    my($self) = @_;
    print "frobnerizing!\n";
}

1;
[download]

This will cause $foo->frobnicate() to fail, because frobnerize() isn't known to the object:

Can't locate object method "frobnerize" via package "Foo" at Mixin.pm line 9.

I could call frobnerize($self, ...) instead, but I'd like to use the object syntax for various reasons. Also, I could add frobnerize() in the class extended by the Mixin, like

use Mixin qw(frobnicate frobnerize);
[download]

(after adding frobnerize to the EXPORTS_OK array) but that would break the encapsulation, as the main program doesn't care about the internal implementation of Foo's frobnicate(). Or, if I could tell Exporter to use both EXPORTS_OK (frobnicate) and EXPORTS (frobnerize), exporting frobnicate() by request and frobnerize() by default, that would somehow work, although Exporter doesn't seem to support it, and it would, again, break the encapsulation. Better ideas, anyone?

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