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Jesse Luehrs > Moose > Moose::Cookbook::Roles::ApplicationToInstance


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Source   Latest Release: Moose-2.2011


Moose::Cookbook::Roles::ApplicationToInstance - Applying a role to an object instance


version 2.0604


  package MyApp::Role::Job::Manager;

  use List::Util qw( first );

  use Moose::Role;

  has 'employees' => (
      is  => 'rw',
      isa => 'ArrayRef[Employee]',

  sub assign_work {
      my $self = shift;
      my $work = shift;

      my $employee = first { !$_->has_work } @{ $self->employees };

      die 'All my employees have work to do!' unless $employee;


  package main;

  my $lisa = Employee->new( name => 'Lisa' );

  my $homer = Employee->new( name => 'Homer' );
  my $bart  = Employee->new( name => 'Bart' );
  my $marge = Employee->new( name => 'Marge' );

  $lisa->employees( [ $homer, $bart, $marge ] );
  $lisa->assign_work('mow the lawn');


In this recipe, we show how a role can be applied to an object. In this specific case, we are giving an employee managerial responsibilities.

Applying a role to an object is simple. The Moose::Meta::Role object provides an apply method. This method will do the right thing when given an object instance.


We could also use the apply_all_roles function from Moose::Util.

  apply_all_roles( $person, MyApp::Role::Job::Manager->meta );

The main advantage of using apply_all_roles is that it can be used to apply more than one role at a time.

We could also pass parameters to the role we're applying:

      -alias => { assign_work => 'get_off_your_lazy_behind' },

We saw examples of how method exclusion and alias working in Moose::Cookbook::Roles::Restartable_AdvancedComposition.


Applying a role to an object instance is a useful tool for adding behavior to existing objects. In our example, it is effective used to model a promotion.

It can also be useful as a sort of controlled monkey-patching for existing code, particularly non-Moose code. For example, you could create a debugging role and apply it to an object at runtime.


Moose is maintained by the Moose Cabal, along with the help of many contributors. See "CABAL" in Moose and "CONTRIBUTORS" in Moose for details.


This software is copyright (c) 2012 by Infinity Interactive, Inc..

This is free software; you can redistribute it and/or modify it under the same terms as the Perl 5 programming language system itself.

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