merge trunk to pluggable errors

[gitmo/Moose.git] / lib / Moose / Cookbook / Roles / Recipe2.pod

=pod

=head1 NAME

Moose::Cookbook::Roles::Recipe2 - Advanced Role Composition - method exclusion and aliasing

=head1 SYNOPSIS

  package Restartable;
  use Moose::Role;

  has 'is_paused' => (
      is      => 'rw',
      isa     => 'Bool',
      default => 0,
  );

  requires 'save_state', 'load_state';

  sub stop { ... }

  sub start { ... }

  package Restartable::ButUnreliable;
  use Moose::Role;

  with 'Restartable' => { alias  => { stop  => '_stop',
                                      start => '_start' } };

  sub stop {
      my $self = shift;

      $self->explode() if rand(1) > .5;

      $self->_stop();
  }

  sub start {
      my $self = shift;

      $self->explode() if rand(1) > .5;

      $self->_start();
  }

  package Restartable::ButBroken;
  use Moose::Role;

  with 'Restartable' => { excludes => [ 'stop', 'start' ] };

  sub stop {
      my $self = shift;

      $self->explode();
  }

  sub start {
      my $self = shift;

      $self->explode();
  }

=head1 DESCRIPTION

Sometimes when you include a role in a class, you may want to leave
out some of its methods. In this example, we have a role C<Restartable>
which provides an C<is_paused> attribute, and two methods, C<stop> and
C<start>. The implementation of those two methods is irrelevant.

Then we have two more roles which also implement the same interface,
each putting their own spin on the C<stop> and C<start> method.

In the C<Restartable::ButUnreliable> role, we want to provide a new
implementation of C<stop> and C<start>, but still have access to the
original implementation. To do this, we alias the methods from
C<Restartable> to private methods, and provide wrappers around the
originals (1).

In the C<Restartable::ButBroken> role, we want to provide an entirely
new behavior for C<stop> and C<start>, so we exclude them when
composing the C<Restartable> role into C<Restartable::ButBroken>.

It's worth noting that the C<excludes> parameter also accepts a single
string as an argument if you just want to exclude one method.

=head1 CONCLUSION

Method exclusion and renaming can come in handy, especially when
building roles out of other roles. In this example, all of our roles
implement the C<Restartable> role. Each role provides same API, but
each has a different implementation under the hood.

You can also use the method aliasing and excluding features when
composing a role into a class.

=head1 FOOTNOTES

=over 4

=item (1)

The mention of wrapper should tell you that we could do the same thing
using method modifiers, but for the sake of this example, we don't.

=back

=head1 AUTHOR

Dave Rolsky E<lt>autarch@urth.orgE<gt>

=head1 COPYRIGHT AND LICENSE

Copyright 2006-2008 by Infinity Interactive, Inc.

L<http://www.iinteractive.com>

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

=cut