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[gitmo/Moose.git] / lib / Moose / Cookbook / Roles / Recipe1.pod

=pod

=head1 NAME

Moose::Cookbook::Roles::Recipe1 - The Moose::Role example

=head1 SYNOPSIS

  package Eq;
  use Moose::Role;

  requires 'equal_to';

  sub not_equal_to {
      my ( $self, $other ) = @_;
      not $self->equal_to($other);
  }

  package Comparable;
  use Moose::Role;

  with 'Eq';

  requires 'compare';

  sub equal_to {
      my ( $self, $other ) = @_;
      $self->compare($other) == 0;
  }

  sub greater_than {
      my ( $self, $other ) = @_;
      $self->compare($other) == 1;
  }

  sub less_than {
      my ( $self, $other ) = @_;
      $self->compare($other) == -1;
  }

  sub greater_than_or_equal_to {
      my ( $self, $other ) = @_;
      $self->greater_than($other) || $self->equal_to($other);
  }

  sub less_than_or_equal_to {
      my ( $self, $other ) = @_;
      $self->less_than($other) || $self->equal_to($other);
  }

  package Printable;
  use Moose::Role;

  requires 'to_string';

  package US::Currency;
  use Moose;

  with 'Comparable', 'Printable';

  has 'amount' => ( is => 'rw', isa => 'Num', default => 0 );

  sub compare {
      my ( $self, $other ) = @_;
      $self->amount <=> $other->amount;
  }

  sub to_string {
      my $self = shift;
      sprintf '$%0.2f USD' => $self->amount;
  }

=head1 DESCRIPTION

Roles have two primary purposes: as interfaces, and as a means of code
reuse. This recipe demonstrates the latter, with roles that define
comparison and display code for objects.

Let's start with CB<Eq>. First, note that we've replaced C<use Moose>
with C<use Moose::Role>. We also have a new sugar function, C<required>:

  requires 'equal_to';

This says that any class which consumes this role must provide an
C<equal_to> method. It can provide this method directly, or by
consuming some other role.

The C<Eq> role defines its C<not_equal_to> method in terms of the
required C<equal_to> method. This lets us minimize the methods that
consuming classes must provide.

The next role, C<Comparable>, builds on the C<Eq> role. We include
C<Eq> in C<Comparable> using C<with>, another new sugar function:

  with 'Eq';

The C<with> function takes a list of roles to consume. In our example,
the C<Comparable> role provides the C<equal_to> method required by
C<Eq>. However, it could opt not to, in which case a class that
consumed C<Comparable> would have to provide its own C<equal_to>. In
other words, a role can consume another role I<without> providing any
required methods.

The C<Comparable> role requires a method,  C<compare>:

  requires 'compare';

The C<Comparable> role also provides a number of other methods, all of
which ultimately rely on C<compare>.

  sub equal_to {
      my ( $self, $other ) = @_;
      $self->compare($other) == 0;
  }

  sub greater_than {
      my ( $self, $other ) = @_;
      $self->compare($other) == 1;
  }

  sub less_than {
      my ( $self, $other ) = @_;
      $self->compare($other) == -1;
  }

  sub greater_than_or_equal_to {
      my ( $self, $other ) = @_;
      $self->greater_than($other) || $self->equal_to($other);
  }

  sub less_than_or_equal_to {
      my ( $self, $other ) = @_;
      $self->less_than($other) || $self->equal_to($other);
  }

Finally, we the C<Printable> role. This role exists solely to provide
an interface. It has no methods, just a list of required methods. In
this case, it just requires a C<to_string> method.

An interface role is useful because it defines both a method and a
I<name>. We know that any class which does this role has a
C<to_string> method, but we can also assume that this method has the
semantics we want. Presumably, in real code we would define those
semantics in the documentation for the C<Printable> role. (1)

Finally, we have the C<US::Currency> class which consumes both the
C<Comparable> and C<Printable> roles.

  with 'Comparable', 'Printable';

It also defines a regular Moose attribute, C<amount>:

  has 'amount' => ( is => 'rw', isa => 'Num', default => 0 );

Finally we see the implementation of the methods required by our
roles. We have a C<compare> method:

  sub compare {
      my ( $self, $other ) = @_;
      $self->amount <=> $other->amount;
  }

By consuming the C<Comparable> role and defining this method, we gain
the following methods for free: C<equal_to>, C<greater_than>,
C<less_than>, C<greater_than_or_equal_to> and
C<less_than_or_equal_to>.

Then we have our C<to_string> method:

  sub to_string {
      my $self = shift;
      sprintf '$%0.2f USD' => $self->amount;
  }

=head1 CONCLUSION

Roles can very powerful. They are a great way of encapsulating
reusable behavior, as well as communicating (semantic and interface)
information about the methods our classes provide.

=head1 FOOTNOTES

=over 4

=item (1)

Consider two classes, C<Runner> and C<Process>, both of which define a
C<run> method. If we just require that an object implements a C<run>
method, we still aren't saying anything about what that method
I<actually does>. If we require an object that implements the
C<Executable> role, we're saying something about semantics.

=back

=head1 AUTHORS

Stevan Little E<lt>stevan@iinteractive.comE<gt>

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

=head1 COPYRIGHT AND LICENSE

Copyright 2006-2009 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