Attempt to explain why a role that just requires methods is useful. Prose cleanups...

[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

In this recipe we examine the role support provided in Moose. "Roles" may be
described in many ways, but there are two main ways in which they are used: as
interfaces, and as a means of code reuse. This recipe demonstrates the
construction and incorporation of roles that define comparison and display of
objects.

Let's start by examining B<Eq>. You'll notice that instead of the familiar C<use
Moose> you might be expecting, here we use C<Moose::Role> to make it clear that
this is a role. We encounter a new keyword, C<requires>:

  requires 'equal_to';

What this does is to indicate that any class which "consumes" (that is to say,
"includes using C<with>", as we'll see a little later) the B<Eq> role I<must>
include an C<equal_to> method, whether this is provided by the class itself, one
of its superclasses, or another role consumed by the class (1).

In addition to requiring an C<equal_to> method, B<Eq> defines a C<not_equal_to>
method, which simply inverts the result of C<equal_to>. Defining additional
methods in this way, by using only a few base methods that target classes must
define, is a useful pattern to provide maximum functionality with minimum
effort.

After the minimal B<Eq>, we next move on to B<Comparable>. The first thing you
will notice is another new keyword, C<with>:

  with 'Eq';

C<with> is used to provide a list of roles which this class (or role) consumes.
Here, B<Comparable> only consumes one role (B<Eq>). In effect, it is as if we
defined a C<not_equal_to> method within Comparable, and also promised to fulfill
the requirement of an C<equal_to> method.

B<Comparable> itself states that it requires C<compare>. Again, it means that
any classes consuming this role must implement a C<compare> method.

  requires 'compare';

B<Comparable> defines an C<equal_to> method which satisfies the B<Eq> role's
requirements. This, along with a number of other methods (C<greater_than>,
C<less_than>, C<greater_than_or_equal_to>, and C<less_than_or_equal_to>) is
simply defined in terms of C<compare>, once again demonstrating the pattern of
defining a number of utility methods in terms of only a single method that the
target class need implement.

  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);
  }

Next up is B<Printable>. This is a very simple role, akin to B<Eq>. It merely
requires a C<to_string> method. Roles that only require methods are very much
like Java's interfaces. If we know that a class does the B<Printable> role, it
not only tells us that we can call the C<to_string> method on it, but also that
C<to_string> has the precise semantics we want (consider classes B<Tree> and
B<Dog>, both with method C<bark>).

Finally, we come to B<US::Currency>, a class that allows us to reap the benefits
of our hard work. This is a regular Moose class, so we include the normal C<use
Moose>. It consumes both B<Comparable> and B<Printable>, as the following line
shows:

  with 'Comparable', 'Printable';

It also defines a regular Moose attribute, C<amount>, with a type constraint of
C<Num> and a default of C<0>:

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

Now we come to the core of the class. First up, we define a C<compare> method:

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

As you can see, it simply compares the C<amount> attribute of this object with
the C<amount> attribute of the other object passed to it. With the single
definition of 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>.

We end the class with a definition of the C<to_string> method, which formats the
C<amount> attribute for display:

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

=head1 CONCLUSION

This recipe has shown that roles can be very powerful and immensely useful, and
save a great deal of repetition.

=head1 FOOTNOTES

=over 4

=item (1)

At present, method requirements from roles cannot be satisfied by attribute
accessors. This is a limitation of Moose, and will most likely be rectified in a
future release.

=back

=head1 AUTHOR

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