Moose::Cookbook::Roles::Comparable_CodeReuse(3)

NAME
Moose::Cookbook::Roles::Comparable_CodeReuse - Using roles for code reuse

VERSION
version 2.2012

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;
  }

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 "Eq". First, note that we've replaced "use Moose" with "use Moose::Role". We also have a new sugar function, "requires":

  requires 'equal_to';

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

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

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

  with 'Eq';

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

The "Comparable" role requires a method, "compare":

  requires 'compare';

The "Comparable" role also provides a number of other methods, all of which ultimately rely on "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 define the "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 "to_string" method.

An interface role is useful because it defines both a method and a name. We know that any class which does this role has a "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 "Printable" role. (1)

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

  with 'Comparable', 'Printable';

It also defines a regular Moose attribute, "amount":

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

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

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

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

Then we have our "to_string" method:

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

CONCLUSION
Roles can be 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.

FOOTNOTES

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

AUTHORS

•   Stevan Little <stevan.little@iinteractive.com>

•   Dave Rolsky <autarch@urth.org>

•   Jesse Luehrs <doy@tozt.net>

•   Shawn M Moore <code@sartak.org>

•   יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>

•   Karen Etheridge <ether@cpan.org>

•   Florian Ragwitz <rafl@debian.org>

•   Hans Dieter Pearcey <hdp@weftsoar.net>

•   Chris Prather <chris@prather.org>

•   Matt S Trout <mst@shadowcat.co.uk>

COPYRIGHT AND LICENSE
This software is copyright (c) 2006 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.