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and 142 contributors-
Stevan Little
-
Dave Rolsky
-
Jesse Luehrs
-
Shawn M Moore
-
יובל קוג'מן (Yuval Kogman)
-
Florian Ragwitz
-
Hans Dieter Pearcey
-
Chris Prather
-
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NAME
Moose::Cookbook::Roles::Comparable_CodeReuse - Using roles for code reuse
VERSION
version 2.2201
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 replaceduse Moose
withuse 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 itsnot_equal_to
method in terms of the requiredequal_to
method. This lets us minimize the methods that consuming classes must provide.The next role,
Comparable
, builds on theEq
role. We includeEq
inComparable
usingwith
, another new sugar function:with 'Eq';
The
with
function takes a list of roles to consume. In our example, theComparable
role provides theequal_to
method required byEq
. However, it could opt not to, in which case a class that consumedComparable
would have to provide its ownequal_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 oncompare
.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 ato_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 thePrintable
role. (1)Finally, we have the
US::Currency
class which consumes both theComparable
andPrintable
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
andless_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
andProcess
, both of which define arun
method. If we just require that an object implements arun
method, we still aren't saying anything about what that method actually does. If we require an object that implements theExecutable
role, we're saying something about semantics.
AUTHORS
Stevan Little <stevan@cpan.org>
Dave Rolsky <autarch@urth.org>
Jesse Luehrs <doy@cpan.org>
Shawn M Moore <sartak@cpan.org>
יובל קוג'מן (Yuval Kogman) <nothingmuch@woobling.org>
Karen Etheridge <ether@cpan.org>
Florian Ragwitz <rafl@debian.org>
Hans Dieter Pearcey <hdp@cpan.org>
Chris Prather <chris@prather.org>
Matt S Trout <mstrout@cpan.org>
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.
Module Install Instructions
To install Moose, copy and paste the appropriate command in to your terminal.
cpanm Moose
perl -MCPAN -e shell install Moose
For more information on module installation, please visit the detailed CPAN module installation guide.