Moose::Manual::Roles(3pm)

NAME

Moose::Manual::Roles - Roles, an alternative to deep hierarchies and
base classes

WHAT IS A ROLE?

A role is something that classes do. Usually, a role encapsulates some piece of behavior or state that can be shared between classes. It is
important to understand that roles are not classes. You cannot inherit from a role, and a role cannot be instantiated. We sometimes say that
roles are consumed, either by classes or other roles.

Instead, a role is composed into a class. In practical terms, this means that all of the methods and attributes defined in a role are
added directly to (we sometimes say "flattened into") the class that
consumes the role. These attributes and methods then appear as if they were defined in the class itself. A subclass of the consuming class
will inherit all of these methods and attributes.

Moose roles are similar to mixins or interfaces in other languages.

Besides defining their own methods and attributes, roles can also
require that the consuming class define certain methods of its own. You could have a role that consisted only of a list of required methods, in which case the role would be very much like a Java interface.

Note that attribute accessors also count as methods for the purposes of satisfying the requirements of a role.

A SIMPLE ROLE

Creating a role looks a lot like creating a Moose class:

package Breakable;

use Moose::Role;

has 'is_broken' => (

is => 'rw',
isa => 'Bool',

);

sub break {

my $self = shift;

print "I broke\n";

$self->is_broken(1);

}

Except for our use of Moose::Role, this looks just like a class
definition with Moose. However, this is not a class, and it cannot be
instantiated.

Instead, its attributes and methods will be composed into classes which use the role:

package Car;

use Moose;

with 'Breakable';

has 'engine' => (

is => 'ro',
isa => 'Engine',

);

The "with" function composes roles into a class. Once that is done, the "Car" class has an "is_broken" attribute and a "break" method. The
"Car" class also "does('Breakable')":

my $car = Car->new( engine => Engine->new );

print $car->is_broken ? 'Busted' : 'Still working';
$car->break;
print $car->is_broken ? 'Busted' : 'Still working';

$car->does('Breakable'); # true

This prints:

Still working
I broke
Busted

We could use this same role in a "Bone" class:

package Bone;

use Moose;

with 'Breakable';

has 'marrow' => (

is => 'ro',
isa => 'Marrow',

);

See also Moose::Cookbook::Roles::Recipe1 for an example.

REQUIRED METHODS

As mentioned previously, a role can require that consuming classes
provide one or more methods. Using our "Breakable" example, let's make it require that consuming classes implement their own "break" methods:

package Breakable;

use Moose::Role;

requires 'break';

has 'is_broken' => (

is => 'rw',
isa => 'Bool',

);

after 'break' => sub {

my $self = shift;

$self->is_broken(1);

};

If we try to consume this role in a class that does not have a "break" method, we will get an exception.

You can see that we added a method modifier on "break". We want classes that consume this role to implement their own logic for breaking, but
we make sure that the "is_broken" attribute is always set to true when "break" is called.

package Car

use Moose;

with 'Breakable';

has 'engine' => (

is => 'ro',
isa => 'Engine',

);

sub break {

my $self = shift;

if ( $self->is_moving ) {

$self->stop;

}

}

Roles Versus Abstract Base Classes

If you are familiar with the concept of abstract base classes in other languages, you may be tempted to use roles in the same way.

You can define an "interface-only" role, one that contains just a list of required methods.

However, any class which consumes this role must implement all of the
required methods, either directly or through inheritance from a parent. You cannot delay the method requirement check so that they can be
implemented by future subclasses.

Because the role defines the required methods directly, adding a base
class to the mix would not achieve anything. We recommend that you
simply consume the interface role in each class which implements that
interface.

Required Attributes

As mentioned before, a role requirement may also be satisfied by an
attribute accessor. But any "has" functions, which will generate
accessors that satisfy the role requirement, must be placed before the "with" function that composes the role.

package Breakable;

use Moose::Role;

requires 'stress';

package Car;

use Moose;

has 'stress' => (

is => 'rw',
isa => 'Int',

);

with 'Breakable';

USING METHOD MODIFIERS

Method modifiers and roles are a very powerful combination. Often, a
role will combine method modifiers and required methods. We already saw one example with our "Breakable" example.

Method modifiers increase the complexity of roles, because they make
the role application order relevant. If a class uses multiple roles,
each of which modify the same method, those modifiers will be applied
in the same order as the roles are used:

package MovieCar;

use Moose;

extends 'Car';

with 'Breakable', 'ExplodesOnBreakage';

Assuming that the new "ExplodesOnBreakage" method also has an "after" modifier on "break", the "after" modifiers will run one after the
other. The modifier from "Breakable" will run first, then the one from "ExplodesOnBreakage".

METHOD CONFLICTS

If a class composes multiple roles, and those roles have methods of the same name, we will have a conflict. In that case, the composing class
is required to provide its own method of the same name.

package Breakdancer;

use Moose::Role

sub break {

}

If we compose both "Breakable" and "Breakdancer" in a class, we must
provide our own "break" method:

package FragileDancer;

use Moose;

with 'Breakable', 'Breakdancer';

sub break { ... }

A role can be a collection of other roles:

package Break::Bundle;

use Moose::Role;

with ('Breakable', 'Breakdancer');

METHOD EXCLUSION AND ALIASING

If we want our "FragileDancer" class to be able to call the methods
from both its roles, we can alias the methods:

package FragileDancer;

use Moose;

with 'Breakable' => { -alias => { break => 'break_bone' } },

'Breakdancer' => { -alias => { break => 'break_dance' } };

However, aliasing a method simply makes a copy of the method with the new name. We also need to exclude the original name:

with 'Breakable' => {

-alias => { break => 'break_bone' },
-excludes => 'break',
},
'Breakdancer' => {
-alias => { break => 'break_dance' },
-excludes => 'break',
};

The excludes parameter prevents the "break" method from being composed into the "FragileDancer" class, so we don't have a conflict. This means that "FragileDancer" does not need to implement its own "break" method.

This is useful, but it's worth noting that this breaks the contract
implicit in consuming a role. Our "FragileDancer" class does both the
"Breakable" and "BreakDancer", but does not provide a "break" method.
If some API expects an object that does one of those roles, it probably expects it to implement that method.

In some use cases we might alias and exclude methods from roles, but
then provide a method of the same name in the class itself.

Also see Moose::Cookbook::Roles::Recipe2 for an example.

ROLE EXCLUSION

A role can say that it cannot be combined with some other role. This
should be used with great caution, since it limits the re-usability of the role.

package Breakable;

use Moose::Role;

excludes 'BreakDancer';

APPLYING ROLES

A role can be applied to a class or an instance in other ways besides
using the 'with' syntax.

To apply a role to a class, use Moose::Util and the 'apply_all_roles'
function. If you apply the role to a class, it will affect all objects of that class. You can't apply a role to a class if it has been made
immutable. In some circumstances it may make sense to make the class
mutable, apply the role, then make the class immutable again.

use Moose::Util;
...
my $class = 'MyApp::Test';
$class->meta->make_mutable;
Moose::Util::apply_all_roles($class->meta, ('MyApp::SomeRole'));
$class->meta->make_immutable;

Do not apply roles to classes that have immutable subclasses, since
that will invalidate the metadata of the subclasses.

If you want the role to be applied only to a particular instance and
not to the class, you can apply the roles to the instance instead of
the class's meta:

Moose::Util::apply_all_roles($instance, ('MyApp::SomeRole'));

Or you can use the role's meta object:

MyApp::SomeRole->meta->apply($instance);

The mutable/immutable state is not relevant to roles applied to
instances. See Moose::Role and Moose::Util for more details and
Moose::Cookbook::Roles::Recipe3 for a more developed example.

ADDING A ROLE TO AN OBJECT INSTANCE

Sometimes you may want to add a role to an object instance, rather than to a class. For example, you may want to add debug tracing to one
instance of an object while debugging a particular bug. Another use
case might be to dynamically change objects based on a user's
configuration, as a plugin system.

The best way to do this is to use the "apply_all_roles" function from
Moose::Util:

use Moose::Util qw( apply_all_roles );

my $car = Car->new;
apply_all_roles( $car, 'Breakable' );

This function can apply more than one role at a time, and will do so
using the normal Moose role combination system. We recommend using this function to apply roles to an object. This is what Moose uses
internally when you call "with".

AUTHOR

Dave Rolsky <autarch@urth.org>

COPYRIGHT AND LICENSE

Copyright 2009 by Infinity Interactive, Inc.

<http://www.iinteractive.com>

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

perl v5.10.1 2010-07-02 Moose::Manual::Roles(3pm)