Moose::Spec::Role(3)					User Contributed Perl Documentation				      Moose::Spec::Role(3)

NAME

       Moose::Spec::Role - Formal spec for Role behavior

VERSION

       version 2.1202

DESCRIPTION

       NOTE: This document is currently incomplete.

   Components of a Role
       Excluded Roles
	   A role can have a list of excluded roles, these are basically roles that they shouldn't be composed with. This is not just direct
	   composition either, but also "inherited" composition.

	   This feature was taken from the Fortress language and is really of most use when building a large set of role "building blocks" some of
	   which should never be used together.

       Attributes
	   A roles attributes are similar to those of a class, except that they are not actually applied. This means that methods that are
	   generated by an attributes accessor will not be generated in the role, but only created once the role is applied to a class.

       Methods
	   These are the methods defined within the role. Simple as that.

       Required Methods
	   A role can require a consuming class (or role) to provide a given method. Failure to do so for classes is a fatal error, while for
	   roles it simply passes on the method requirement to the consuming role.

       Required Attributes
	   Just as a role can require methods, it can also require attributes.	The requirement fulfilling attribute must implement at least as
	   much as is required. That means, for instance, that if the role requires that the attribute be read-only, then it must at least have a
	   reader and can also have a writer. It means that if the role requires that the attribute be an ArrayRef, then it must either be an
	   ArrayRef or a subtype of an ArrayRef.

       Overridden Methods
	   The "override" and "super" keywords are allowed in roles, but their behavior is different from that of its class counterparts.  The
	   "super" in a class refers directly to that class's superclass, while the "super" in a role is deferred and only has meaning once the
	   role is composed into a class. Once that composition occurs, "super" then refers to that class's superclass.

	   It is key to remember that roles do not have hierarchy, so they can never have a super role.

       Method Modifiers
	   These are the "before", "around" and "after" modifiers provided in Moose classes. The difference here is that the modifiers are not
	   actually applied until the role is composed into a class (this is just like attributes and the "override" keyword).

   Role Composition
       Composing into a Class

       Excluded Roles
       Required Methods
       Required Attributes
       Attributes
       Methods
       Overridden methods
       Method Modifiers (before, around, after)

       Composing into a Instance

       Composing into a Role

       Excluded Roles
       Required Methods
       Required Attributes
       Attributes
       Methods
       Overridden methods
       Method Modifiers (before, around, after)

       Role Summation

       When multiple roles are added to another role (using the "with @roles" keyword) the roles are composed symmetrically.  The product of the
       composition is a composite role (Moose::Meta::Role::Composite).

       Excluded Roles
       Required Methods
       Required Attributes
       Attributes
	   Attributes with the same name will conflict and are considered a unrecoverable error. No other aspect of the attribute is examined, it
	   is enough that just the attribute names conflict.

	   The reason for such early and harsh conflicts with attributes is because there is so much room for variance between two attributes that
	   the problem quickly explodes and rules get very complex. It is my opinion that this complexity is not worth the trouble.

       Methods
	   Methods with the same name will conflict, but no error is thrown, instead the method name is added to the list of required methods for
	   the new composite role.

	   To look at this in terms of set theory, each role can be said to have a set of methods. The symmetric difference of these two sets is
	   the new set of methods for the composite role, while the intersection of these two sets are the conflicts. This can be illustrated like
	   so:

	      Role A has method set { a, b, c }
	      Role B has method set { c, d, e }

	      The composite role (A,B) has
		  method   set { a, b, d, e }
		  conflict set { c }

       Overridden methods
	   An overridden method can conflict in one of two ways.

	   The first way is with another overridden method of the same name, and this is considered an unrecoverable error. This is an obvious
	   error since you cannot override a method twice in the same class.

	   The second way for conflict is for an overridden method and a regular method to have the same name. This is also an unrecoverable error
	   since there is no way to combine these two, nor is it okay for both items to be composed into a single class at some point.

	   The use of override in roles can be tricky, but if used carefully they can be a very powerful tool.

       Method Modifiers (before, around, after)
	   Method modifiers are the only place where the ordering of role composition matters. This is due to the nature of method modifiers
	   themselves.

	   Since a method can have multiple method modifiers, these are just collected in order to be later applied to the class in that same
	   order.

	   In general, great care should be taken in using method modifiers in roles. The order sensitivity can possibly lead to subtle and
	   difficult to find bugs if they are overused. As with all good things in life, moderation is the key.

       Composition Edge Cases

       This is a just a set of complex edge cases which can easily get confused. This attempts to clarify those cases and provide an explanation
       of what is going on in them.

       Role Method Overriding
	   Many people want to "override" methods in roles they are consuming.	This works fine for classes, since the local class method is
	   favored over the role method. However in roles it is trickier, this is because conflicts result in neither method being chosen and the
	   method being "required" instead.

	   Here is an example of this (incorrect) type of overriding.

	       package Role::Foo;
	       use Moose::Role;

	       sub foo { ... }

	       package Role::FooBar;
	       use Moose::Role;

	       with 'Role::Foo';

	       sub foo { ... }
	       sub bar { ... }

	   Here the "foo" methods conflict and the Role::FooBar now requires a class or role consuming it to implement "foo". This is very often
	   not what the user wants.

	   Now here is an example of the (correct) type of overriding, only it is not overriding at all, as is explained in the text below.

	       package Role::Foo;
	       use Moose::Role;

	       sub foo { ... }

	       package Role::Bar;
	       use Moose::Role;

	       sub foo { ... }
	       sub bar { ... }

	       package Role::FooBar;
	       use Moose::Role;

	       with 'Role::Foo', 'Role::Bar';

	       sub foo { ... }

	   This works because the combination of Role::Foo and Role::Bar produce a conflict with the "foo" method. This conflict results in the
	   composite role (that was created by the combination of Role::Foo and Role::Bar using the with keyword) having a method requirement of
	   "foo". The Role::FooBar then fulfills this requirement.

	   It is important to note that Role::FooBar is simply fulfilling the required "foo" method, and **NOT** overriding "foo". This is an
	   important distinction to make.

	   Now here is another example of a (correct) type of overriding, this time using the excludes option.

	       package Role::Foo;
	       use Moose::Role;

	       sub foo { ... }

	       package Role::FooBar;
	       use Moose::Role;

	       with 'Role::Foo' => { -excludes => 'foo' };

	       sub foo { ... }
	       sub bar { ... }

	   By specifically excluding the "foo" method during composition, we allow Role::FooBar to define its own version of "foo".

SEE ALSO

       Traits
	   Roles are based on Traits, which originated in the Smalltalk community.

	   <http://www.iam.unibe.ch/~scg/Research/Traits/>
	       This is the main site for the original Traits papers.

	   Class::Trait
	       I created this implementation of traits several years ago, after reading the papers linked above. (This module is now maintained by
	       Ovid and I am no longer involved with it).

       Roles
	   Since they are relatively new, and the Moose implementation is probably the most mature out there, roles don't have much to link to.
	   However, here is some bits worth looking at (mostly related to Perl 6)

	   <http://www.oreillynet.com/onlamp/blog/2006/08/roles_composable_units_of_obje.html>
	       This is chromatic's take on roles, which is worth reading since he was/is one of the big proponents of them.

	   <http://svn.perl.org/perl6/doc/trunk/design/syn/S12.pod>
	       This is Synopsis 12, which is all about the Perl 6 Object System.  Which, of course, includes roles.

AUTHORS

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

       o   Dave Rolsky <autarch@urth.org>

       o   Jesse Luehrs <doy@tozt.net>

       o   Shawn M Moore <code@sartak.org>

       o   XXXX XXX'XX (Yuval Kogman) <nothingmuch@woobling.org>

       o   Karen Etheridge <ether@cpan.org>

       o   Florian Ragwitz <rafl@debian.org>

       o   Hans Dieter Pearcey <hdp@weftsoar.net>

       o   Chris Prather <chris@prather.org>

       o   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.

perl v5.18.2							    2014-01-19						      Moose::Spec::Role(3)