Страницы справочника OS X

Эта страница руководства для  версии 10.9 Mac OS X

Читать страницы руководства

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



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

VERSION
       version 2.0604

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.

AUTHOR
       Moose is maintained by the Moose Cabal, along with the help of many contributors. See "CABAL" in
       Moose and "CONTRIBUTORS" in Moose for details.

COPYRIGHT AND LICENSE
       This software is copyright (c) 2012 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 v5.16.2                                     2012-09-19                             Moose::Spec::Role(3)