NAME

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

DESCRIPTION

Components of a Role

Excluded Roles

Attributes

Methods

Required Methods

Overriden Methods

The override and super keywords are allowed in roles, but thier behavior is different from that of it's 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

Role to Role Composition Rules

When a role is added to another role (using the with keyword) the two roles are composed symmetrically. The product of the composition is a third composite role.

Attributes

Attributes with the same name will conflict and are considered a un-recoverable error. No other aspect of the attribute is examained, 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 }

Overriden methods

An overriden method can conflict in one of two ways.

The first way is with another overriden method of the same name, and this is considered an un-recoverable 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 overriden method and a regular method to have the same name. This is also an un-recoverable 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.

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 maintatined 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

Stevan Little <stevan@iinteractive.com>

COPYRIGHT AND LICENSE

Copyright 2007 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.