--- /dev/null
+
+=pod
+
+=head1 NAME
+
+Moose::Spec::Role - Formal spec for Role behavior
+
+=head1 DESCRIPTION
+
+B<NOTE:> This document is currently incomplete.
+
+=head2 Components of a Role
+
+=over 4
+
+=item 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.
+
+=item 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.
+
+=item Methods
+
+These are the methods defined within the role. Simple as that.
+
+=item 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.
+
+=item 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.
+
+=item Overridden Methods
+
+The C<override> and C<super> keywords are allowed in roles, but
+their behavior is different from that of it's class counterparts.
+The C<super> in a class refers directly to that class's superclass,
+while the C<super> in a role is deferred and only has meaning once
+the role is composed into a class. Once that composition occurs,
+C<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 I<super> role.
+
+=item Method Modifiers
+
+These are the C<before>, C<around> and C<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 C<override> keyword).
+
+=back
+
+=head2 Role Composition
+
+=head3 Composing into a Class
+
+=over 4
+
+=item Excluded Roles
+
+=item Required Methods
+
+=item Required Attributes
+
+=item Attributes
+
+=item Methods
+
+=item Overridden methods
+
+=item Method Modifiers (before, around, after)
+
+=back
+
+=head3 Composing into a Instance
+
+=head3 Composing into a Role
+
+=over 4
+
+=item Excluded Roles
+
+=item Required Methods
+
+=item Required Attributes
+
+=item Attributes
+
+=item Methods
+
+=item Overridden methods
+
+=item Method Modifiers (before, around, after)
+
+=back
+
+=head3 Role Summation
+
+When multiple roles are added to another role (using the
+C<with @roles> keyword) the roles are composed symmetrically.
+The product of the composition is a composite role
+(L<Moose::Meta::Role::Composite>).
+
+=over 4
+
+=item Excluded Roles
+
+=item Required Methods
+
+=item Required Attributes
+
+=item 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.
+
+=item Methods
+
+Methods with the same name will conflict, but no error is
+thrown, instead the method name is added to the list of
+I<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 }
+
+=item 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.
+
+=item 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.
+
+=back
+
+=head3 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.
+
+=over 4
+
+=item 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 C<foo> methods conflict and the Role::FooBar now requires a
+class or role consuming it to implement C<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 C<foo> method. This conflict results in the
+composite role (that was created by the combination of Role::Foo
+and Role::Bar using the I<with> keyword) having a method requirement
+of C<foo>. The Role::FooBar then fulfills this requirement.
+
+It is important to note that Role::FooBar is simply fulfilling the
+required C<foo> method, and **NOT** overriding C<foo>. This is an
+important distinction to make.
+
+Now here is another example of a (correct) type of overriding, this
+time using the I<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 C<foo> method during composition,
+we allow B<Role::FooBar> to define it's own version of C<foo>.
+
+=back
+
+=head1 SEE ALSO
+
+=over 4
+
+=item Traits
+
+Roles are based on Traits, which originated in the Smalltalk
+community.
+
+=over 4
+
+=item L<http://www.iam.unibe.ch/~scg/Research/Traits/>
+
+This is the main site for the original Traits papers.
+
+=item L<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).
+
+=back
+
+=item 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)
+
+=over 4
+
+=item L<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.
+
+=item L<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.
+
+=back
+
+=back
+
+=head1 AUTHOR
+
+Stevan Little E<lt>stevan@iinteractive.comE<gt>
+
+=head1 COPYRIGHT AND LICENSE
+
+Copyright 2007-2009 by Infinity Interactive, Inc.
+
+L<http://www.iinteractive.com>
+
+This library is free software; you can redistribute it and/or modify
+it under the same terms as Perl itself.
+
+=cut
+
projects / catagits/Gitalist.git / blobdiff