=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.
+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
+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
+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 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
+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 fufilling attribute must implement at least as much
+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 readonly, 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
+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 Overriden Methods
+=item Overridden Methods
-The C<override> and C<super> keywords are allowed in roles, but
-thier behavior is different from that of it's class counterparts.
-The C<super> in a class refers directly to that class's superclass,
+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.
+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
+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
+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 Role
+=head3 Composing into a Class
=over 4
=item Methods
-=item Overriden methods
+=item Overridden methods
=item Method Modifiers (before, around, after)
=back
-=head3 Composing into a Class
+=head3 Composing into a Instance
+
+=head3 Composing into a Role
=over 4
=item Methods
-=item Overriden methods
+=item Overridden methods
=item Method Modifiers (before, around, after)
=back
-=head3 Composing into a Instance
-
-=head2 Role Summation
+=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
+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 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.
+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
+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
+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
+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
+
+ The composite role (A,B) has
method set { a, b, d, e }
conflict set { c }
-=item Overriden methods
+=item Overridden methods
-An overriden method can conflict in one of two ways.
+An overridden 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
+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.
+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 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.
+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
+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
+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.
+Roles are based on Traits, which originated in the Smalltalk
+community.
-=over 4
+=over 4
=item L<http://www.iam.unibe.ch/~scg/Research/Traits/>
=item L<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).
+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
+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)
=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
+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>
=head1 COPYRIGHT AND LICENSE
-Copyright 2007-2008 by Infinity Interactive, Inc.
+Copyright 2007-2009 by Infinity Interactive, Inc.
L<http://www.iinteractive.com>
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