From: Dave Rolsky <autarch@urth.org>
Date: Sat, 14 Feb 2009 17:53:08 +0000 (+0000)
Subject: Clarify how roles do and do not participate in inheritance.
X-Git-Tag: 0.70~6
X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=c89a5d69d49ca4a7850adf01125219e8ee412836;p=gitmo%2FMoose.git

Clarify how roles do and do not participate in inheritance.

Add a section on abstract base classes vs roles
---

diff --git a/lib/Moose/Manual/Roles.pod b/lib/Moose/Manual/Roles.pod
index 73b08df..b1e45b7 100644
--- a/lib/Moose/Manual/Roles.pod
+++ b/lib/Moose/Manual/Roles.pod
@@ -8,15 +8,16 @@ Moose::Manual::Roles - Roles, an alternative to deep hierarchies and base classe
 
 A role is something that classes do. Usually, a role encapsulates some
 piece of behavior or state that can be shared between classes. It is
-important to understand that I<roles are not classes>. Roles do not
-participate in inheritance, and a role cannot be instantiated. We
-sometimes say that classes I<consume> roles.
+important to understand that I<roles are not classes>. You cannot
+inherit from a role, and a role cannot be instantiated. We sometimes
+say that roles are I<consumed>, either by classes or other roles.
 
 Instead, a role is I<composed> into a class. In practical terms, this
 means that all of the methods and attributes defined in a role are
 added directly to (we sometimes say "flattened into") the class that
 consumes the role. These attributes and methods then appear as if they
-were defined in the class itself.
+were defined in the class itself. A subclass of the consuming class
+will inherit all of these methods and attributes.
 
 Moose roles are similar to mixins or interfaces in other languages.
 
@@ -147,6 +148,24 @@ set to true when C<break> is called.
       }
   }
 
+=head2 Roles Versus Abstract Base Classes
+
+If you are familiar with the concept of abstract base classes in other
+languages, you may be tempted to use roles in the same way.
+
+You I<can> define a "interface-only" role, one that contains I<just> a
+list of required methods.
+
+However, any class which consumes this role must implement all of the
+required methods, either directly or through inheritance from a
+parent. You cannot delay the method requirement check so that they can
+be implemented by future subclasses.
+
+Because the role defines the required methods directly, adding a base
+class to the mix would not achieve anything. We recommend that you
+simply consume the interface role in each class which implements that
+interface.
+
 =head1 USING METHOD MODIFIERS
 
 Method modifiers and roles are a very powerful combination.  Often, a