From: Daniel DeLorme <dan-ml@...42.com>
Date: 2006-07-11T09:13:24+09:00
Subject: include vs. extend

I was under the impression that include and extend were basically module-level 
and object-level versions of the same thing, that including a module at the 
class level had the same effect as extending an object. But it seems there is a 
big difference:

class Foo
   include Enumerable
end
class Bar
   def initialize
     extend Enumerable
   end
end
(class << Foo.new;self;end).ancestors #=> [Foo, Enumerable, Object, Kernel]
(class << Bar.new;self;end).ancestors #=> [Enumerable, Bar, Object, Kernel]

This makes a whole world of difference when you want to wrap certain methods 
with additional functionality. In such cases you *have* to use extend (or ugly 
techniques like aliasing the old method). Why the difference? In what cases 
should I use include and what cases should I use extend?

Daniel