From: dblack@...
Date: 2006-07-11T09:25:13+09:00
Subject: Re: include vs. extend

Hi --

On Tue, 11 Jul 2006, Daniel DeLorme wrote:

> 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?

It's understood that if you extend an object, it's because you *want*
to put the module ahead of the object's class in the method look-up
path.  Otherwise, you'd put the functionality you want in the class
:-)  By the same token, it would be very unusual to put extend in
initialize.

Basically you use extend when you want to control behavior via modules
on a per-object basis.  A common use is for classes:

   module M
     def meth
       puts "Hi"
     end
   end

   class C
     extend M
   end

   C.meth   # Hi

obj.extend(Mod) is equivalent to:

   class << obj
     include Mod
   end

(or very nearly equivalent; there may be some arcane difference I'm
not remembering).  So it's all really include operations -- but, when
done with extend, the class doing the including is a singleton class.


David

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