From: Vincent Fourmond <vincent.fourmond@9online.fr>
Date: 2006-12-22T01:59:37+09:00
Subject: Re: extend weirdness?

Ian White wrote:
> Hi Group,
> 
> This code:
>     module Foo
>       def self.included(base)
>         base.class_eval <<-end_eval
>           def self.meth
>             "Foo"
>           end
>         end_eval
>       end
>     end
> 
>     module Bar
>       def self.included(base)
>         base.class_eval do
>           extend ClassMethods
>         end
>       end
> 
>       module ClassMethods
>         def meth
>           "Bar"
>         end
>       end
>     end
> 
>     class A
>       include Foo
>       include Bar
>     end
> 
>     puts A.meth
> 
> produces:
>     Foo
> 
> whereas I would expect it to produce "Bar"
> 
> Can anyone explain what's going on here?  i.e. Why the latter extend
> with Bar doesn't over-write the class method introduced by Foo

  The answer is simple: extend doesn't overwrite methods, it just
includes the module for the singleton object. As there is already a
singleton method 'meth' defined in the singleton object A, this method
is used systematically in method calls.

  Does that explain your result ? For information, extend is

rb_extend_object(obj, module)
    VALUE obj, module;
{
    rb_include_module(rb_singleton_class(obj), module);
}

  It is basically like

class << self
  include Something
end

  Cheers,

	Vince

-- 
Vincent Fourmond, PhD student
http://vincent.fourmond.neuf.fr/