From: Paul Brannan <pbrannan@...>
Date: 2003-12-04T02:36:21+09:00
Subject: Re: mixin vs. multiple inheritence

On Wed, Dec 03, 2003 at 11:51:22PM +0900, Dave Thomas wrote:
> module A; end
> module B; include A; end
> module C; include A; end
> class D; include B; include C; end
> 
> You can build as complicated a mixin graph as you can a MI graph. The 
> difference is that because mixins are modules, they don't have their 
> own state, but instead parasitically use the state of the class into 
> which they are mixed. This eliminates the underlying issue with MI, 
> where there may be two _instances_ (logically) of the class at the top 
> of the diamond.

Mixins in Ruby always create a linear graph.  Consider your example with
a method foo added to each class that prints the name of the class and
then calls foo in the base class:

  module A; def foo; puts "A"; end; end
  module B; include A; def foo; puts "B"; super; end; end
  module C; include A; def foo; puts "C"; super; end; end
  class D; include B; include C; def foo; puts "D"; super; end; end

  D.new.foo

The resulting output is:

  D
  C
  B
  A

I know of no way to get C#foo to call A#foo here.

Paul