From: "Mauricio Fernández" <batsman.geo@...>
Date: 2003-04-07T18:36:10+09:00
Subject: Re: To inherit or to include? That is the question.

On Mon, Apr 07, 2003 at 04:47:25PM +0900, Robert Klemme wrote:
> > So, the module Foo was able to introduce some state into the class Bar.
> 
> Oops!  Something learned again.  Obviously I misread something.  Maybe the
> difference then is that a module can't introduce state on initialization?
> 

???
batsman@kodos:/tmp$ expand -t 2 a.rb
module A
  def initialize(*args)
    args.each do |a|
      puts "Adding i.v. #{a}"
      instance_eval "@#{a} = 1"
    end
  end
end

class B
  include A
end

a = B.new("a")
puts a.instance_variables
batsman@kodos:/tmp$ ruby a.rb
Adding i.v. a
@a

The more I think about it, the more it seems to me that the following
are the only differences:
 * a module cannot be instantiated
 * there is no relation between the singleton method of a class and that
  of an included module (as opposed to what happens in normal
  inheritance)
 * a module cannot be inherited from ;)

We can then choose between classes and modules to model different
designs, but as far as the interpreter is concerned these seems to be
the only differences (gotta check the source).

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