From: Joel VanderWerf <vjoel@...>
Date: 2005-07-29T02:04:30+09:00
Subject: Re: What's so special about operators,      built-in classes and modules?

Jim Weirich wrote:

> I found this particular entry to be less helpful than other resources. 
> The problem is rarely which method to call (which actually is not a
> diamond inheritance problem ... it can arise whenever two base classes
> share a method with a common name).  The big problem with diamond
> inheritance is the question of instance data ... do you have one or two
> copies?
> 
> (and given the way Ruby handles instance variables, I find it hard to
> imagine the answer to this question would ever be two).

Dim memory... wasn't there a proposal to have per-module (or class)
instance variables, so that different parts of the inheritance chain
don't step on each other?

Wouldn't this introduce the diamond problem?

Take a case like this:

  module Counts
    def inc
      @count ||= 0
      @count += 1
    end

    def cur
      @count
    end
  end

  module CountsDucks
    include Counts

    def saw_a_duck
      inc
    end

    def duck_count
      cur
    end
  end

  module CountsChickens
    include Counts

    def saw_a_chicken
      inc
    end

    def chicken_count
      cur
    end
  end

  class C
    include CountsDucks
    include CountsChickens
  end

  c = C.new

  c.saw_a_duck
  c.saw_a_duck
  c.saw_a_chicken

  p c.duck_count
  p c.chicken_count


Now, the output is

3
3

But using per-module instance variables, you would want this to be

2
1

This seems like a case where you would want there to be two copies of
the (ivars defined by the) Counts module.