From: Dave Thomas <dave@...>
Date: 2003-12-03T23:51:22+09:00
Subject: Re: mixin vs. multiple inheritence


On Dec 3, 2003, at 5:46, Christoph wrote:

> T. Onoma wrote:
> ...
>>
>> How are mix-in more restricted than multiple inheritance?
>
> There is non such thing as a diamond shape
> mixin graph ...
>

Actually, that's not really true.

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.

Cheers


Dave