From: Paul Brannan <pbrannan@...>
Date: 2003-12-04T03:45:37+09:00
Subject: Re: mixin vs. multiple inheritence

On Thu, Dec 04, 2003 at 03:09:52AM +0900, T. Onoma wrote:
> What you say is true enough. But I am wondering what it is that supposedly 
> Mutiple-in gives you that Mix-in does not, which was implied by matz, albiet 
> that the it is better Ruby's way.

I'm going to speak about inheritance of implementation here; inheritance
of interface is a separate issue that doesn't really apply to Ruby.

With multiple inheritance, every class knows exactly what its immediate
ancestors are and can explicitly call functions in those classes.
However, complex hierarchies that depend on subtleties of relationships
between base class and derived class are generally brittle and hard to
follow.

With Ruby's mixins, because inheritance is always single-inheritance,
Ruby may adjust the order of inheritance so that a class or module
doesn't necessarily know what its immediate ancestors are.  This is less
flexible, but it's conceptually much simpler and easier to implement
than MI.

I'm yet to see an example using inheritance of implementation where
multiple inheritance would work but Ruby mixins would not that didn't
make me cringe.

Paul