From: Rick DeNatale <rick.denatale@...>
Date: 2007-09-27T02:28:24+09:00
Subject: Re: difference between inheritance and mixin

On 9/26/07, David A. Black <dblack@rubypal.com> wrote:
> Hi --
>
> On Wed, 26 Sep 2007, Phrogz wrote:
>
> > On Sep 26, 12:06 am, Vellingiri Arul <hariharan....@rediffmail.com>
> > wrote:
> >> Hai friends,
> >>       Can anybody can answer this question.
> >> what is the difference between inheritance and mixin.
> >>     what is the use of mixin and what is the use of inheritance.
> >
> > Mixin modules are searched before the parent class.
> > http://phrogz.net/RubyLibs/RubyMethodLookupFlow.png
> >
> > Also, you can only inherit from one parent class, but you can mixin
> > many modules to the same class.
> >
> > Because modules cannot inherit from other modules or classes, there is
> > no problem of 'diamond' inheritance.
>
> Modules can mix in modules, though, so you could have:
>
>               module M
>
>      module N         module O
>
>                class C
>
>
> But the diamond problem is avoided by having the order of mixing in be
> significant, so that there's no ambiguity about which module/class is
> to be searched.

On the other hand, Ruby has varied semantics  for different versions
of structures like this:

       Module M
                     \
                      Class C
                           |
                      Class D
      Module M  |
                    \     |
                    Class E

If M defines a method m and D overrides m, it's version dependent
which definition instances of E get.  At least for a while in the 1.9
history, it would get the original version back, which seems to be the
'correct' semantics to me.  Last time I checked though 1.9 went back
to ignoring the re-inclusion.
-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/