From: Sylvain Joyeux <sylvain.joyeux@...>
Date: 2007-10-07T16:50:16+09:00
Subject: Re: The Case for Multiple-Inheritance

> You could, of course, override #kind_of? to do what you want.
Yes. I could build a MI-like pattern from scratch. I does not seem to be 
the "right way" ...

> What Mentalguy's suggesting is that the relationships may not be best
> modeled as an is-a relationship, as I suggested to Trans early on in
> the discussion. Is-a relationships are very tightly coupled and should
> only be used where such a tight relationship is warranted.
I'm modelling activities (moving to a point, moving the camera, taking a 
picture at a location, this kind of things). To keep things manageable, 
use the inheritance relationship to express that we have abstract actions 
(the generic MoveTo(x, y)) and implementation of those actions 
(MyWayToMoveTo(x, y)).

I need to reason about these relationships because you can use a specific 
movement method in place of a generic one, but you can't use a 
TakePictureNow activity in place of a MoveTo(x, y). However, you could use 
a TakePictureAt(x, y) in place of a MoveTo(x, y) since it is both 
modelling a MoveTo(x, y) *and* a TakePicture.

In most cases, it is not a problem: I represent the fact that 
TakePictureAt(x, y) is in fact a sequence of MoveTo(x, y) and a 
TakePictureNow. It is therefore possible to repesent the thing with single 
inheritance. But there are cases where this kind of separation cannot be 
done -- hence the need for one class to be an implementation of multiple 
models.

Now: why classes ? Some code is tied to the various activities (code to 
start the activity, to stop it, handlers when some event appear) and I do 
want the children models to inherit the methods of their parent models. 

I actuall started to write parts of my framework without using method 
dispatch and inheritance. It began quickly horrible, so I rewrote it using 
it. I'm more than happy now. I would just be happier with MI.

Sylvain