Re: Is the Writing on the Wall for Ada?

["Robert I. Eachus" <rieachus@attbi.com>]

Hyman Rosen wrote:

> Thereby exhibiting the simple lack of courage to add multiple
> inheritance. Why should we limit to inheriting from one type?
> What is it about one that makes it better than two or more?

Actually with interfaces added, Ada will have two ways of doing multiple 
inheritance, mix-ins and interfaces.  This is fine because each is a 
solution for different design idioms.

Why not add "full" multiple inheritance and be done with it?  Sorry 
there is no such thing.  Or if you prefer, deriving from several 
interface classes with no privileged parent does exactly that.

What having a single parent allows is inheriting both an interface and 
an implementation.  You can't inherit multiple implementations, at least 
not and have a single concrete implementation.  Let me give an example 
so you can see the problems.  Let's say I have two point types, one of 
which uses Cartesian coordinates, one of which uses polar coordinates.

How do I implement multiple inheritance of both representations?  It 
can't be done.  A representation that stored both cartesian and polar 
coordinates would be possible.  But the compiler can't automatically 
generate the code to insure that both representations agree.  In fact if 
you think about the representable points in both representations, they 
are different sets!  Some points are in both sets, for example those on 
the X and Y axis but almost all points in one representation do not have 
exact representations in the other.  In addition, some of the points in 
the cartesian domain will be at least (Sqrt(2)-1) * Element'Last away 
from any point in the polar domain.

Notice that if I do interface inheritance from the two concrete types, I 
can program the underlying routines to do something sensible when say 
adding a polar displacement to a cartesian point.  By the way, notice 
that in Ada if I want I can provide a type with both cartesian and polar 
implementations.  As a programmer, I can go further and add the 
"missing" operations like adding a cartesian point to a polar point.  Do 
everything right and "+" will just work.  That is a kind of multiple 
implementations of a type that I can live with, and I don't know of any 
other language where you can do such overloading.  (It is tricky enough 
to get right in Ada...)

Interface inheritance will make doing this sort of thing easier in Ada.
But even now, Ada is providing more multiple inheritance than many 
languages which are accepted as having "full" multiple inheritance. 
Sorry if that doesn't satisfy you.

--
                                          Robert I. Eachus

"As far as I'm concerned, war always means failure." -- Jacques Chirac, 
President of France
"As far as France is concerned, you're right." -- Rush Limbaugh