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From: "Robert I. Eachus" <rieachus@attbi.com>
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Newsgroups: comp.lang.ada
Subject: Re: MI in Ada 200X (was: Suggestion for Ada 200x - Interface
 inheritance)
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Xref: archiver1.google.com comp.lang.ada:37965
Date: 2003-05-29T02:13:07+00:00
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M�rio Amado Alves wrote:

> Everybody wants class MI. The reasons it was left out of Ada 95 and
> 83, the cons, are weaker than the pros. So every Adaist whishes to see
> it in Ada 200X.

I must not be everybody.  Ada currently supports the mix-in model of 
multiple inheritance.  I sometimes have a type declaration followed by 
half a dozen mixins:

   type Visible is tagged private;
private
   type Foo is abstract tagged record...end record;
   type Foo1 is new Mixin1(Foo,...);
   type Foo2 is new Mixin2(Foo1,...);
   type Foo3 is new Mixin3(Foo2,...);
   ...
   type Visible type is new MixinN(FooN,...);

I've even used multiple instances of the same mixin, for example a 
sparse array implementation which used two instances of the same linked 
list package. (The matricies I was working with were sparse enough that 
walking a list was probably faster than using a tree index.  But I could 
have added two AVL tree mixins if necessary with little change to the code.)

The only problem I have with this approach is the need to come up with 
all those intermediate type names.  (But the names are necessary, you 
have to use them in the calls to the various mixins.  Even if you could 
use the name of the generic, it could be ambiguous, as in the sparse 
matrix example.  But Bar(Foo2(Object)) calls the right Bar operation.

I have no objection to adding the interface abstraction to Ada, but to 
me in is a solution to problems other than MI.