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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: [Newbie] doubly constrained array, dumb question
Date: Tue, 27 Feb 2018 16:13:18 +0100
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Date: 2018-02-27T16:13:18+01:00
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On 27/02/2018 15:34, Jere wrote:
> So the design idea is anything that you actually want to dispatch
> on is relegated to an interface (or multiple interfaces), and you use
> classwide types of the interface to do the dispatching.

I do that in Ada quite customary by declaring an interface before most 
of concrete types.

> This makes for
> what is practically a max 1 level deep extension tree.

Not really. It is not a directly related thing.

> Composition
> allows for you to hide non dispatching operations as you see fit (I.E.
> you only implement the ones you want your client to see) while the
> interfaces allow you to cleanly dispatch for operations.  You are also
> free to only implement the interfaces you want for each level of
> inheritance.  So parent classes can implement interfaces that child
> classes do not.

This is delegation I wanted Ada to have for a long time. Thus would be a 
way to work around limitations put upon multiple inheritance. Combined 
with declaring interfaces it could be a quite powerful model:

    type Interface_T is interface;
    procedure Foo (X : Interface_T) is abstract;

    type T is new Interface_T ...;
    overriding procedure Foo (X : T);

    type S is new Controlled and Interface_T ... with record
       X : S; -- ARG won't let me derive from T, I do it anyway
    end record;
    overriding procedure Foo (X : S) is delegated S.X;

Even more flexible if Ada allowed getting interfaces from concrete 
types. E.g. instead of

    type Interface_T is interface;
    procedure Foo (X : Interface_T) is abstract;

    type T is new Interface_T ...;
    overriding procedure Foo (X : T);

You do

    type T is tagged ...;
    procedure Foo (X : T);

and use T'Interface instead of explicit declaration of Interface_T.

> I'm not saying this is the end all be all method, but it is definitely
> interesting to tinker with and makes you consider designs a bit
> differently (much like pure composition does).

Everything must be a method = there must be a class for any 
type-operation pair.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de