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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Class-wide types algebra
Date: Tue, 27 Sep 2016 19:05:43 -0500
Organization: JSA Research & Innovation
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Date: 2016-09-27T19:05:43-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:nr8rah$14gp$1@gioia.aioe.org...
> ... E.g.
>
>    type Root_Stream_Type is ...
>
> OK, that was a mistake, it should have been
>
>    type Root_Stream_Interface is limited interface;
>    type Root_Stream_Type is abstract new Root_Stream_Interface
>       with private;

We would have made the above change years ago if it wasn't severely 
incompatible.

> With T'Interface you could still work it around:

But you'd still have the problem that interfaces can't be hidden (which is 
the source of the incompatibility). And lifting that restriction would be a 
nightmare (we've tried on several occasions), as you quickly get scenarios 
where you have to have two different copies of the same interface in order 
to keep sanity about the operations. (Else one has to totally abandon 
privacy, allowing hidden operations to be overridden. That leads to usage 
madness...)

The problem with all forms of MI is diamond inheritance, which get much 
worse in Ada because of our strict privacy rules. I don't think that could 
ever be solved for Ada (perhaps a new language could solve it, but not Ada).

                                         Randy.