Re: Multiple Interface Inheritance (was Re: JGNAT and ObjectAdaand...)

["Ehud Lamm" <mslamm@mscc.huji.ac.il>]

Brian Rogoff <bpr@shell5.ba.best.com> wrote in message
news:Pine.BSF.4.21.0012241221330.24815-100000@shell5.ba.best.com...
> On Wed, 20 Dec 2000, Tucker Taft wrote:
> > It was not responding for a while, but it seems better now.
> > I was able to find out that it is AI 251 (it badly needs an
> > example, but otherwise is pretty complete).
> >
> > Here is a direct link to the most recent version of this AI:
> >
> >
http://www.ada-auth.org/cgi-bin-acats/cvsweb.cgi/AIs/AI-00251.DOC?rev=1.3
>
> I've been pondering this a little and while it seems to capture the
desired
> Java functionality I wonder if it is possible to aim higher and get some
of
> the additional power of the old GNU C++ signature feature set, namely the
> ability to get the signature (abstract interface type in the AI
terminology)
> of an existing class (tagged type) and use this anonymous signature. Maybe
> a 'Interface attribute or somesuch corresponding to the sigof of GNU C++.
> As it never made it into a working GNU C++ that I used I can't say much
> about its use in practice but Sather had some similar capabilities.
>

My feeling is that as a porposal for an extension to the existing language,
this seems quite nice (I didn't consider all the details yet, so I don't
want to say anything more definite/interesting).
To really achive the state-of-the-art interface support, I think many many
features in the language should be redesigned, and I think doing this is
unreasonable. These ideas are best left for a new language to implement.
Notice that Ada is not orthogonal when it comes to interfaces: there are
many kinds of Ada abstractions, each with its own kind of interface. (I seem
to remember an MRT document listing some 7 methods of building Ada ADTs).
From where I am standing, the importance of interface/implemention
seperation and the importance of SPECIFYING good interface is so high, that
I'd prefer a lanuage more refined in its OVERALL apporahc to interfaces.
(e.g, supose we have 'Interface. What operations are allowed on this? Are
interface first class? I guess not)

It is quite problematic for a multi-paradigm language like Ada to handle
such issues consistently. For example, just consider all the work about
protected types and inheritance. Both supply interfaces - it is just that
the interaction of these causes so much trouble... (and remember: we still
want to be able to reason about interfaces, with out relying on impl.
details).

All these fascinating topics, IMO, but I think for Ada0X we must be modest.
Ada9X did a wonderful job incorporating inheritance into Ada, and I'd love
to see a solution of this quality to these issues. But even the best
solutions (and the AI does indeed look convincing) still seem to me to be
limited in scope. I am afraid I can't offer something better (or even
somethng that SEEMS better), without proposing a major change in the
language, and this is simply not in the cards...


--
Ehud Lamm   mslamm@mscc.huji.ac.il
http://lambda.weblogs.com  <== The Programming Languages Weblog