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From: Robert A Duff <bobduff@world.std.com>
Subject: Re: Disallowing Pre-Defined Operations
Date: 2000/03/21
Message-ID: <wccvh2gxw7v.fsf@world.std.com>#1/1
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Sender: bobduff@world.std.com (Robert A Duff)
References: <8a9eeg$qtv$1@newpoisson.nosc.mil> <38CA05AF.7E77790D@online.no>
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Organization: The World Public Access UNIX, Brookline, MA
Newsgroups: comp.lang.ada
Date: 2000-03-21T00:00:00+00:00
List-Id: <comp.lang.ada>

claveman@cod.nosc.mil (Charles H. Sampson) writes:

>      But it doesn't exactly meet my needs, Robert.  I don't even think 
> it's correct to say that it comes close to meeting my needs.

Not having a certain "*" operator seems very close indeed to having it,
but not being allowed to call it.  Yes, it's different in certain cases
that cause ambiguity, but I can't get excited about these rather rare
cases.

>      For all I know, the abstract approach can cause problems other than 
> ambiguities as well.

Well, you have to admit, *that's* a pretty weak argument.  ;-)

>      The behavior of generics is even further away from what I want.

The issue with generics is the same, whether abstract works the way it
does, or the way you would like it to, and whether you spell it
"abstract" or "disllowed" or whatever.  The problem is that inside the
generic, we have this numeric type, and we legally call "*".  Now what
should that do in an instance where there is no "*" (or where there is a
"*" but it's supposed to be illegal to call it)?  Do you have a proposed
solution to that?  I agree that the current Ada solution is less than
satisfactory.

>...  I 
> haven't verified it myself yet, but several people in this thread have 
> said that an abstract operation reverts to non-abstract in a generic 
> instantiation.  For an operation that has been disallowed, there is 
> nothing to revert to.

So what does it do, in your proposal?  You have to give run-time
semantics, or invent some way to make it illegal.

- Bob