Re: Rational for not making cursor tagged in Containers

["Randy Brukardt" <randy@rrsoftware.com>]

"Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
news:wcc8xclrz1j.fsf@shell01.TheWorld.com...
> "Randy Brukardt" <randy@rrsoftware.com> writes:
>
> > "Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
> > news:wccbqhizsh0.fsf@shell01.TheWorld.com...
> > ...
> >> I also thought (and continue to think) that array types should be
> >> allowed to have discriminants.  The quote above was a jab at people
> >> who thought otherwise.  I lost that argument.
> >
> > That would have been me. I think I was arguing simply from the
> > implementation complexity standpoint
>
> Implementation complexity for existing (Ada 83) implementations.
> A newly-designed implementation would have had no trouble.
> 20-20 hindsight!

That's probably true, but there were almost no new implementations. I think
GNAT was the only one. We forked our compiler, but we certainly did not
start from scratch. Implementing expression resolution once was enough (by
far!).

> I claim that it's much easier to implement Ada 95 than it is to
> implement Ada 83 and then adapt that implementation to Ada 95.

That has to be true, simply because there are rules in Ada 83 that don't
exist in Ada 95; clearly there is an effort to support those rules. But I
don't think it matters in any important practical way; most everything was
an addition, not a change.

> The same applies to 95/2005.  I sweated blood implementing the AI-318
> features (limited constructor functions) for Ada 2005 in GNAT, but I'm
> pretty sure that AI-318 would be no extra trouble in a from-scratch
> implementation of Ada 2005.

I suspect that you are being kind; it would be an amazing amount of work
either way. Passing finalization chains, task masters, and *moving* them is
outside of the normal intent in any language design - finalization chains
are pretty much the only way to implement that, no matter what bizarre
requirements are added in unusual cases. (My intent is simply to add a
number of new calling (sub)conventions; the only issue is if they are used
for any existing subprograms written in assembler. But those new conventions
will take a lot of work to make them work properly.) But the big gain in not
implementing Ada 95 would not be implementing all of that hair-brained
return-by-reference stuff.

                      Randy.