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From: bobduff@world.std.com (Robert A Duff)
Subject: Re: Q: access to subprogram
Date: 1996/07/28
Message-ID: <Dv99nL.6uu@world.std.com>#1/1
X-Deja-AN: 170653998
references: <4rb9dp$qe6@news1.delphi.com> <JSA.96Jul2221357@organon.com>
 <4t9un9$imn@goanna.cs.rmit.edu.au>
organization: The World Public Access UNIX, Brookline, MA
newsgroups: comp.lang.ada
Date: 1996-07-28T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <4t9un9$imn@goanna.cs.rmit.edu.au>,
Richard A. O'Keefe <ok@goanna.cs.rmit.edu.au> wrote:
>>How much are upward funargs used in day to day functional programming? I 
>>think Bob Duff is still looking for a compelling example of why they are 
>>so much better than the explicitly simulated closure via an instance of a 
>>tagged type.
>
>Whenever you simulate any mechanism,
>...[all manner of evil happens]

True, but the logical conclusion is that all useful mechanisms that
anybody has ever thought of should be included in the language.  One has
to make a trade-off between a useful gizmo and language complexity
(etc).

Anyway, I think this argument is getting confused between "downward
closures" and "full closures".  If I understand your example correctly,
it needs downward closures, but not full closures.  True?  I have argued
in favor of downward closures (because in my experience they have been
very useful, and they are very cheap to implement).  But I remain
unconvinced about full closures (because they seem less commonly needed,
and they are substantially more expensive to implement -- although your
benchmark shows that in at least one case, the run-time cost is a myth).
And the only serious example anybody posted was Ackermann's function,
which is useful in explaining how full closures work, but doesn't
exactly make me want to use them every day.  I have never in my entire
career needed to compute Ackermann's function, as far as I can recall.
;-)

- Bob