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From: Robert A Duff <bobduff@world.std.com>
Subject: Re: Compile time executed functions
Sender: bobduff@world.std.com (Robert A Duff)
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Date: Fri, 30 Mar 2001 00:06:05 GMT
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Date: 2001-03-30T00:06:05+00:00
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Mark Biggar <mark.a.biggar@home.com> writes:

> It's been done many times.  For example go read "Godel, Esher and
> Bach" by Hofsteder, the section on the *LOOP languages.  Obviously you
> must outlaw abritrary "goto" statements, recursive procedures and
> while loops, but if (case) statements and Ada like "for" loops are fine.
> The trick it that you can only have looping construsts that allow
> the computation of a strict bound on the number of iterations, 
> before the loop is entered.

But if you give me a language like that, and what I really want is a
while loop, I'll end up writing something like:

    for I in 1..10**100 loop -- or maybe just 2**31-1 ;-)
        exit when ...;
        ...
    end loop;

It's easy to prove that the above loop is not infinite, but if I make a
mistake in the exit condition, I'll end up with a very long loop, which
is just as bad in practise.

So I think it's better just to allow while loops, and live with the fact
that they might be infinite.  Unless you have a fancy proof engine that
can prove termination of while loops most of the time.

- Bob