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From: koopman@a.gp.cs.cmu.edu (Philip Koopman)
Newsgroups: comp.lang.forth
Subject: Re: Data determines correct operation?
Summary: why it is scary
Message-ID: <9898@pt.cs.cmu.edu>
Date: 13 Jul 90 20:40:20 GMT
References: <9811@pt.cs.cmu.edu> <1326.UUL1.3#5129@willett.UUCP>
Organization: Carnegie-Mellon University, CS/RI
Lines: 35

In article <1326.UUL1.3#5129@willett.UUCP>, dwp@willett.UUCP (Doug Philips) writes:
> Is it that "works with positive numbers only" is scary because you
> think that the author is lying, or is it scary because you didn't think
> it should be necessary to put such a restriction on it?  Or am I missing
> some other reason?

The caveat "works with positive numbers only" is much preferable to
undocumented limitations.  So, this restriction is not nearly as bad
as the cmForth "< bug".  However, as an outsider (say, a C programmer)
I would not be accustomed to things as basic as division having
data restrictions not taught in grade school.

If you bought a calculator for $5, would you be upset if it said
in the instruction manual that division with negative operands gave
unspecified results?  Would you even think to look for such a thing?
Of course you could try it out and see what it did.  But wouldn't
such a limitation (if you happened to see it in the manual in
the store) make you shy away from purchase out of anxiety over
what other limitations (documented or not) you didn't think to
look for?

I have had several email conversations with Mitch Bradley in which
we seem to agree that /MOD should have "reasonable" behavior for
negative inputs.  For example, one could place restrictions on
allowable remainders and quotients.  Or, one could simply say that
it is equivalent to one of the fully defined divisions (which one
might be implementation dependent).

BUT, leaving it undefined translates to "broken" in my book.

  Phil Koopman                koopman@greyhound.ece.cmu.edu   Arpanet
  2525A Wexford Run Rd.
  Wexford, PA  15090
Senior scientist at Harris Semiconductor, and adjunct professor at CMU.
I don't speak for them, and they don't speak for me.