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From: koopman@a.gp.cs.cmu.edu (Philip Koopman)
Newsgroups: comp.lang.forth
Subject: Data determines correct operation?
Keywords: / NOT scariness
Message-ID: <9811@pt.cs.cmu.edu>
Date: 5 Jul 90 11:05:16 GMT
Organization: Carnegie-Mellon University, CS/RI
Lines: 34


Now that we all seem to be talking about /, NOT, and compromises
again, here is a point to consider:

When I was looking for a minimal kernel to port to a new stack
CPU design, I got a copy of cmForth to evaluate.  I liked the
fact that it was small and (probably) easy to port.  Then I
noticed the following definition:
  : <  - 0< ;
This definition only works when the input data are closer
than $8000 apart!  (I speculate that Chuck did this because the
Novix doesn't have an overflow flag, and a "correct" implementation 
of < is complex).  The circumstances of finding the same bug
in another Forth cost me a day or so of debugging.  Consequently,
I rejected out-of-hand cmForth because it had at least one
case of correctness determined by input data values (and I didn't
want to spend the time to try and find all the others that might
be there).

My current gut reaction is to reject without further examination
any language, system, or implementation that does not work 
correctly when fed reasonable data sets (especially if not
documented to do so for extremely important reasons).
I find definitions of the form "works with positive numbers only"
scary.  How many outsiders share my fears?  Remember, even if
C does not specify division at a very detailed level, common
practice is to use hardware-supplied division, and most (if not
all) machines do it the same way.

  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.