Path: utzoo!mnetor!uunet!husc6!cmcl2!rutgers!ucla-cs!arman
From: arman@CS.UCLA.EDU
Newsgroups: comp.lang.prolog
Subject: Arithmetic problems with Quintus Prolog
Message-ID: <11111@shemp.CS.UCLA.EDU>
Date: 11 Apr 88 04:51:10 GMT
Sender: news@CS.UCLA.EDU
Reply-To: arman@CS.UCLA.EDU (Arman Bostani)
Organization: UCLA Computer Science Department
Lines: 57

Please don't take this message as a flame against Quintus. I'd like to
discuss some problems that I think Qprolog should address. Overall,
though, Qprolog is one of the best prolog systems I have used.
The following examples are taken from Quintus prolog release 2.2
running on a Sun-3, Unix 3.5.

Firstly, Qprolog seems to maintain both 0 and -0. Here's what I mean:

| ?- X is 0.0, Y is -X, X=Y.

no

This is a bug! Qprolog works fine though if you replace "0.0" with "0"
in the above example.

Also, the Qprolog lexical analyzer seems to be somewhat brain-damaged.
For example:

| ?- X is 1000000000.

X = -73741824 

I think that in this case the system should have either trapped with
an arithmetic overflow, or it should have unified X with "Infinity" as
C-Prolog does it. Granted "Infinity" is not a great solution, but it's
better than random answers.

Here is a misfeature that really bugged me. 

| ?- Op=..[+,2,2], X is Op.
[ERROR: variable is not bound to a number: 2+2 (error 303)]
[ERROR: invalid arithmetic expression: 2+2 (error 302)]

no

The Qprolog manual mentions this misfeature. The problem is that
is/2 will not accept expressions that contain variables not bound to
numbers. This is bad. It means that if I want a C-Prolog like is/2,
I'll have to write my own! (note that is/2 works as expected with SICS
prolog which is a Quintus look alike).

I should also note that floating point numbers are not automatically
coerced into integers. For example:

| ?- 0 is 0.0.

no

This is a matter of taste I guess. I personally prefer the way that
C-Prolog does it (i.e. when a floating point number fits in an
integer, it is automatically converted) This can become a big
performance issue if one is trying to do number crunching. (Most prologs
chew everything very slowly anyway, just being polite I guess :-)

-- Arman Bostani // UCLA Computer Science Department // +1 213-825-3194
	3417 Boelter Hall // Los Angeles, California 90024-1596 // USA
	arman@CS.UCLA.EDU   ...!(ucbvax,rutgers)!ucla-cs!arman