Path: utzoo!attcan!uunet!zaphod.mps.ohio-state.edu!rpi!batcomputer!munnari.oz.au!goanna!ok
From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.prolog
Subject: Re: Standards question: behavior of arg/3
Keywords: ANSI, standard
Message-ID: <4036@goanna.cs.rmit.oz.au>
Date: 22 Oct 90 03:19:08 GMT
References: <9888@bunny.GTE.COM> <3992@goanna.cs.rmit.oz.au> <1920@tuvie> <5816@munnari.oz.au>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 29

In article <5816@munnari.oz.au>, lee@munnari.oz.au (Lee Naish) writes:
> I have had the
> "pleasure" of porting a 30,000 line program written in (an old version
> of) Quintus Prolog to NU-Prolog.  One of the nastiest bugs was a call to
> functor/3 which failed in Quintus because it was insuffiiently
> instantiated, but delayed in NU-Prolog.  I'm not sure what the latest
> version of Quintus does.

It reports the instantiation fault.  Silent failure when the real problem
is not that there are _no_ solutions but that there are infinitely many
is clearly (a) wrong and (b) unhelpful.

> I hope more systems start producing error
> messages in situations like this.

QP3.0 is really thorough about this, and to be fair, IF/Prolog always
did produce error messages.  (To be even fairer, IF/Prolog also produced
error messages in several cases where the query made sense and had a
definite answer...)

> I haven't seen any recent draft standards - do they discuss the
> general class of errors called instantiation faults?

Yes, EPs in the ISO draft do report instantiation faults.
It's not described as a general rule; you have to check each separate
predicate description.

-- 
Fear most of all to be in error.	-- Kierkegaard, quoting Socrates.