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From: mitanu@goedel.top.cis.syr.edu (Mitanu Paul)
Newsgroups: comp.lang.prolog
Subject: Question about operators
Message-ID: <1991Feb13.123619.23855@rodan.acs.syr.edu>
Date: 13 Feb 91 17:57:03 GMT
References: <12166@darkstar.ucsc.edu> <4764@goanna.cs.rmit.oz.au>
Reply-To: mitanu@top.cis.syr.edu (Mitanu Paul)
Organization: CIS Dept., Syracuse University
Lines: 35


Hi,

I came across this problem. The following operators are defined:

	:- op(500, fx, +).
	:- op(500, yfx, +).
	:- op(200, xfy, ^).
	:- op(550, xf, !).
	:- op(550, xfx, !).

The query

	?- X = (a+b!).

produces the following error:

	** Syntax error: **
	X=(a+b!
	** here **
	)

Similarly, the query "X=(a^b!)" gives a syntax error at the same location.

Why? I guess that it is because of two op declarations for !, since
I don't get the error when the op(550,xfx,!) declaration is removed.
Does the parser get confused when confronted with such operators?

Both C-prolog and Quintus prolog produce this error.

Please reply by e-mail.

Thank you.

-- Mitanu	(mitanu@top.cis.syr.edu)