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From: bimbart@kulcs.cs.kuleuven.ac.be (Bart Demoen)
Newsgroups: comp.lang.prolog
Subject: Re: Does Prolog have an unambiguous syntax definition?
Message-ID: <1945@n-kulcs.cs.kuleuven.ac.be>
Date: 14 Feb 91 07:52:52 GMT
Sender: news@cs.kuleuven.ac.be
Organization: Dept. of Computer Science (K.U.Leuven)
Lines: 34
Originator: bimbart@kulcs.cs.kuleuven.ac.be

I send the following on behalf of Roger Scowen.

------------------------------------------


Kjell (University of California, Santa Cruz) asks:

> Is there a document that describes the reader's behaviour when it
> sees an ambiguous term?  Here's an example to show what I mean:
> 
> :- op(300,fx,#).        % prefix
> :- op(500,xf,#).        % postfix
> :- op(400,fx,@).        % prefix
> 
> display(@ #).
> 
> Will Prolog display @(#) or #(@), and why is one preferred over the other?

Syntactic ambiguities in Prolog have been considered by the ISO Prolog 
standardization group. Different Prolog systems give different
results, so there is no right answer.

The latest draft (SC22 WG17 N70) forbids all ambiguous cases from
occurring in a standard-conforming program.  There are several 
restrictions, for example the case you have quoted is not allowed because
the draft states (in 6.3.1.3):
"an atom which is an operator shall not be the immediate operand of an
operator".

Roger Scowen, SC22 WG17 (Prolog) Convener,
DITC/93, National Physical Laboratory, Teddington, Middlesex, England  TW11 0LW
13 February 1991
Tel (Abroad): +44 81 943 6956      Fax (Abroad): +44 81 977 7091 
Email: rss@seg.npl.co.uk           Telex: 262344