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From: ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe)
Newsgroups: comp.lang.prolog
Subject: Re: Does Prolog have an unambiguous syntax definition?
Message-ID: <4764@goanna.cs.rmit.oz.au>
Date: 13 Feb 91 06:31:16 GMT
References: <12166@darkstar.ucsc.edu>
Organization: Comp Sci, RMIT, Melbourne, Australia
Lines: 28

In article <12166@darkstar.ucsc.edu>, kjell@terra.ucsc.edu writes:
> Is there a document that describes the reader's behaviour when it
> sees an ambiguous term?

Which Prolog?  There *is* a document:  the public-domain DEC-10 Prolog
parser, published in 1984 (or was it 1983)?

> :- 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?

In a Prolog system whose reader is based on the public-domain parser (the
*point* of providing that was to help implementors tell the truth when they
claimed "Edinburgh" compatibility), the answer must be #(@).
THERE IS NOTHING AMBIGUOUS ABOUT THIS TERM:
    if a possible prefix operator is followed by
	- a right bracket of some sort , ) | ] } .
	- a possible infix operator of wider scope (bigger number)
	- or a possible postfix operator of wider scope (bigger number)
    then the possible prefix operator is read as an atom.
    (In the case of a following infix or postfix operator, this forces
    the operator reading.)
-- 
Professional programming is paranoid programming