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From: kjell@saturn.ucsc.edu (Kjell Post)
Newsgroups: comp.lang.prolog
Subject: Re: Does Prolog have an unambiguous syntax definition?
Message-ID: <12388@darkstar.ucsc.edu>
Date: 15 Feb 91 08:48:57 GMT
Sender: usenet@darkstar.ucsc.edu
Organization: University of California, Santa Cruz
Lines: 75

In article <4764@goanna.cs.rmit.oz.au> ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe) writes:
>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.)

Interesting.  I hadn't seen that before and it certainly takes care of
some problems.  But what about this example?

	:- op(500,xfy,@).	% infix
	:- op(600,xf,@).	% postfix
	:- op(700,xf,#).	% postfix

	display(a @ #).

Quintus Prolog Release 2.2 (Sun-3, Unix 3.5)	=>	@(a,#)
SICStus 0.6 #18: Mon Oct 22 13:46:21 PDT 1990	=>	#(@(a))

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PS: Pnews didn't like my citation/comment-ratio so I included this picture:

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--Kjell
--
      For athletes and programmers,  ! Kjell E. Post
a woman is the end of their career.  ! CIS/CE
                                     ! University of California, Santa Cruz
              -- A.Wickberg          ! Email: kjell@saturn.ucsc.edu