Path: utzoo!censor!geac!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!ukc!tcdcs!swift.cs.tcd.ie!brady
From: brady@swift.cs.tcd.ie
Newsgroups: comp.lang.prolog
Subject: Re: Does Prolog have an unambiguous syntax definition?
Message-ID: <1991Feb13.174251.7784@swift.cs.tcd.ie>
Date: 13 Feb 91 17:42:51 GMT
References: <12166@darkstar.ucsc.edu> <4764@goanna.cs.rmit.oz.au>
Organization: Computer Science Department, Trinity College Dublin
Lines: 37

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


Surely there _is_ an ambiguity here. If there wasn't, one wouldn't need to
consult a programmed parser to resolve it!

The DECsystem-10 User's Manual says that under certain circumstances a prefix
operator can be taken as an atom. One can reasonably deduce that those
circumstances are when there is no valid argument following the possible
prefix operator - e.g. where there is a closing bracket, or a stop, or
a term of a higher scope following.
If that is the case, then, by taking the prefix interpretation of the #,
and by turning it into an atom (because it has no following argument),
then it seems legal for the prefix @ to take it (i.e. what is now the _atom_ # )
as its argument, yielding @(#).

If seems that what Richard is saying is that this issue has been resolved
de-facto (which is fine with me), but the ambiguity remains.

I haven't thought of any 'collateral' consequences of this, but maybe there
aren't any?

Mike
brady@cs.tcd.ie