Path: utzoo!attcan!uunet!snorkelwacker.mit.edu!bloom-beacon!eru!hagbard!sunic!sics.se!sics.se!alf
From: alf@sics.se (Thomas Sj|land)
Newsgroups: comp.lang.prolog
Subject: Re: Prolog parser bug in reading conditional? Or not?
Message-ID: <1991Mar1.102711.7580@sics.se>
Date: 1 Mar 91 10:27:11 GMT
References: <1991Feb22.172635.16587@and.cs.liv.ac.uk>
	<4826@goanna.cs.rmit.oz.au>
Sender: news@sics.se
Organization: Swedish Institute of Computer Science, Kista
Lines: 43
In-Reply-To: ok@goanna.cs.rmit.oz.au's message of 26 Feb 91 05:16:47 GMT


in his article ok@goanna.cs.rmit.oz.au commenting on the questions of
dave8@and.cs.liv.ac.uk:

>Parentheses in Prolog have *NO* semantic effect,
>they are *solely* a device for controlling grouping.  It follows that
>	( P -> Q ) ; R
>*MUST* be the data structure
>	;( ->( P, Q ), R )
>and it is this data structure which bears the meaning "if then else",
>*not* the surface syntax.  

This last point shows a design decision in earlier Prologs which now
and then causes problems.

ok ends with some words of wisdom:

>The rule of thumb is to never use P->Q on its own.  This is doubly
>advisable because there are otherwise excellent Prolog systems
>which interpret a bare P->Q as P->Q;true.
>
>> Is this a parser problem or not?
>
>No, it is a parser WORKING.

Just because it is "WORKING" doesn't mean the design is not 
unnecessarily confusing.

The design decision to use A->B;C for if-then-else wasn't very clever
in my opinion. I hope the standard will not be backwards compatible on
this point. Even 'A->B|C' would be much better, if '|' was not immediately
translated into ';'. Maybe we could use some other operator when we are not 
talking about disjunctions.

---Read Richard's book, someone might quote it, and you better be informed.


--
Thomas Sjoeland
SICS, PO Box 1263, S-164 28 KISTA, SWEDEN
Tel: +46 8 752 15 42  Fax: +46 8 751 72 30
Internet: alf@sics.se 
......