Path: utzoo!attcan!uunet!mcsun!sunic!sics.se!sics.se!matsc
From: matsc@sics.se (Mats Carlsson)
Newsgroups: comp.lang.prolog
Subject: DCG expansion pitfall
Message-ID: <MATSC.89Sep5092248@skarabe.sics.se>
Date: 5 Sep 89 07:22:48 GMT
Sender: news@sics.se
Distribution: comp
Organization: Swedish Institute of Computer Science, Kista
Lines: 22

Implementors and users, check how your favourite Prolog expands the
DCG clause:

p(X) -->
	(   {test(X)} -> []
	;   [token(X)]
	).

It should expand to something equivalent to

p(A, B, C) :-
        (    test(A) ->
             B = C
        ;    B = [token(A)|C]
        ).

This is yet another example of the unfortunate fact that (P;Q) is not
equivalent to ((true,P);Q) if P = (If->Then).
--
Mats Carlsson
SICS, PO Box 1263, S-164 28  KISTA, Sweden    Internet: matsc@sics.se
Tel: +46 8 7521543      Ttx: 812 61 54 SICS S      Fax: +46 8 7517230