From: Markus Triska <triska@metalevel.at>
Date: Sun, 21 Aug 2022 12:29:32 +0000 (+0200)
Subject: ENHANCED: Improve determinism of ... //0 and seq//1.
X-Git-Tag: v0.9.1^2~32
X-Git-Url: https://git.sagredo.dev/?a=commitdiff_plain;h=47892bf24a8c4becb196f1b400fe40d5b50be9fa;p=scryer-prolog.git

ENHANCED: Improve determinism of ... //0 and seq//1.

Example:

    ?- phrase(..., "hello").
       true.

This addresses #1577.
---

diff --git a/src/lib/dcgs.pl b/src/lib/dcgs.pl
index 54160cf6..ae996cf0 100644
--- a/src/lib/dcgs.pl
+++ b/src/lib/dcgs.pl
@@ -132,16 +132,29 @@ user:term_expansion(Term0, Term) :-
     dcg_rule(Term0, Term).
 
 % Describes a sequence
+seq(Xs) -->
+   call(det_end_(Xs)),
+   !.
 seq([]) --> [].
 seq([E|Es]) --> [E], seq(Es).
 
+det_end_(Xs, Cs0,Cs) :-
+   var(Xs),
+   Cs0 == [],
+   Xs = [],
+   Cs0 = Cs.
+
 % Describes a sequence of sequences
 seqq([]) --> [].
 seqq([Es|Ess]) --> seq(Es), seqq(Ess).
 
 % Describes an arbitrary number of elements
+... --> call(det_end), !.
 ... --> [] | [_], ... .
 
+det_end(Cs0,Cs) :-
+   Cs0 == [],
+   Cs0 = Cs.
 
 error_goal(error(E, must_be/2), error(E, must_be/2)).
 error_goal(error(E, (=..)/2), error(E, (=..)/2)).