From: Markus Triska <triska@metalevel.at>
Date: Wed, 12 Jul 2023 05:26:52 +0000 (+0200)
Subject: ENHANCED: improved determinism of member/2
X-Git-Tag: v0.9.2~64^2
X-Git-Url: https://git.sagredo.dev/?a=commitdiff_plain;h=refs%2Fheads%2Flookahead_indexing;p=scryer-prolog.git

ENHANCED: improved determinism of member/2

Example:

    ?- member(X, "abc").
    %@    X = a
    %@ ;  X = b
    %@ ;  X = c.

This addresses #750.
---

diff --git a/src/lib/lists.pl b/src/lib/lists.pl
index 815e2b2a..8a94ed6a 100644
--- a/src/lib/lists.pl
+++ b/src/lib/lists.pl
@@ -122,8 +122,13 @@ length_addendum([_|Xs], N, M) :-
 %    X = h
 % ;  ... .
 % ```
-member(X, [X|_]).
-member(X, [_|Xs]) :- member(X, Xs).
+
+member(X, [L|Ls]) :-
+        member_(Ls, L, X).
+
+member_(_, X, X).
+member_([L|Ls], _, X) :-
+        member_(Ls, L, X).
 
 %% select(X, Xs0, Xs1).
 %