From: Mark Thom <markjordanthom@gmail.com>
Date: Sun, 3 Mar 2019 17:37:34 +0000 (-0700)
Subject: tighten reverse/2
X-Git-Tag: v0.8.110~216
X-Git-Url: https://git.sagredo.dev/?a=commitdiff_plain;h=3fc0a230faf1758a70091f4c406ee660ccbcc696;p=scryer-prolog.git

tighten reverse/2
---

diff --git a/src/prolog/lib/lists.pl b/src/prolog/lib/lists.pl
index e96d677d..7382a144 100644
--- a/src/prolog/lib/lists.pl
+++ b/src/prolog/lib/lists.pl
@@ -41,7 +41,10 @@ append([X|L], R, [X|S]) :- append(L, R, S).
 
 memberchk(X, Xs) :- member(X, Xs), !.
 
-reverse(Xs, Ys) :- reverse(Xs, Ys, [], [], Xs).
+reverse(Xs, Ys) :-
+    (  var(Ys) -> reverse(Xs, Ys, [], [], Xs)
+    ;  reverse(Ys, Xs, [], [], Ys)
+    ).
 
 reverse([], [], XsRev, YsRev, YsRev).
 reverse([X1|Xs], [Y1|Ys], XsPreludeRev, YsPreludeRev, Xss) :-