From: Mark Thom <markjordanthom@gmail.com>
Date: Thu, 14 Feb 2019 08:19:11 +0000 (-0700)
Subject: fix bug in dif/2 implementation
X-Git-Tag: v0.8.110~255
X-Git-Url: https://git.sagredo.dev/?a=commitdiff_plain;h=8958a563dea9a511ad38a7f2d04f5c1f7f710a49;p=scryer-prolog.git

fix bug in dif/2 implementation
---

diff --git a/src/prolog/lib/dif.pl b/src/prolog/lib/dif.pl
index 4bb44c0d..7d36d0b8 100644
--- a/src/prolog/lib/dif.pl
+++ b/src/prolog/lib/dif.pl
@@ -35,9 +35,10 @@ verify_attributes(Var, Value, Goals) :-
 % Probably the world's worst dif/2 implementation. I'm open to
 % suggestions for improvement.
 
-dif(X, Y) :- (   X \= Y -> true
+dif(X, Y) :- X \== Y,
+             (   X \= Y -> true
              ;   term_variables(X, XVars), term_variables(Y, YVars),
-		 (   XVars == [], YVars == [] -> true
+		 (   XVars == [], YVars == [] -> false
 		 ;   dif_set_variables(XVars, X, Y),
 		     dif_set_variables(YVars, X, Y)
 		 )