From: Markus Triska <triska@metalevel.at>
Date: Sat, 22 Aug 2020 15:47:42 +0000 (+0200)
Subject: dif/2: Succeed unconditionally if the arguments are not unifiable.
X-Git-Tag: v0.9.0~174^2~8^2~1
X-Git-Url: https://git.sagredo.dev/?a=commitdiff_plain;h=e75ebd9b6e947fdc2bca6dacad88012290a28f19;p=scryer-prolog.git

dif/2: Succeed unconditionally if the arguments are not unifiable.

This addresses one part of #135.
---

diff --git a/src/lib/dif.pl b/src/lib/dif.pl
index 347e9d13..ad3053ec 100644
--- a/src/lib/dif.pl
+++ b/src/lib/dif.pl
@@ -38,11 +38,14 @@ 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,
-             (   term_variables(X, XVars), term_variables(Y, YVars),
-	         dif_set_variables(XVars, X, Y),
-		 dif_set_variables(YVars, X, Y)
-	     ).
+dif(X, Y) :-
+    X \== Y,
+    (   X \= Y -> true
+    ;   (   term_variables(X, XVars), term_variables(Y, YVars),
+            dif_set_variables(XVars, X, Y),
+            dif_set_variables(YVars, X, Y)
+        )
+    ).
 
 gather_dif_goals([]) --> [].
 gather_dif_goals([(X \== Y) | Goals]) -->