From: Mark Thom <markjordanthom@gmail.com>
Date: Wed, 22 Feb 2023 03:53:13 +0000 (-0700)
Subject: assign responsibility for emitting dif goal to the first variable of the left-hand... 
X-Git-Tag: v0.9.2~178
X-Git-Url: https://git.sagredo.dev/?a=commitdiff_plain;h=95f6ebc0002778b71d402e149e01b2031f74083a;p=scryer-prolog.git

assign responsibility for emitting dif goal to the first variable of the left-hand term (#1739)
---

diff --git a/src/lib/dif.pl b/src/lib/dif.pl
index 33e3e44b..a59052ac 100644
--- a/src/lib/dif.pl
+++ b/src/lib/dif.pl
@@ -40,9 +40,6 @@ verify_attributes(Var, Value, Goals) :-
     ;   Goals = []
     ).
 
-% Probably the world's worst dif/2 implementation. I'm open to
-% suggestions for improvement.
-
 %% dif(?X, ?Y).
 %
 % True iff X and Y are different terms. Unlike `\=/2`, `dif/2` is more declarative because if X and Y can
@@ -69,12 +66,16 @@ dif(X, Y) :-
         )
     ).
 
-gather_dif_goals([]) --> [].
-gather_dif_goals([(X \== Y) | Goals]) -->
-    [dif:dif(X, Y)],
-    gather_dif_goals(Goals).
+gather_dif_goals(_, []) --> [].
+gather_dif_goals(V, [(X \== Y) | Goals]) -->
+    (  { term_variables(X, [V0 | _]),
+         V == V0 } ->
+       [dif:dif(X, Y)]
+    ;  []
+    ),
+    gather_dif_goals(V, Goals).
 
 attribute_goals(X) -->
     { get_atts(X, +dif(Goals)) },
-    gather_dif_goals(Goals),
+    gather_dif_goals(X, Goals),
     { put_atts(X, -dif(_)) }.