prepare to add project_attributes/2 and attribute_goals/2

src/prolog/lib/dif.pl

@@ -0,0 +1,57 @@
+[:- module(dif, [dif/2]).
+
+:- use_module(library(atts)).
+:- use_module(library(control), [(\=)/2]).
+
+:- attribute dif/2.
+
+dif_set_variables([], _, _).
+dif_set_variables([Var|Vars], X, Y) :-
+    put_atts(Var, dif(X, Y)),
+    dif_set_variables(Vars, X, Y).
+
+verify_dif_attrs([dif(X, Y) | Attrs], Var, [X \== Y | Goals]) :-
+    (   get_atts(Var, +dif(X, Y)) -> true
+    ;   put_atts(Var, +dif(X, Y))
+    ),
+    verify_dif_attrs(Attrs, Var, Goals).
+verify_dif_attrs([_ | Attrs], Var, Goals) :-
+    verify_dif_attrs(Attrs, Var, Goals).
+verify_dif_attrs([], _, []).
+
+verify_dif_attrs_no_var([dif(X, Y) | Attrs], [X \== Y | Goals]) :-
+    verify_dif_attrs_no_var(Attrs, Goals).
+verify_dif_attrs_no_var([_ | Attrs], Goals) :-
+    verify_dif_attrs_no_var(Attrs, Goals).
+verify_dif_attrs_no_var([], []).
+
+verify_attributes(Var, Value, Goals) :-
+    (   get_atts(Var, Attrs) ->
+	(   var(Value) -> verify_dif_attrs(Attrs, Value, Goals)
+	;   verify_dif_attrs_no_var(Attrs, Goals)
+	)
+    ).
+
+% Probably the world's worst dif/2 implementation. I'm open to
+% suggestions for improvement.
+
+dif(X, Y) :- (   X \= Y -> true
+	     ;   term_variables(X, XVars), term_variables(Y, YVars),
+		 (   XVars == [], YVars == [] -> false
+		 ;   dif_set_variables(XVars, X, Y),
+		     dif_set_variables(YVars, X, Y)
+		 )
+	     ).
+
+gather_dif_goals(Attrs, Goal, Goal) :-
+    var(Attrs), !.
+gather_dif_goals([dif(X, Y)|Attrs], OldGoal, Goal) :-
+    (   var(OldGoal), !, gather_dif_goals(Attrs, dif(X, Y), Goal)
+    ;   !, gather_dif_goals(Attrs, (dif(X, Y), OldGoal), Goal)
+    ).
+gather_dif_goals([_|Attrs], OldGoal, Goal) :-
+    gather_dif_goals(Attrs, OldGoal, Goal).
+
+attribute_goals(X, Goal) :-
+    '$get_attr_list'(X, Attrs),
+    gather_dif_goals(Attrs, _, Goal).