fix group_by_variants/4 and keysort in setof/3 (#1440, #1856)

src/lib/builtins.pl

@@ -883,12 +883,45 @@ set_difference([X|Xs], [Y|Ys], Zs) :-
 set_difference([], _, []) :- !.
 set_difference(Xs, [], Xs).
 
+
+% variant/2 checks whether X is a variant of Y per the definition in
+% 7.1.6.1 of the ISO standard.
+
+:- non_counted_backtracking variant/4.
+
+variant(X,Y,VPs,VPs0) :-
+    (  var(X) ->
+       var(Y),
+       VPs = [X-Y|VPs0]
+    ;  var(Y) ->
+       false
+    ;  X =.. [FX | XArgs],
+       Y =.. [FX | YArgs],
+       lists:foldl('$call'(builtins:variant), XArgs, YArgs, VPs, VPs0)
+    ).
+
+:- non_counted_backtracking variant/2.
+
+singleton([_]).
+
+variant(X, Y) :-
+    variant(X,Y, VPs, []),
+    keysort(VPs, SVPs),
+    pairs:group_pairs_by_key(SVPs, SVPKs),
+    pairs:pairs_values(SVPKs, Vals),
+    lists:maplist('$call'(builtins:term_variables), Vals, Vs),
+    lists:maplist('$call'(builtins:singleton), Vs),
+    term_variables(Vs, YVars),
+    lists:length(SVPKs, N),
+    lists:length(YVars, N).
+
+
 :- non_counted_backtracking group_by_variant/4.
 
 group_by_variant([V2-S2 | Pairs], V1-S1, [S2 | Solutions], Pairs0) :-
-    V1 = V2, % \+ \+ (V1 = V2), % (2) % iso_ext:variant(V1, V2), % (1)
+    variant(V1, V2),
     !,
-    % V1 = V2, % (3)
+    V1 = V2,
     group_by_variant(Pairs, V2-S2, Solutions, Pairs0).
 group_by_variant(Pairs, _, [], Pairs).
 
@@ -1008,7 +1041,7 @@ setof(Template, Goal, Solution) :-
     term_variables(TemplateVars+GoalVars, TGVs),
     lists:append(TemplateVars, Witnesses0, TGVs),
     findall_with_existential(Template, Goal, PairedSolutions0, Witnesses0, Witnesses),
-    keysort(PairedSolutions0, PairedSolutions),
+    '$keysort_with_constant_var_ordering'(PairedSolutions0, PairedSolutions), % see 7.2.1
     group_by_variants(PairedSolutions, GroupedSolutions),
     iterate_variants_and_sort(GroupedSolutions, Witnesses, Solution).