[m-rev.] diff: add maybe.fold2_maybe/6

[Julien Fischer]

Add the predicate maybe.fold2_maybe/6 to the standard library.

Add a predicate version of ground_pseudo_type_desc_to_type_desc.

library/maybe.m:
 	Add the above predicate.

 	Re-order some existing declarations as per the coding standard.

library/type_desc.m:
 	Add a predicate verison of ground_pseudo_type_desc_to_type_desc.

NEWS:
 	Announce the above.

diff --git a/NEWS b/NEWS
index 4bf175c..0685004 100644
--- a/NEWS
+++ b/NEWS
@@ -116,6 +116,10 @@ Changes to the Mercury standard library:

  * We have a added a new type, maybe_errors, to the maybe module.

+  The following predicate has been added to the maybe module:
+
+   - fold2_maybe/6
+
  * The following predicates and functions have been added to the calendar module

     - int_to_month/2
diff --git a/library/maybe.m b/library/maybe.m
index bf39dfd..31d951a 100644
--- a/library/maybe.m
+++ b/library/maybe.m
@@ -79,6 +79,11 @@
      %
  :- func map_maybe(func(T) = U, maybe(T)) = maybe(U).

+    % fold_maybe(_, no, Acc) = Acc.
+    % fold_maybe(F, yes(Value), Acc0) = F(Acc0).
+    %
+:- func fold_maybe(func(T, U) = U, maybe(T), U) = U.
+
      % fold_maybe(_, no, !Acc).
      % fold_maybe(P, yes(Value), !Acc) :- P(Value, !Acc).
      %
@@ -90,10 +95,21 @@
  :- mode fold_maybe(pred(in, mdi, muo) is semidet, in, mdi, muo) is semidet.
  :- mode fold_maybe(pred(in, di, uo) is semidet, in, di, uo) is semidet.

-    % fold_maybe(_, no, Acc) = Acc.
-    % fold_maybe(F, yes(Value), Acc0) = F(Acc0).
+    % As above, but with two accumulators.
      %
-:- func fold_maybe(func(T, U) = U, maybe(T), U) = U.
+:- pred fold2_maybe(pred(T, U, U, V, V), maybe(T), U, U, V, V).
+:- mode fold2_maybe(pred(in, in, out, in, out) is det, in, in, out,
+    in, out) is det.
+:- mode fold2_maybe(pred(in, in, out, mdi, muo) is det, in, in, out,
+    mdi, muo) is det.
+:- mode fold2_maybe(pred(in, in, out, di, uo) is det, in, in, out,
+    di, uo) is det.
+:- mode fold2_maybe(pred(in, in, out, in, out) is semidet, in, in, out,
+    in, out) is semidet.
+:- mode fold2_maybe(pred(in, in, out, mdi, muo) is semidet, in, in, out,
+    mdi, muo) is semidet.
+:- mode fold2_maybe(pred(in, in, out, di, uo) is semidet, in, in, out,
+    di, uo) is semidet.

      % map_fold_maybe(_, no, no, !Acc).
      % map_fold_maybe(P, yes(Value0), yes(Value), !Acc) :-
@@ -164,12 +180,17 @@ map_maybe(P, yes(T0), yes(T)) :- P(T0, T).
  map_maybe(_, no) = no.
  map_maybe(F, yes(T)) = yes(F(T)).

-fold_maybe(_, no, !Acc).
-fold_maybe(P, yes(Value), !Acc) :- P(Value, !Acc).
-
  fold_maybe(_, no, Acc) = Acc.
  fold_maybe(F, yes(Value), Acc0) = F(Value, Acc0).

+fold_maybe(_, no, !Acc).
+fold_maybe(P, yes(Value), !Acc) :-
+    P(Value, !Acc).
+
+fold2_maybe(_, no, !Acc1, !Acc2).
+fold2_maybe(P, yes(Value), !Acc1, !Acc2) :-
+    P(Value, !Acc1, !Acc2).
+
  map_fold_maybe(_, no, no, Acc, Acc).
  map_fold_maybe(P, yes(T0), yes(T), Acc0, Acc) :-
      P(T0, T, Acc0, Acc).
diff --git a/library/type_desc.m b/library/type_desc.m
index d09b6db..bb96ac1 100644
--- a/library/type_desc.m
+++ b/library/type_desc.m
@@ -63,6 +63,8 @@
      %
  :- func ground_pseudo_type_desc_to_type_desc(pseudo_type_desc) = type_desc
      is semidet.
+:- pred ground_pseudo_type_desc_to_type_desc(pseudo_type_desc::in,
+    type_desc::out) is semidet.

      % Convert a pseudo_type_desc describing a ground type to a type_desc.
      % If the pseudo_type_desc describes a non-ground type, abort.
@@ -438,6 +440,9 @@ type_desc_to_pseudo_type_desc(_TypeDesc) = _PseudoTypeDesc :-
      private_builtin.sorry("type_desc_to_pseudo_type_desc").

  ground_pseudo_type_desc_to_type_desc(PseudoTypeDesc) = TypeDesc :-
+    ground_pseudo_type_desc_to_type_desc(PseudoTypeDesc, TypeDesc).
+
+ground_pseudo_type_desc_to_type_desc(PseudoTypeDesc, TypeDesc) :-
      ( pseudo_type_desc_is_ground(PseudoTypeDesc) ->
          private_builtin.unsafe_type_cast(PseudoTypeDesc, TypeDesc)
      ;