[m-rev.] diff: add list.map_foldr/5

[Julien Fischer]

Branches: main

--------

Add list.map_foldr/5.

library/list.m:
 	As above.

NEWS:
 	Announce the addition.

Julien.

diff --git a/NEWS b/NEWS
index e77be99..e34d201 100644
--- a/NEWS
+++ b/NEWS
@@ -18,8 +18,8 @@ Changes to the Mercury standard library:
  * We have added the following predicates and functions to the map module:
    det_min_key/1, det_max_key/1, foldl2_values/6 and foldl3_values/8.

-* We have added the following predicates to the list module: foldr2/6 and
-  foldr3/8.
+* We have added the following predicates to the list module: foldr2/6,
+  foldr3/8 and map_foldlr/5.

  * We have added the following predicates to the bag module: foldl/4 and
    foldl2/6.
diff --git a/library/list.m b/library/list.m
index d3f1856..0ace715 100644
--- a/library/list.m
+++ b/library/list.m
@@ -1170,12 +1170,12 @@
      % returned in End.
      %
  :- pred list.map_foldl(pred(L, M, A, A), list(L), list(M), A, A).
-:- mode list.map_foldl(pred(in, out, di, uo) is det, in, out, di, uo)
-    is det.
  :- mode list.map_foldl(pred(in, out, in, out) is det, in, out, in, out)
      is det.
  :- mode list.map_foldl(pred(in, out, mdi, muo) is det, in, out, mdi, muo)
      is det.
+:- mode list.map_foldl(pred(in, out, di, uo) is det, in, out, di, uo)
+    is det.
  :- mode list.map_foldl(pred(in, out, in, out) is semidet, in, out, in, out)
      is semidet.
  :- mode list.map_foldl(pred(in, out, mdi, muo) is semidet, in, out, mdi, muo)
@@ -1470,6 +1470,28 @@
  :- mode list.map4_foldl(pred(in, out, out, out, out, di, uo) is cc_multi,
      in, out, out, out, out, di, uo) is cc_multi.

+    % list.map_foldr(Pred, InList, OutList, Start, End) calls Pred
+    % with an accumulator (with the initial value of Start) on
+    % each element of InList (working right-to-left) to transform
+    % InList into OutList.  The final value of the accumulator is
+    % returned in End.
+    %
+:- pred list.map_foldr(pred(L, M, A, A), list(L), list(M), A, A).
+:- mode list.map_foldr(pred(in, out, in, out) is det, in, out, in, out)
+    is det.
+:- mode list.map_foldr(pred(in, out, mdi, muo) is det, in, out, mdi, muo)
+    is det.
+:- mode list.map_foldr(pred(in, out, di, uo) is det, in, out, di, uo)
+    is det.
+:- mode list.map_foldr(pred(in, out, in, out) is semidet, in, out, in, out)
+    is semidet.
+:- mode list.map_foldr(pred(in, out, mdi, muo) is semidet, in, out, mdi, muo)
+    is semidet.
+:- mode list.map_foldr(pred(in, out, di, uo) is semidet, in, out, di, uo)
+    is semidet.
+:- mode list.map_foldr(pred(in, in, di, uo) is semidet, in, in, di, uo)
+    is semidet.
+
      % list.filter_map_foldl(Transformer, List, TrueList, Start, End):
      % Takes a predicate with one input argument, one output argument and an
      % accumulator. It is called with each element of List. If a call succeeds,
@@ -2780,6 +2802,11 @@ list.map4_foldl(P, [H0 | T0], [H1 | T1], [H2 | T2], [H3 | T3], [H4 | T4], !A) :-
      P(H0, H1, H2, H3, H4, !A),
      list.map4_foldl(P, T0, T1, T2, T3, T4, !A).

+list.map_foldr(_, [], [], !A).
+list.map_foldr(P, [H0 | T0], [H | T], !A) :-
+    list.map_foldr(P, T0, T, !A),
+    P(H0, H, !A).
+
  list.filter_map_foldl(_, [], [], !A).
  list.filter_map_foldl(P, [X | Xs], True, !A) :-
      ( P(X, Y, !A) ->