Merge pull request #2350 from pguyot/w27/estdlib-lists-mapfoldr

estdlib: add lists:mapfoldr/3

These changes are made under both the "Apache 2.0" and the "GNU Lesser General
Public License 2.1 or later" license terms (dual license).

SPDX-License-Identifier: Apache-2.0 OR LGPL-2.1-or-later

Author: bettio

CHANGELOG.md

@@ -33,6 +33,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Added a Linux `gpio` driver for the generic_unix port (in `avm_unix`) using sysfs
 - Added `console:print_err/1` to write to standard error
 - Added `erlang:term_to_binary/2`, `erlang:is_builtin/3` and `erlang:bitstring_to_list/1`
+- Added `lists:mapfoldr/3`
 
 ### Changed
 - Updated network type db() to dbm() to reflect the actual representation of the type

libs/estdlib/src/lists.erl

@@ -50,6 +50,7 @@
     foldl/3,
     foldr/3,
     mapfoldl/3,
+    mapfoldr/3,
     all/2,
     any/2,
     flatten/1,
@@ -412,6 +413,22 @@ mapfoldl0(Fun, {List1, Acc0}, [H | T]) ->
     {B, Acc1} = Fun(H, Acc0),
     mapfoldl0(Fun, {[B | List1], Acc1}, T).
 
+%%-----------------------------------------------------------------------------
+%% @param   Fun the function to apply
+%% @param   Acc0 the initial accumulator
+%% @param   List the list over which to fold
+%% @returns the result of mapping and folding Fun over List, from right to left
+%% @doc     Combine `map/2' and `foldr/3' in one pass.
+%% @end
+%%-----------------------------------------------------------------------------
+-spec mapfoldr(fun((A, Acc) -> {B, Acc}), Acc, [A]) -> {[B], Acc}.
+mapfoldr(_Fun, Acc0, []) ->
+    {[], Acc0};
+mapfoldr(Fun, Acc0, [H | T]) ->
+    {Bs, Acc1} = mapfoldr(Fun, Acc0, T),
+    {B, Acc2} = Fun(H, Acc1),
+    {[B | Bs], Acc2}.
+
 %%-----------------------------------------------------------------------------
 %% @equiv   foldl(Fun, Acc0, reverse(List))
 %% @doc     Fold over a list of terms, from right to left, applying Fun(E, Accum)

tests/libs/estdlib/test_lists.erl

@@ -477,6 +477,17 @@ test_mapfoldl() ->
         lists:mapfoldl(fun(X, A) -> {X * A, A + 1} end, 1, [1, 2, 3])
     ),
     ?ASSERT_ERROR(lists:mapfoldl(fun(X, A) -> {X * A, A + 1} end, 1, foo), function_clause),
+    ok = test_mapfoldr(),
+    ok.
+
+test_mapfoldr() ->
+    ?ASSERT_MATCH({[], 1}, lists:mapfoldr(fun(X, A) -> {X * A, A + 1} end, 1, [])),
+    %% Right-to-left: the accumulator visits 3, then 2, then 1.
+    ?ASSERT_MATCH(
+        {[3, 4, 3], 4},
+        lists:mapfoldr(fun(X, A) -> {X * A, A + 1} end, 1, [1, 2, 3])
+    ),
+    ?ASSERT_ERROR(lists:mapfoldr(fun(X, A) -> {X * A, A + 1} end, 1, foo), function_clause),
     ok.
 
 test_append() ->