Add list.map_reduce

2 files changed, 36 insertions, 0 deletions

diff --git a/src/gleam/list.gleam b/src/gleam/list.gleam
index 7df9862..6d9c709 100644
--- a/src/gleam/list.gleam
+++ b/src/gleam/list.gleam
@@ -246,6 +246,36 @@ pub fn map(list: List(a), with fun: fn(a) -> b) -> List(b) {
   do_map(list, fun, [])
 }
 
+/// Similar to map but also lets you pass around an accumulated value.
+///
+/// ## Examples
+///
+/// ```
+/// > map_reduce(
+///     over: [1, 2, 3],
+///     from: 100,
+///     with: fn(memo, i) { tuple(memo + i, i * 2) }
+///  )
+///  tuple(106, [2, 4, 6])
+/// ```
+///
+pub fn map_reduce(
+  over list: List(a),
+  from memo: memo,
+  with fun: fn(memo, a) -> tuple(memo, b),
+) -> tuple(memo, List(b)) {
+  fold(
+    over: list,
+    from: tuple(memo, []),
+    with: fn(item, acc) {
+      let tuple(current_memo, items) = acc
+      let tuple(next_memo, next_item) = fun(current_memo, item)
+      tuple(next_memo, [next_item, ..items])
+    },
+  )
+  |> pair.map_second(reverse)
+}
+
 fn do_index_map(
   list: List(a),
   fun: fn(Int, a) -> b,
diff --git a/test/gleam/list_test.gleam b/test/gleam/list_test.gleam
index b561d33..fa87952 100644
--- a/test/gleam/list_test.gleam
+++ b/test/gleam/list_test.gleam
@@ -99,6 +99,12 @@ pub fn map_test() {
   |> should.equal([0, 8, 10, 14, 6])
 }
 
+pub fn map_reduce_test() {
+  [1, 2, 3, 4]
+  |> list.map_reduce(from: 0, with: fn(acc, i) { tuple(acc + i, i * 2) })
+  |> should.equal(tuple(10, [2, 4, 6, 8]))
+}
+
 pub fn try_map_test() {
   let fun = fn(x) {
     case x == 6 || x == 5 || x == 4 {