Flip order of args passed to fn in map_reduce. Item first and memo second.

2 files changed, 9 insertions, 9 deletions

diff --git a/src/gleam/list.gleam b/src/gleam/list.gleam
index 6d9c709..e911b80 100644
--- a/src/gleam/list.gleam
+++ b/src/gleam/list.gleam
@@ -262,18 +262,18 @@ pub fn map(list: List(a), with fun: fn(a) -> b) -> List(b) {
 pub fn map_reduce(
   over list: List(a),
   from memo: memo,
-  with fun: fn(memo, a) -> tuple(memo, b),
-) -> tuple(memo, List(b)) {
+  with fun: fn(a, memo) -> tuple(b, memo),
+) -> tuple(List(b), memo) {
   fold(
     over: list,
-    from: tuple(memo, []),
+    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])
+      let tuple(items, current_memo) = acc
+      let tuple(next_item, next_memo) = fun(item, current_memo)
+      tuple([next_item, ..items], next_memo)
     },
   )
-  |> pair.map_second(reverse)
+  |> pair.map_first(reverse)
 }
 
 fn do_index_map(
diff --git a/test/gleam/list_test.gleam b/test/gleam/list_test.gleam
index fa87952..b7470ac 100644
--- a/test/gleam/list_test.gleam
+++ b/test/gleam/list_test.gleam
@@ -101,8 +101,8 @@ pub fn map_test() {
 
 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]))
+  |> list.map_reduce(from: 0, with: fn(i, acc) { tuple(i * 2, acc + i) })
+  |> should.equal(tuple([2, 4, 6, 8], 10))
 }
 
 pub fn try_map_test() {