Change `iterator.flatten`'s type

1 files changed, 16 insertions, 1 deletions

diff --git a/test/gleam/iterator_test.gleam b/test/gleam/iterator_test.gleam
index 8b7e01d..5d10cb2 100644
--- a/test/gleam/iterator_test.gleam
+++ b/test/gleam/iterator_test.gleam
@@ -193,7 +193,7 @@ pub fn append_test() {
   test([1, 2], [3, 4])
 }
 
-// a |> list.map(from_list) |> concat |> to_list == list.concat(a)
+// a |> list.map(from_list) |> from_list |> concat |> to_list == list.concat(a)
 pub fn concat_test() {
   let test = fn(lists) {
     lists
@@ -209,6 +209,21 @@ pub fn concat_test() {
   test([[1, 2], [3, 4]])
 }
 
+// a |> list.map(from_list) |> flatten |> to_list == list.concat(a)
+pub fn flatten_test() {
+  let test = fn(lists) {
+    lists
+    |> list.map(iterator.from_list)
+    |> iterator.flatten
+    |> iterator.to_list
+    |> should.equal(list.concat(lists))
+  }
+
+  test([[], []])
+  test([[1], [2]])
+  test([[1, 2], [3, 4]])
+}
+
 // a |> from_list |> filter(f) |> to_list == a |> list.filter(_, f)
 pub fn filter_test() {
   let test = fn(subject, f) {