list.partition

1 files changed, 18 insertions, 0 deletions

diff --git a/src/gleam/list.gleam b/src/gleam/list.gleam
index d9296da..4ca39f4 100644
--- a/src/gleam/list.gleam
+++ b/src/gleam/list.gleam
@@ -1034,3 +1034,21 @@ pub fn each(list: List(a), f: fn(a) -> b) -> Nil {
     }
   }
 }
+
+pub fn partition(
+  list: List(a),
+  with categorise: fn(a) -> Bool,
+) -> tuple(List(a), List(a)) {
+  do_partition(list, categorise, [], [])
+}
+
+fn do_partition(list, categorise, trues, falses) {
+  case list {
+    [] -> tuple(reverse(trues), reverse(falses))
+    [x, ..xs] ->
+      case categorise(x) {
+        True -> do_partition(xs, categorise, [x, ..trues], falses)
+        False -> do_partition(xs, categorise, trues, [x, ..falses])
+      }
+  }
+}