From 34cfbd5cdc4c2d05bfc9abd80272ce4d9e52d719 Mon Sep 17 00:00:00 2001
From: Kento Okura <kentookura@gmail.com>
Date: Sat, 22 Aug 2026 15:09:25 +0200
Subject: [PATCH] Add Eio.Fiber.List.partition_map

---
 lib_eio/core/eio__core.mli |  5 +++++
 lib_eio/core/fiber.ml      |  1 +
 tests/fiber.md             | 20 ++++++++++++++++++++
 3 files changed, 26 insertions(+)

diff --git a/lib_eio/core/eio__core.mli b/lib_eio/core/eio__core.mli
index dbcd4fa06..19b449635 100644
--- a/lib_eio/core/eio__core.mli
+++ b/lib_eio/core/eio__core.mli
@@ -350,6 +350,11 @@ module Fiber : sig
         invocations of [f] are run concurrently in separate fibers.
         @param max_fibers Maximum number of fibers to run concurrently *)
 
+    val partition_map : ?max_fibers:int -> ('a -> ('b, 'c) Either.t) -> 'a list -> 'b list * 'c list
+    (** [partition_map f x] is like [List.partition_map f x] except that the
+        invocations of [f] are run concurrently in separate fibers.
+        @param max_fibers Maximum number of fibers to run concurrently *)
+
     val iter : ?max_fibers:int -> ('a -> unit) -> 'a list -> unit
     (** [iter f x] is like [List.iter f x] except that the invocations of [f] are
         run concurrently in separate fibers.
diff --git a/lib_eio/core/fiber.ml b/lib_eio/core/fiber.ml
index 1e241606e..97123d909 100644
--- a/lib_eio/core/fiber.ml
+++ b/lib_eio/core/fiber.ml
@@ -260,6 +260,7 @@ module List = struct
       aux items
 
   let map ?max_fibers fn = filter_map ?max_fibers (fun x -> Some (fn x))
+  let partition_map ?max_fibers fn items = List.partition_map Fun.id (map ?max_fibers fn items) 
   let filter ?max_fibers fn = filter_map ?max_fibers (fun x -> if fn x then Some x else None)
 
   let iter ?(max_fibers=max_int) fn items =
diff --git a/tests/fiber.md b/tests/fiber.md
index ad42a54fe..ade4da6c8 100644
--- a/tests/fiber.md
+++ b/tests/fiber.md
@@ -452,6 +452,10 @@ let string_even x =
 let crash_on_three x =
   if x = 3 then failwith "Simulated error"
   else string_even x
+
+let even_or_odd x =
+  if is_even x then Either.Right (string_of_int x)
+  else Either.Left (string_of_int x)
 ```
 
 ```ocaml
@@ -502,6 +506,22 @@ let crash_on_three x =
 - : unit = ()
 ```
 
+```ocaml
+# Eio_mock.Backend.run @@ fun () ->
+  Fiber.List.partition_map (process even_or_odd) [1; 2; 3; 4]
+  |> traceln "%a" Fmt.Dump.(pair (list string) (list string));;
++Start 1
++Start 2
++Start 3
++Start 4
++Finished 1
++Finished 2
++Finished 3
++Finished 4
++(["1"; "3"], ["2"; "4"])
+- : unit = ()
+```
+
 If any fiber raises, everything is cancelled:
 
 ```ocaml