Add bind_error to Lwt_result

CHANGES

@@ -9,6 +9,7 @@
 
   * In the Lwt_io module, add `?cloexec:bool` optional arguments to functions that create file descriptors (`pipe`). The `?cloexec` argument is simply forwarded to the wrapped Lwt_unix function. (#872, #911, Antonin Décimo)
   * Add Lwt_result.error, Lwt_result.iter, and Lwt_result.iter_error for consistency with Stdlib. (#927, Antonin Décimo)
+  * Lwt_result.bind_error (#943, Boning Dong)
 
 ====== Misc ======
 

src/core/lwt_result.ml

@@ -53,7 +53,13 @@ let bind e f =
     (function
       | Error e -> Lwt.return (Error e)
       | Ok x -> f x)
-
+
+let bind_error e f =
+  Lwt.bind e
+    (function
+      | Error e -> f e
+      | Ok x -> Lwt.return (Ok x))
+
 let bind_lwt e f =
   Lwt.bind e
     (function

src/core/lwt_result.mli

@@ -39,6 +39,9 @@ val map_error : ('e1 -> 'e2) -> ('a,'e1) t -> ('a,'e2) t
 
 val bind : ('a,'e) t -> ('a -> ('b,'e) t) -> ('b,'e) t
 
+val bind_error : ('a,'e1) t -> ('e1 -> ('a,'e2) t) -> ('a,'e2) t
+(** @since 5.6.0 *)
+
 val bind_lwt : ('a,'e) t -> ('a -> 'b Lwt.t) -> ('b,'e) t
 
 val bind_lwt_error : ('a,'e1) t -> ('e1 -> 'e2 Lwt.t) -> ('a,'e2) t

test/core/test_lwt_result.ml

@@ -65,7 +65,22 @@ let suite =
          let actual = Lwt_result.bind x (fun y -> Lwt_result.return (y + 1)) in
          Lwt.return (actual = x)
       );
+
+    test "bind_error"
+      (fun () ->
+         let x = Lwt_result.return 0 in
+         let actual = Lwt_result.bind_error x (fun y -> Lwt_result.return (y + 1)) in
+         Lwt.return (actual = x)
+      );
 
+    test "bind_error, error case"
+      (fun () ->
+         let x = Lwt_result.fail 0 in
+         let correct = Lwt_result.return 1 in
+         let actual = Lwt_result.bind_error x (fun y -> Lwt_result.return (y + 1)) in
+         Lwt.return (actual = correct)
+      );
+
     test "ok"
       (fun () ->
          let x = Lwt.return 0 in