Add Lwt_result.error

For consistency with Stdlib.

CHANGES

@@ -3,7 +3,7 @@
 ====== Additions ======
 
   * 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.iter and Lwt_result.iter_error for consistency with Stdlib. (#927, Antonin Décimo)
+  * Add Lwt_result.error, Lwt_result.iter, and Lwt_result.iter_error for consistency with Stdlib. (#927, Antonin Décimo)
 
 ====== Fixes ======
 

src/core/lwt_result.ml

@@ -14,6 +14,7 @@ let fail e = Lwt.return (Error e)
 
 let lift = Lwt.return
 let ok x = Lwt.map (fun y -> Ok y) x
+let error x = Lwt.map (fun y -> Error y) x
 
 let map f e =
   Lwt.map

src/core/lwt_result.mli

@@ -20,6 +20,9 @@ val lift : ('a, 'b) Result.result -> ('a, 'b) t
 
 val ok : 'a Lwt.t -> ('a, _) t
 
+val error : 'b Lwt.t -> (_, 'b) t
+(** @since 5.6.0  *)
+
 val catch : 'a Lwt.t -> ('a, exn) t
 (** [catch x] behaves like [return y] if [x] evaluates to [y],
     and like [fail e] if [x] raises [e] *)

test/core/test_lwt_result.ml

@@ -66,6 +66,12 @@ let suite =
          Lwt.return (Lwt_result.ok x = Lwt_result.return 0)
       );
 
+    test "error"
+      (fun () ->
+        let x = Lwt.return 0 in
+        Lwt.return (Lwt_result.error x = Lwt_result.fail 0)
+      );
+
     test "catch"
       (fun () ->
          let x = Lwt.return 0 in