Lwt_result.catch takes a function as parameter now

CHANGES

@@ -1,6 +1,10 @@
 ===== dev =====
 
-===== Build =====
+====== Breaking API changes ======
+
+  * Lwt_result.catch now takes a function (unit -> 'a t) rather than a promise ('a t) (#965)
+
+====== Build ======
 
   * Remove unused dependency in dune file. (#969, Kate Deplaix)
 

src/core/lwt_result.ml

@@ -31,7 +31,7 @@ let map_err f e = map_error f e
 
 let catch e =
   Lwt.catch
-    (fun () -> ok e)
+    (fun () -> ok (e ()))
     fail
 
 let get_exn e =

src/core/lwt_result.mli

@@ -23,9 +23,9 @@ 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] *)
+val catch : (unit -> '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] *)
 
 val get_exn : ('a, exn) t -> 'a Lwt.t
 (** [get_exn] is the opposite of {!catch}: it unwraps the result type,

test/core/test_lwt_result.ml

@@ -87,13 +87,25 @@ let suite =
 
     test "catch"
       (fun () ->
-         let x = Lwt.return 0 in
+         let x () = Lwt.return 0 in
          Lwt.return (Lwt_result.catch x = Lwt_result.return 0)
       );
 
     test "catch, error case"
       (fun () ->
-         let x = Lwt.fail Dummy_error in
+         let x () = Lwt.fail Dummy_error in
+         Lwt.return (Lwt_result.catch x = Lwt_result.fail Dummy_error)
+      );
+
+    test "catch, bound raise"
+      (fun () ->
+         let x () = Lwt.bind Lwt.return_unit (fun () -> raise Dummy_error) in
+         Lwt.return (Lwt_result.catch x = Lwt_result.fail Dummy_error)
+      );
+
+    test "catch, immediate raise"
+      (fun () ->
+         let x () = raise Dummy_error in
          Lwt.return (Lwt_result.catch x = Lwt_result.fail Dummy_error)
       );