add Result.{to,of}_err

src/core/CCResult.ml

@@ -245,6 +245,16 @@ let to_seq e k = match e with
   | Ok x -> k x
   | Error _ -> ()
 
+type ('a, 'b) error = [`Ok of 'a | `Error of 'b]
+
+let of_err = function
+  | `Ok x -> Ok x
+  | `Error y -> Error y
+
+let to_err = function
+  | Ok x -> `Ok x
+  | Error y -> `Error y
+
 (** {2 IO} *)
 
 let pp pp_x buf e = match e with

src/core/CCResult.mli

@@ -181,6 +181,14 @@ val of_opt : 'a option -> ('a, string) t
 
 val to_seq : ('a, _) t -> 'a sequence
 
+type ('a, 'b) error = [`Ok of 'a | `Error of 'b]
+
+val of_err : ('a, 'b) error -> ('a, 'b) t
+(** @since NEXT_RELEASE *)
+
+val to_err : ('a, 'b) t -> ('a, 'b) error
+(** @since NEXT_RELEASE *)
+
 (** {2 IO} *)
 
 val pp : 'a printer -> ('a, string) t printer