Add Lwt_result.collect without deps

src/core/lwt_result.ml

@@ -104,6 +104,14 @@ let iter_error f r =
       | Error e -> f e
       | Ok _ -> Lwt.return_unit)
 
+let collect x =
+  let rec aux oks errors = function
+    | [] -> if errors = [] then Ok (List.rev oks) else Error (List.rev errors)
+    | Ok o :: t -> aux (o :: oks) errors t
+    | Error e :: t -> aux oks (e :: errors) t
+  in
+  x |> Lwt.all |> Lwt.map (fun r -> aux [] [] r)
+
 module Infix = struct
   let (>>=) = bind
   let (>|=) e f = map f e

src/core/lwt_result.mli

@@ -71,6 +71,14 @@ val iter_error : ('e -> unit Lwt.t) -> ('a, 'e) t -> unit Lwt.t
     @since Lwt 5.6.0
 *)
 
+val collect : ('a, 'b) result Lwt.t list -> ('a list, 'b list) result Lwt.t
+(** [collect r] is a single promise resolved with [Ok u] if all promises
+    in [r] resolve with [Ok v] and a single promise resolved with [Error w] 
+    otherwise.
+
+    @since NEXT_RELEASE
+*)
+
 module Infix : sig
   val (>|=) : ('a,'e) t -> ('a -> 'b) -> ('b,'e) t
   val (>>=) : ('a,'e) t -> ('a -> ('b,'e) t) -> ('b,'e) t