add Result.flatten_l to turn a list of results into a result of list

src/core/CCResult.ml

@@ -1,4 +1,3 @@
-
 (* This file is free software, part of containers. See file "license" for more details. *)
 
 (** {1 Error Monad} *)
@@ -218,6 +217,19 @@ let map_l f l =
         | Ok y -> map (y::acc) l'
   in map [] l
 
+let flatten_l l =
+  let rec loop acc l = match l with
+    | [] -> Ok (List.rev acc)
+    | Ok x::l' -> loop (x::acc) l'
+    | Error e::_ -> Error e
+  in loop [] l
+
+(*$=
+  (Ok []) (flatten_l [])
+  (Ok [1;2;3]) (flatten_l [Ok 1; Ok 2; Ok 3])
+  (Error "ohno") (flatten_l [Ok 1; Error "ohno"; Ok 2; Ok 3; Error "wut"])
+*)
+
 exception LocalExit
 
 let fold_seq f acc seq =

src/core/CCResult.mli

@@ -1,4 +1,3 @@
-
 (* This file is free software, part of containers. See file "license" for more details. *)
 
 (** {1 Error Monad}
@@ -187,6 +186,12 @@ end
 
 (** {2 Collections} *)
 
+val flatten_l : ('a, 'err) t list -> ('a list, 'err) t
+(** Same as [map_l id]: returns [Ok [x1;…;xn]] if [l=[Ok x1; …; Ok xn]],
+    or the first error otherwise.
+    @since NEXT_RELEASE
+*)
+
 val map_l : ('a -> ('b, 'err) t) -> 'a list -> ('b list, 'err) t
 (** [map_l f [a1; ...; an]] applies the function [f] to [a1, ..., an] , and, in case of
     success for every element, returns the list of [Ok]-value.