Simplify closure calculation with Result.iter

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/lib.ml

@@ -874,16 +874,10 @@ and closure_with_overlap_checks db ts ~stack =
       >>= fun () ->
       Dep_stack.push stack (to_id t) >>= fun stack ->
       t.requires >>= fun deps ->
-      iter deps ~stack >>| fun () ->
+      Result.iter deps ~f:(loop ~stack) >>| fun () ->
       res := t :: !res
-  and iter ts ~stack =
-    match ts with
-    | [] -> Ok ()
-    | t :: ts ->
-      loop t ~stack >>= fun () ->
-      iter ts ~stack
   in
-  iter ts ~stack >>| fun () ->
+  Result.iter ts ~f:(loop ~stack) >>| fun () ->
   List.rev !res
 
 let closure_with_overlap_checks db l =

src/stdune/result.ml

@@ -59,4 +59,11 @@ let concat_map =
   in
   fun l ~f -> loop f [] l
 
+let rec iter t ~f =
+  match t with
+  | [] -> Ok ()
+  | x :: xs ->
+    f x >>= fun () ->
+    iter xs ~f
+
 type ('a, 'error) result = ('a, 'error) t

src/stdune/result.mli

@@ -23,6 +23,8 @@ val map_error : ('a, 'error1) t -> f:('error1 -> 'error2) -> ('a, 'error2) t
 
 val all : ('a, 'error) t list -> ('a list, 'error) t
 
+val iter : 'a list -> f:('a -> (unit, 'error) t) -> (unit, 'error) t
+
 val concat_map
   :  'a list
   -> f:('a -> ('b list, 'error) t)