Improve promote_map implementation

plugins/qcheck-stm/src/reserr.ml

@@ -286,7 +286,19 @@ let rec promote = function
       let* _ = warns ws and* _ = filter_errs errs in
       promote xs
 
-let promote_map f l = List.map f l |> promote
+let promote_map f =
+  let rec aux = function
+    | [] -> ok []
+    | x :: xs -> (
+        match f x with
+        | (Ok _, _) as x ->
+            let* y = x and* ys = aux xs in
+            ok (y :: ys)
+        | Error errs, ws ->
+            let* _ = warns ws and* _ = filter_errs errs in
+            aux xs)
+  in
+  aux
 
 let promote_mapi f =
   let rec aux i = function

plugins/qcheck-stm/src/reserr.mli

@@ -66,6 +66,7 @@ val promote : 'a reserr list -> 'a list reserr
     [Warning] in the warnings list *)
 
 val promote_map : ('a -> 'b reserr) -> 'a list -> 'b list reserr
+(** [promote_map f xs] is [List.map f xs |> promote] with only one traversal *)
 
 val promote_mapi : (int -> 'a -> 'b reserr) -> 'a list -> 'b list reserr
 (** [promote_mapi f xs] is [List.mapi f xs |> promote] with only one traversal *)