let rec map_filter =
  fun f l ->
  match l with
    [] -> []
  | h::t ->
    try (f h)::map_filter f t
    with _ -> map_filter f t

exception NoResult

let rec f = fun n l ->
  match l with
    [] -> if n = 0 then [] else raise NoResult
  | h::t ->
    try h::(f (n - h) t)
    with _ -> f n t