filter in Ocaml

let filter p l =
    let rec filter_int g ll rr =
          match ll with
        | [] -> rr
        | h::r -> if (g h) then filter_int g r (h::rr) else filter_int g r rr
      in
      let rec map_int g ll rr =
      match ll with
          | [] -> rr
        | h::r -> map_int g r ((g h)::rr)
      in
  map_int (fun i -> i) (filter_int p l []) [] ;;

实现的功能如下，给定一个判别函数p，对列表list的每个元素，如果p执行为真，则输出到结果列表中。