filter_take.ml

let filter_take ~f:valid n list =
  let rec take_from ?(taken = []) ?(n_taken = 0) list =
    if n_taken = n then taken
    else
      match list with
      | [] -> taken
      | e :: remaining ->
          let taken, n_taken =
            if valid e then (e :: taken, n_taken + 1) else (taken, n_taken)
          in
          take_from remaining ~taken ~n_taken
  in
  take_from list |> List.rev

# let numbers = List.(1--100);;

# filter_take 3 numbers ~f:(fun n -> n mod 10 = 0);;
- : int list = [10; 20; 30]

# numbers |> filter_take 3 ~f:(fun n -> n mod 10 = 0);;
- : int list = [10; 20; 30]