Solution for problem 34 in OCaml

problem34.ml

let fast_fact =
    let rec fact n = if n > 1 then n * fact (n - 1) else 1
    in Array.map fact [|0; 1; 2; 3; 4; 5; 6; 7; 8; 9|];;

let digits num =
  let rec helper num result =
    if num < 10 then num :: result else helper (num / 10) ((num mod 10) :: result)
  in helper num [];;

let test_number num =
  num == (List.fold_left (+) 0 (List.map (fun x -> fast_fact.(x)) (digits num))) ;;

let (--) i j = 
    let rec aux n acc =
      if n < i then acc else aux (n-1) (n :: acc)
    in aux j [] ;;

let _ = print_endline (string_of_int (List.fold_left (+) 0 (List.filter test_number (10--10000000)))) ;;