let evens n = 
  let rec loop k () =
    if k == n then
      None
    else
      Some (k , loop (k + 2))
  in loop 2

let test () =
  let rec loop next =
    match next () with
      | None -> ()
      | Some (res, next') ->
          printf "%i\n" res;
          loop next'
  in loop (evens 10)

let _ = test ()