let program =
  Lwt_io.with_file ~mode:Lwt_io.Output "some_code" begin fun oc ->
    let rec forever () =
      Lwt_io.read_line Lwt_io.stdin >>= Lwt_io.write_line oc >>= forever
    in
    Lwt.async (fun () -> forever ());
    Lwt.return_unit

  end

let () =
  Lwt_main.run program