let log f s =
  Lwt_unix.openfile f [Lwt_unix.O_APPEND; Lwt_unix.O_CREAT] 0o640 >>= fun o ->
  Lwt_unix.write o s 0 (String.length s)

let () = Lwt_main.run (log "/var/log/cwb.log" "some message\n");;
Error: This expression has type int but an expression was expected of type unit

let t = Lwt_main.run (log "/var/log/cwb.log" "some message\n");;
Exception: Unix.Unix_error (Unix.EBADF, "write", "").