def ackermann(m, n) {
    | (0, n) => n + 1
    | (m, 0) when m > 0 => ackermann(m - 1, 1)
    | (m, n) when m > 0 && n > 0 => ackermann(m - 1, ackermann(m, n - 1))
    | _ => throw Exception("invalid inputs");
}