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