Ackermann function

func A(m, n) {
    m == 0 ? (n + 1)
           : (n == 0 ? (A(m - 1, 1))
                     : (A(m - 1, A(m, n - 1))));
}

Alternatively, using multiple dispatch:

func A((0), n) { n + 1 }
func A(m, (0)) { A(m - 1, 1) }
func A(m,  n)  { A(m-1, A(m, n-1)) }

Calling the function:

say A(3, 2);     # prints: 29