Multi-Dispatch Ackermann function at Perl 6.

multi-ack.pl6

# ~~~            | n + 1                 :@: if m = 0            ~~~ #
# ~~~  A(m, n) = | A(m - 1, 1)           :@: if m > 0 and n = 0  ~~~ #
# ~~~            | A(m - 1, A(m, n - 1)  :@: if m > 0 and n > 0  ~~~ #

multi A($m where(* == 0), $n              ) { $n + 1 }
multi A($m where(*  > 0), $n where(* == 0)) { A($m - 1, 1) }
multi A($m where(*  > 0), $n where(*  > 0)) { A($m - 1, A($m, $n - 1)) }

say A(1, 2);
# say A(4, 3);