Some more Perl 6 testing.

Ackermann.p6

use v6;

say 'Ackermann function: A(m,n)';
print 'Enter m = ';
my $M = $*IN.get;
print 'Enter n = ';
my $N = $*IN.get;

sub A($m, $n) {
    $m == 0  ??    $n + 1                   !!
    $n == 0  ??  A($m - 1, 1            )   !!
                 A($m - 1, A($m, $n - 1));
}

A($M, $N).say;