=head1 METHODS

=head2 factorial

INPUT : a non-negative integer, n

OUTPUT: the factorial of n

=cut

sub factorial {
    my $number = shift;

    return 1 if $number <= 1;

    return ($number * factorial($number-1)) ;
}

our %validate = 
    (
     non_negative => sub { $_[0] >= 0 },
     integer      => sub { 1 } 
     );

sub validate {
    my $input = shift;
    my %arg = @_;

    for (@{$arg{as}}) {
	my $func = $validate{$_};
	my $r    =  $func->($input);
	die "$_ validation failed" unless $r ;
    }

    return 1;
}

sub get {
    print "input? ";
    $_[0] = <>;
}

my $input;
get($input) 
    and validate($input, as => [qw(non_negative integer)])
    and print factorial($input);