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2009年02月14日

Bounds checking with Variable::Magic

Vincent Pit's Variable::Magic is an evil module that allows you to define Perl variable magic in Perl space. I hope to be able to use it for new types of join points in aspect-oriented programming (see Aspect).

Playing around with the module, I thought it could reproduce the effects of tie() without having to tie objects. For example, let's enhance a variable so that it can only take values that lie within given lower and upper bounds.

use Variable::Magic qw(wizard cast);
use Carp qw(croak);

sub bounds_spell {
    my ($lower, $upper) = @_;

    wizard
        set => sub {
            my $value = ${$_[0]};
            return if $value >= $lower && $value <= $upper;
            croak "bounds violation: $lower <= $value <= $upper is not true\n";
        };
}

my $foo;
cast $foo, bounds_spell(2,8) or die "wizard FAIL\n";

$foo = 7;    # OK
$foo += 2;   # exception

Setting the variable to 7 works fine, but increasing its value to 9 produces the following error:

bounds violation: 2 <= 9 <= 8 is not true
 at test.pl line 15
    main::__ANON__('SCALAR(0x800c6c)', 'undef') called at test.pl line 23

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