package Tie::Constrained;

##</code><code>##

use Errno qw/EINVAL EDOM ERANGE/;

sub TIESCALAR {
    my $class = shift;
    my $self = { test => defined $_[0]? $_[0]: \&validate };
    $self->{test}($_[1]) or invalid(EINVAL) if defined $_[1];
    $self->{val} = $_[1];
    bless $self, $class;
}

sub STORE {
    my ($self, $try) = @_;
    $self->{test}($try) or invalid(EINVAL);
    $self->{val} = $try;
}

sub FETCH {
    $_[0]->{val};
}

sub DESTROY {}

sub validate { 1 }

sub invalid {
    $! = shift;
    die sprintf("Constraint violation: %s by %s::%s in %s line %s.\n",
        $!,
        map { qq($_) } (caller 1)[0,3,1,2] );
}

1;
__DATA__
##</code><code>##

Usage:
    use Tie::Constrained;
    tie my $var, Tie::Constrained => \&mytest, $initval;

Both arguments are optional, but the default validator
function always says yes.  mytest() should be designed
to return true for valid data and false for data to reject.