package Tie::Scalar::Watch;

use Carp;
use strict;

# rich man's Exporter ;)
# this just puts 'watch()' in the caller's namespace
sub import {
  my $pkg = caller;
  no strict 'refs';
  *{ $pkg . "::watch" } = \&watch;
}

sub TIESCALAR {
  my ($class, $val, $rule) = @_;
  my $self = bless [ undef, $rule ], $class;
  $self->STORE($val);
  return $self;
}

sub FETCH { $_[0][0] }

sub STORE {
  my ($self, $val) = @_;
  return $self->[0] = $val if $self->[1]->($val);
  croak "Value $val is out of bounds";
}

sub watch (&$) {
  tie $_[1], __PACKAGE__, $_[1], $_[0];
}

1;