validator on a second field

Field::inst( 'casetype' )->validator( 'Validate::required' ),
Field::inst( 'factsnum' )->validator( function($val, $data, $opts) {

if ('casetype' == 'this' && strlen($val) < 3) {
return "Value <b>must</b> is required!";


}
return true;

} ),

How do I enter the input field of another value. In this case casetype!

In plain english if casetype is this the length of the string must be greater than 3.