Closures inside property initializations inside the class body don't work. Not even in an array.

So this code throws up a "Parse error: syntax error, unexpected 'function' (T_FUNCTION)"

    class User
    {
        public static $mapping = array(
            'username' => array(
                'type' => 'string',
                'length' => 32,
                'unique' => true,
                'nullable' => function() {
                    // Some logic to determine value.  
                }
            )
        );  
    }