static public function directory($value, $message = null, $propertyPath = null)
        {
            static::string($value, $message);

            if (!is_dir($value)) {
                $message = $message ?: sprintf(
                    'Path "%s" was expected to be a directory.',
                    static::stringify($value)
                );

                throw static::createException($value, $message, static::INVALID_DIRECTORY, $propertyPath);
            }
        }