static public function string($value, $message = null, $propertyPath = null)
        {
            if ( ! is_string($value)) {
                $message = $message ?: sprintf(
                    'Value "%s" expected to be string, type %s given.',
                    static::stringify($value),
                    gettype($value)
                );

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