public function setValueType($valueType)
    {
        if (!in_array(
            $valueType,
            [
                \Parable\Console\Parameter::OPTION_VALUE_REQUIRED,
                \Parable\Console\Parameter::OPTION_VALUE_OPTIONAL,
            ]
        )) {
            throw new \Parable\Console\Exception('Value type must be one of the OPTION_* constants.');
        }

        $this->valueType = $valueType;
        return $this;
    }