public function onFunctionCallExpression($functionDescription, $params)
    {
        if (!isset($functionDescription)) {
            throw new \InvalidArgumentException('onFunctionCallExpression');
        }
        if (!array_key_exists($functionDescription->name, $this->functionDescriptionParsers)) {
            throw new \InvalidArgumentException('onFunctionCallExpression');
        }
        return $this->functionDescriptionParsers[$functionDescription->name]($params);
    }