public function getFunctionInstance() {
		if (is_string($this->argument))
			$this->argument = new Attr($this->argument);
		if (!$this->argument instanceof Field)
			throw new \InvalidArgumentException(sprintf("Function '%s' expects an argument of type string or \eMapper\Query\Field", $this->getName()));
		return new Func($this->getName(), [$this->argument]);
	}