private function addDefaultFunction()
    {
        $this->addFunction('app', function () {
            return app();
        });

        $this->addFunction('url', function ($url, $absolute = false, array $params = array()) {
            if ($absolute) {
                return Url::createAbsolute($url, $params);
            } else {
                return Url::create($url, $params);
            }
        });

        $this->addFunction('assets', function ($path) {
            return Url::createAbsolute($path);
        });
    }