public function rememberForever($key, Closure $callback)
    {
        $value = $this->get($key);

        // If the item exists in the cache we will just return this immediately
        // and if not we will execute the given Closure and cache the result
        // of that forever so it is available for all subsequent requests.
        if (! is_null($value)) {
            return $value;
        }

        $this->forever($key, $value = $callback());

        return $value;
    }