public function call($key, $callable)
    {
        if (!is_callable($callable)) {
            throw new \InvalidArgumentException('Not a valid callable.');
        }

        if ($this->exists($key)) {
            return $this->get($key);
        }

        $args = func_get_args();
        array_shift($args);
        array_shift($args);

        $value = call_user_func_array($callable, $args);

        $this->set($key, $value);

        return $value;
    }