https://blog.wongcw.com/2019/04/20/ruby-3-%e6%9c%89%e6%9c%9b%e5%bc%95%e5%85%a5%e9%9d%9c%e6%85%8b%e9%a1%9e%e5%9e%8b/
Ruby 3 有望引入靜態類型