By "classes are open" you mean that you can add methods to classes at will by just implementing a func with the right receiver? In that case they're not truly

By "classes are open" you mean that you can add methods to classes at will by just implementing a func with the right receiver? In that case they're not truly open: Go does not allow you to declare methods on receivers from other packages, which means you can't extend anything which wasn't written by you. Which makes open classes almost useless.