If I write my happy_or_sad() callback and pass it to the Shape, it would be nice if I could get some exhaustiveness checking, but like the blog says, C++ and other popular languages intentionally left it out.
One nice thing about the visitor pattern is that it doesn’t have to match the type hierarchy. For example, you could have a visitor interface method that is invoked for blue shapes, even if there is no BlueShape type. Similarly, the same type hierarchy can support multiple visitor interfaces, so that you can perform different case distinctions on the same value. This is something that sum types can’t do.
Haskell's pattern synonyms should be able to handle this?
The problem with the visitor pattern is that it doesn't compose well. So if you want to match on two values at once, or match deeper into a value, that works well for most pattern matching, but is annoying to piece together with the visitor pattern.
I agree that syntactic sugar for visitor ”matching” would be nice, and it’s something a language could add. The visitor pattern itself doesn’t prevent adding such syntax.
It's not that bad in practice.