I’m not really familiar with Haskell’s pattern synonyms, but maybe yes. What the visitor pattern gives you is that in principle the implementation structure can

I’m not really familiar with Haskell’s pattern synonyms, but maybe yes. What the visitor pattern gives you is that in principle the implementation structure can be orthogonal to the visitor case distinction. You can have objects encapsulate (and possibly hide) one structure while exposing different structural views via visitor. This also allows to re-arrange the implementation of the alternatives without breaking client code. In functional languages, you’d probably rather have functions converting the source value to a different (sum) type to perform the case distinction on. 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.