No I think you're right, but this is fine. There may be instances where you don't want to handle every possible data type and other values should result in a no

No I think you're right, but this is fine. There may be instances where you don't want to handle every possible data type and other values should result in a no-op. If you want full coverage then visitation guarantee that statically. Multi-visitation over m variants each with N possible types also does this statically with N^m instantiations. This balloons quickly, which is why I mentioned yomm2, which can add these handlers dynamically at runtime for polymorphic types.