Annoying and the performance is terrible, since pretty much every instance ends up being an allocation, and allocations in Go are quite a lot more expensive tha

Annoying and the performance is terrible, since pretty much every instance ends up being an allocation, and allocations in Go are quite a lot more expensive than in other GC languages. Further, it doesn't quite have the same semantics--if you return one of these interfaces, your caller still doesn't know what all of the permutations they have to check for.