[bPspGiJT8Y]

> If you have pickRandom<A|B, B|C> x y you would get nested Either's

If this wasn't the case, how would the information about what you got be retained? It's either positional, or by a tag/key (row-polymorphic variants), or none retained.

I don't see why would you want to use monadic API for approaching an "anonymous sum type" problem in the first place. As I said before, there are fundamentally just 2 operations you would want to use: inject and project. Maybe you could also mention assoc for re-association but I'd say if you're using it you're likely handling the problem the wrong way. So I still don't see how monad transformers play into this. They are a nice (decent, at least) trick for dealing with some situations but the problem we're talking about here isn't one of them.