Well, I'd say the real challenge is figuring out what it means once you have written something that type checks. I'm not there yet !

Whilst useful, I think the _Left and _Right exercises are a bit too hard, especially since the types are pretty hard to read, and it's non-obvious what those Prisms are al about.

Did you consider monomorphing the p to -> (assuming that's possible)? This might make types easier to grok. If not, maybe adding another example of a p instance could be useful.