Resolve Hedgehog StateT-outside-of-GenT issue

[mhuesch]

Background

From the hedgehog-1.0 Changelog, we see:

Simplify MonadGen, this breaks the use of StateT on the outside of a GenT for the time being, it still works fine on the inside though and you can use distributeT to run it

It turns out that we need this exact configuration for Goblins' TinkerM:

type TinkerM g = StateT (GoblinData g) Gen

More context is given here:

We also simplify MonadGen significantly, with all functions being implemented in terms of toGen / fromGen. This comes at the cost of StateT and RWST instances, but you can still run these on the inside of the transformer and get a MonadGen, it's just slightly less conveniant. If anyone can figure out how to implement MonadTransControl for TreeT then they will be easy possible on the back of that.

I (@mhuesch) have spent a few days equivalent trying to hack around this issue with MonadTransControl, and haven't made much progress.

Proposed Solution

Make it work 🙂

I see 3 possible routes:

1. Solve MonadTransControl in full generality (what Jacob describes above)
2. Write instances specific to our TinkerM case, which might be able to cheat and avoid issues which arise for full generality.
3. Come up with an alternative way to thread genomes around.

I considered ReaderT for a bit, but we only want to consume genome when we tinker/conjure, and a ReaderT solution would have to meter out genome each time it did a bind (I think).

[mhuesch]

Next Action

• Try each approach above

[mhuesch]

Closing this and continuing the issue in #556