Hello, I have a problem with implementing a MonadBaseControl instance for freer monad (Eff from extensible-effects). Specifically, I don't really get what the associated type StM should be. As far as I understand, MonadBaseControl class does the following: captures the current state. Performs the action passed to lifeBaseWith or other wrapper functions. Returns the result wrapped in the captured state. Here's the instance I could come up with. instance (MonadBase m (Eff r), Typeable m, SetMember Lift (Lift m) r) => MonadBaseControl m (Eff r) where type StM (Eff r) a = Eff r a liftBaseWith f = lift (f return) restoreM = id It obviously doesn't work, but I currently have no idea how to fix it, because `Eff r a' contains the state that needs to be captured and cannot be decomposed without losing data as far as I can see. The code can be found in this branch: https://github.com/greydot/extensible-effects/tree/monadbasecontrol As a side matter, I couldn't find any tests for MonadBaseControl instances, and monad-control package itself lacks any tests whatsoever. I'm curious whether there's a way to test instance correctness without plugging it into working code, e.g. something using lifted-base, and hoping for the best.