This mtl instance is in conflict with a usage of shift/reset that I'm porting from ML (Sheard's type-directed partial evaluator).
instance (MonadReader r' m) => MonadReader r' (ContT r m) where local f m = ContT $ \c -> do r <- ask local f (runContT m (local (const r) . c))
I need to remove the "local (const r)" bit. I think this is just a different point in the diverse design space of dynamic binding/delimited control. Since Haskell lacks visibility management for importing instances, I'm forced to duplicate the rest of the ContT code in order to replace this instance with my own. 1) I am hoping this list can recall the reason for putting this design decision into the mtl. Perhaps there's a reason to prefer this particular side-effect interaction. 2) I am suggesting that the mtl be restructured such that this instance be delegated to a separate module so that i) I can avoid importing it along with the other useful instances ii) and the mtl could provide alternative instances. An alternative to breaking out the modules would be to not provide the instance. Another, more radical alternative could involve phantom types. I don't know of any in the mtl, but this same consideration might be applicable to other libraries with "arbitrary" instances.