MonadTransControl instance for ProxyFast/ProxyCorrect

Question

Using pipes, I'm trying to write an instance of MonadTransControl for the ProxyFast or ProxyCorrect type. This is what I've got:

instance MonadTransControl (ProxyFast a' a b' b) where
  data StT (ProxyFast a' a b' b) a = StProxy { unStProxy :: ProxyFast a' a b' b Identity a}
  liftWith = undefined
  restoreT = undefined

I have no idea how to write liftWith or restoreT. The instances for the other monad transformers all use a function that "swaps" the monads, for example EitherT e m a -> m (EitherT e Identity a), but I couldn't find any such function in pipes. How does the instance for MonadTransControl for ProxyCorrect / ProxyFast look like? Or is it impossible to write one? (If yes, is it possible in pipes 4.0?)

Answer 1

Thanks for the link, and now I can give a better answer.

No, there is no way to implement this, using either version of pipes. The reason why is that MonadTransControl expects a monad transformer to be built on top of a single layer of the underlying base monad. This is true for all the monad transformers that MonadTransControl currently implements, such as:

ErrorT  ~ m (Either e r)
StateT  ~ s -> m (r, s)
WriterT ~ m (r, w)
ReaderT ~ i -> m r
ListT   ~ m [r]  -- This version of ListT is wrong, and the true ListT
                 -- would not work for `MonadTransControl`

However, a Proxy does not wrap a single layer of the base monad. This is true for both pipes versions where you can nest as many layers of the base monad as you want.

In fact, any monad transformer that nests the base monad multiple times will defy a MonadTransControl instance, such as:

FreeT     -- from the `free` package
ListT     -- when done "right"
ConduitM  -- from the `conduit` package

However, just because pipes does not implement MonadTransControl doesn't mean that all hope is lost. pipes-safe implements many of the operations that one would typically expect from MonadTransControl, such as bracketing resource acquisitions, so if you can elaborate on your specific use case I can tell you more if there is an appropriate pipes-based solution for your problem.