[Haskell-cafe] MonadPeelIO instance for monad transformers on top of "forall"

[Max Bolingbroke]

On 4 February 2011 21:41, Anders Kaseorg <andersk at mit.edu> wrote:
> On Fri, 4 Feb 2011, Max Bolingbroke wrote:
>> data M a = M { unM :: forall m. MonadPeelIO m => Reader.ReaderT () m a }
>
> Maybe this won’t help in your actual code, but isn’t M isomorphic to IO
> (via unM :: M a -> IO a, M . liftIO :: IO a -> M a)?

Well, yes :-). My real code actually has a non-trivial ReaderT
transformer and a StateT transformer it reaches "m". I had hoped that
by restricting to ReaderT () the problem would be simpler and hence
clearer.

> instance MonadPeelIO M where
>   peelIO = M (liftIO (liftM (\k (M mx) -> liftM (\my -> (M (liftIO my))) (k mx)) peelIO))

This doesn't type check for me (I think the root of the trouble is you
peelIO in the IO monad, but then later do (k mx) where mx is not an IO
computation). Is this definition trying to exploit the isomorphism, or
do you think that this is a solution to the general class of problems
I'm having trouble with?

Thanks for your reply!
Max