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

Max Bolingbroke batterseapower at hotmail.com
Fri Feb 4 21:19:47 CET 2011

Hi Anders,

I'm using your monad-peel package to good effect in my project, but
I'm having trouble lifting peelIO through a particular monad. What
follows is a simplified description of my problem. Say I have this

data M a = M { unM :: forall m. MonadPeelIO m => Reader.ReaderT () m a }

instance Monad M where
    return x = M (return x)
    M mx >>= fxmy = M $ mx >>= unM . fxmy

instance MonadIO M where
    liftIO io = M (liftIO io)

It seems "clear" that there should be a MonadPeelIO instance for M,
but I can't for the life of me figure it out. I've tried:

instance MonadPeelIO M where
    peelIO = M (fmap (\peel (M mx) -> liftM M (peel mx)) peelIO)

But this is not polymorphic enough: the peelIO gives me back a
function (ReaderT () m a -> IO (ReaderT () m a)) for some *fixed* m,
so I can't pack the result of that function into an M again using
(liftM M).

Have you (or the big brains on Haskell-Cafe, who are CCed) come across
this before? Is there an obvious solution I am missing?


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