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

[Anders Kaseorg]

On Fri, 4 Feb 2011, Max Bolingbroke wrote:
> 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.

Oh sorry, I further simplified your problem by stripping off the ReaderT 
() and forgot to undo that before sending my reply.  Pretend that I sent 
the following instead:  :-)

Maybe this won’t help in your actual code, but isn’t M isomorphic to 
ReaderT () IO (via unM :: M a -> ReaderT () IO a, M . mapReaderT liftIO :: 
IO a -> M a)?

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

> 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?

It does exploit the isomorphism.  I’m not sure whether the isomorphism 
will generalize to your case; I assume there’s probably some reason you 
have a polymorphic type instead of replacing it with an equivalent 
nonpolymorphic type?

Anders