Am 21.04.10 05:01, Anders Kaseorg wrote:
On Tue, 20 Apr 2010, wren ng thornton wrote:
-- | The isomorphic Haskell98 version class Monad m => MonadJoinIO m where -- | Embed the IO into the monad m joinIO :: IO (m a) -> m a
-- | Extract the IO computation to the top level, -- rendering the m pure from IO. partIO :: m a -> IO (m a)
-- | The isomorphisms joinIO' m = morphIO (m >>=) morphIO' f = joinIO (f partIO)
To establish an isomorphism, you also need to define partIO from morphIO. For example, I don’t see how I could define partIO :: IO a -> IO (ReaderT r IO a) that extracts effects into the outer IO, because the effects depend on some unknown state of type r.
I think you mean partIO :: ReaderT r IO a -> IO (ReaderT r IO a) This does not affect the impossibility of pushing the effect to the outer IO. On a side note, joinIO = join . liftIO Regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com