[Haskell-cafe] Re: Move MonadIO to base

[Heinrich Apfelmus]

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

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