[Haskell-cafe] ReaderT and concurrency

[Tomasz Zielonka]

On Wed, Nov 16, 2005 at 09:45:17AM -0800, Andrew Pimlott wrote:
> On Wed, Nov 16, 2005 at 11:51:19AM -0500, Kurt Hutchinson wrote:
> > I have to perform another runReaderT when forking? Or is there a way
> > to get the ReaderT environment automatically carried over to the newly
> > created Set B thread?
> 
> This is an unavoidable pain as far as I know.  It would be nice if
> forkIO were defined in terms of MonadIO:
> 
>     forkIO :: MonadIO m => m () -> m ThreadId
> 
> (Same with forkProcess.)  I haven't thought too hard about it, but it
> seems that it should be possible.

I think it wouldn't be possible using only methods in MonadIO. Besides,
what should be the semantics of forkIO for (StateT s IO)? I can't think
of anything reasonable.

I played with this idea a bit, and below is the result. I'm not sure
there are any meaningful instances of UnliftIO (bad name) other than
the two below.

    class Monad m => UnliftIO m where
        unliftIO :: m a -> m (IO a)

    instance UnliftIO IO where
        unliftIO io = return io

    instance UnliftIO m => UnliftIO (ReaderT r m) where
        unliftIO m = do
            r <- ask
            lift (unliftIO (runReaderT m r))

    forkIO' :: (UnliftIO m, MonadIO m) => m () -> m ThreadId
    forkIO' m = do
        io <- unliftIO m
        liftIO (forkIO io)

Best regards
Tomasz