David Feuer wrote:
Spectacular! That looks like just what I wanted! Unless there are some hidden gotchas, it might be worth making a package of that.
I can think of one nasty surprise: if liftIO is implemented using `unsafeIOToST` then it becomes dupable in the `unsafeDupablePerformIO` sense. It's quite delicate but something along the lines of r <- newIORef "" replicateM 2 $ forkIO $ <some code polling r> runMyIO $ do ((), _) <- mfix $ \(x, y) -> do x <- liftIO $ writeIORef r y y <- getLine return (x,y) last [1..] `seq` return () could trigger the `getLine` operation from different threads simultaneously. The upshot is that `liftIO` should employ `noDuplicate#` to prevent this scenario. Cheers, Bertram For reference:
newtype MyIO a = MyIO (LST.ST RealWorld a) deriving (Functor, Applicative, Monad, MonadFix)
instance MonadIO MyIO where liftIO = MyIO . LSTU.unsafeIOToST
runMyIO :: MyIO a -> IO a runMyIO (MyIO f) = stToIO f
main = runMyIO $ do l <- (2:) `fmap` liftIO readLn m <- replicateM (head l) (liftIO readLn) liftIO (print (l :: [Int],m :: [Int]))