Spectacular! That looks like just what I wanted! Unless there are some
hidden gotchas, it might be worth making a package of that. The bit
that's strangest to me is how stToIO (seems to) make sure that all the
actions actually get performed.
David
On Sun, Apr 24, 2016 at 6:08 PM, Bertram Felgenhauer
I see what I missed here: The IO part of `y` should also force the IO part of `x` to be performed, and that is not captured by `unsafeInterleaveIO`, since it preserves the order of the IO actions. In fact such a monad already exists in the form of the lazy ST monad. So...
import Control.Monad.ST.Lazy as LST import Control.Monad.ST.Lazy.Unsafe as LSTU import Control.Monad.IO.Class import Control.Monad.Fix
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]))
Cheers,
Bertram