On Tue, Aug 11, 2015 at 5:32 AM, Ben
In particular, this version of <$!> wouldn't help with the lazy IO problem described in that stackoverflow question Christopher linked earlier.
We can say more. The version of <$!> that the SO question author himself gives as a solution to the lazy IO problem doesn't really work. I'm referring to: forceM :: Monad m => m a -> m a forceM m = do v <- m; return $! v (<$!>) :: Monad m => (a -> b) -> m a -> m b f <$!> m = liftM f (forceM m) and changing out <$> for <$!> in getLines h = lines <$> hGetContents h With the change, all of short files get read, thanks to file buffering. Like the first 4K or so. Bad surprise when you try it with longer files. There's a right path through lazy I/O and a wrong one. Rather than take the wrong one, better the following instead: main = mapM_ putStrLn =<< lines <$> readFile "test.txt" But if withFile is absolutely needed (experts only): main = withFile "test.txt" ReadMode getAndPutLines where getAndPutLines :: Handle -> IO () getAndPutLines h = mapM_ putStrLn =<< lines <$> hGetContents h -- Kim-Ee