Re: [Haskell-beginners] IO action on a list of [IO a]

On Sat, Oct 06, 2012 at 07:35:13PM +0200, Henk-Jan van Tuyl wrote:

On Sat, 06 Oct 2012 16:16:18 +0200, Manfred Lotz wrote:

...

myfor :: (a -> IO () ) -> [IO a] -> IO () myfor _ [] = return () myfor f (x:xs) = do x' <- x f x' myfor f xs

Is there a library function doing just this?

You could use this: import Control.Monad myfor :: (a -> IO () ) -> [IO a] -> IO () myfor f (x:xs) = mapM_ (liftM f) xs

This should be myfor f xs = mapM_ (>>= f) xs using (liftM f) will result in the right type but it does the wrong thing, as Manfred observed: f :: a -> IO () liftM f :: IO a -> IO (IO a) So mapping liftM f over a list of IO actions results in a list of IO actions with no effects, whose results are the IO actions you really wanted. Then mapM_ throws away those IO actions you really wanted, resulting in essentially (return () :: IO ()). -Brent