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

[Brent Yorgey]

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
> <manfred.lotz at arcor.de> 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