Simple IO Monad problem

[Derek Elkins]

On 12 Sep 2003 22:08:30 -0000
kuq32tr02 at sneakemail.com wrote:

> Hello,
> 
> I'm starting to use Haskell for writing actual programs using monads
> and I'm already lost.
> 
> I have the following script:
> 
> #!/usr/bin/runhugs
>                                                                     
           
> > module Main where
> > import System(getArgs)
> > main = do putStr "Hello, World\n"
> >           strs <- getArgs
> >           map putStrLn strs
> 
> 
> Which gives the following error:
> 
> runhugs: Error occurred
> Reading file "./mailalias.lhs":
> Reading file "/usr/lib/hugs/lib/System.hs":
> Reading file "./mailalias.lhs":
> Type checking
> ERROR "./mailalias.lhs":5 - Type error in final generator
> *** Term           : map putStrLn strs
> *** Type           : [IO ()]
> *** Does not match : IO a
> 
> Can someone please explain what I'm doing wrong?
> 

map :: (a -> b) -> [a] -> [b]
putStrLn :: String -> IO ()
therefore map putStrLn :: [String] -> [IO ()]
map maps a -pure function- over a list.  What you want is to map a
-monadic computation- over the list, further you don't care about the
result.
mapM_ :: Monad m => (a -> m b) -> [a] -> m ()
mapM_ putStrLn :: [String] -> IO ()