sequence :: [IO a] -> IO [a]
> sequence [getLine, getLine]
> hallo
> du
["hallo","du"]
> sequence $ map print [1,2,3]
> 1
> 2
> 3
[(),(),()]
sequence_ :: [IO a] -> IO ()
> sequence_ $ map print [1,2,3]
> 1
> 2
> 3