Hi Roland,
> However my question is: How to do a little bit more complex IO:
> Assume I have a file called 'filenames.txt' that contains a
> list of filenames eg:
> file1.txt
> file2.txt
> file3.txt
>
> I now want to write a simple program, that prints out the first
> line of every file mentioned in 'filenames.txt'.
here we are ...
> module Main
> where
> printFirstLineOf :: FilePath -> IO ()
> printFirstLineOf filePath
> = do contents <- readFile filePath
> putStrLn ((lines contents ++ ["<empty>"])!!0)
> main :: IO ()
> main = do contents <- readFile "filenames.txt"
> mapM_ printFirstLineOf (lines contents)
The function `printFirstLineOf' prints the first line of the given file
(no error handling). If the file is empty its prints `<empty>'. The
predefined function `mapM_ f x' applies `f' to every element in `x' and
sequences the `IO' actions.
Ralf
P.S.: Your code does not work since `processSingleFile' does
`IO' but the type pretends it's a pure function.
> processSingleFile :: String -> String
> processSingleFile s =
> do
> inh <- openFile s ReadMode
> cont <- hGetContents inh
> return (head (lines cont))
