Here's a two-line, mildly obfuscated version of the same
main = applyM putStr $ map unlines $ applyM
(mapM (\ x -> map (head.lines) $ readFile x)) $ (map lines $ readFile
"filenames.txt")
sorry, couldn't resist :)
--Sigbjorn
Sven Panne writes:
> Roland Senn wrote:
> > [...] I now want to write a simple program, that prints out the first
> > line of every file mentioned in 'filenames.txt'.
> > [...]
> > Everybody immediately sees, that the last function 'processSingleFile'
> > is wrong! But how, has this example to be changed, to compile and
> > run successfully ??
>
> This can be done using mapM:
>
> ---------------------------------------------------------------------
> import IO
>
> main :: IO ()
> main = do fromHandle <- openFile "filenames.txt" ReadMode
> contents <- hGetContents fromHandle
> firstLines <- mapM processSingleFile (lines contents)
> putStr (unlines firstLines)
>
> processSingleFile :: String -> IO String
> processSingleFile s = do inh <- openFile s ReadMode
> cont <- hGetContents inh
> return (head (lines cont))
> ---------------------------------------------------------------------
>
> mapM is in the standard Prelude and has the following signature:
>
> mapM :: Monad m => (a -> m b) -> [a] -> m [b]
>
> Specialized to the IO monad this means:
>
> Given an IO action expecting an "a" and returning a "b"
> and a list of "a"s,
> yield an IO action returning a list of "b"s.
>
> Here, both "a" and "b" are String.
>
>
> P.S.: Closing the files with hClose would be a good idea...
>
> --
> Sven Panne Tel.: +49/89/2178-2235
> LMU, Institut fuer Informatik FAX : +49/89/2178-2211
> LFE Programmier- und Modellierungssprachen Oettingenstr. 67
> mailto:[EMAIL PROTECTED] D-80538 Muenchen
> http://www.pms.informatik.uni-muenchen.de/mitarbeiter/panne
