On Wed, Jan 28, 2009 at 6:14 PM, Erik de Castro Lopo
Alexander Dunlap wrote:
It seems like foldM ought to do what you want. Could you post some more details please?
This is a function that I have working in Ocaml which is a little more lenient about IO :-).
This is what I have but won't compile:
fileNames :: ([FilePath] -> FilePath -> FilePath -> [FilePath]) -> [FilePath] -> FilePath -> IO [FilePath] fileNames builder builder_accum topdir = do names <- getDirectoryContents topdir let properNames = filter (`notElem` [".", ".."]) names (dirs, files) <- splitDirFile properNames let accum <- foldl' (\ acc f -> builder acc topdir f) builder_accum files return $ foldM (\ acc d -> fileNames builder accum (topdir d)) accum dirs
I get following error on the foldM:
Couldn't match expected type `[FilePath]' against inferred type `IO [FilePath]' Expected type: IO [FilePath] Inferred type: IO (IO [FilePath])
Thinking about it some more, I can see the problem; accum is an "IO [FilePath]" and my builder function requires a "[FilePath]".
Once I get the function working I would like to generalize it to:
fileNames :: (a -> FilePath -> FilePath -> a) -> a -> FilePath -> IO a
Cheers, Erik -- ----------------------------------------------------------------- Erik de Castro Lopo
Try removing the "return $" on the last line. foldM ... will already be in the IO monad; return will lift the IO a into the IO monad again, so you'll have IO (IO a), which you don't want. Alex