P.P.S. Strange it does not seem to work with the paste. So here comes the solution by mail: module Consolidator.BusinessLogic.ConflictsResolved (consolidateDuplicates) where import System.FilePath import System.Directory import Control.Monad (filterM) import Control.Exception (throwIO) import System.Environment import Data.Maybe import Control.Monad import Control.Monad.Trans newtype MaybeT m a = MaybeT { runMaybeT :: m (Maybe a) } instance (Monad m) => Monad (MaybeT m) where (>>=) tmb_v f = MaybeT (runMaybeT tmb_v >>= \b_v -> case b_v of Nothing -> return Nothing Just v -> runMaybeT $ f v ) return = MaybeT . return . return instance MonadTrans MaybeT where lift mon = MaybeT (mon >>= return . Just) abort :: String -> MaybeT IO a abort reason = do lift . putStrLn $ reason MaybeT (return Nothing) {- The traversal is one directory deep only. I try to find out if every immediate subdirectory contains exactly one "*.gdr" file, and collect the path names in a list, sgls. Afterwards I append the contents of each such file to another file. I want to abort the whole process as soon as I encounter a directory that does not include exactly one *.gdr file. Currently I'm throwing exceptions but I'd prefer to rewrite this code to use continuations. -} consolidateDuplicates :: FilePath -> MaybeT IO () consolidateDuplicates fp = do dirs <- lift (getDirectoryContents fp) recs <- lift (filterM doesDirectoryExist $ map (fp ) $ filter (not . flip elem [".", ".."]) dirs) sgls <- mapM checkForSingle recs let cpy = fp "Korrigiert.gdr" lift (copyFile (fp "Konsolidiert.gdr") cpy) lift (mapM_ (\sgl -> do str <- readFile sgl appendFile cpy str) sgls) checkForSingle :: FilePath -> MaybeT IO FilePath checkForSingle fp = do cnt <- lift (getDirectoryContents fp) let fltr = filter ((== ".gdr") . takeExtension) case fltr cnt of [] -> abort ("The directory " ++ fp ++ " is empty") [f] -> return (fp f) _ -> abort ("There is more than one file in the directory " ++ fp)