gistfile1.hs

walk :: FilePath -> ((FilePath, [FilePath], [FilePath]) -> IO ()) -> IO ()
walk root action = do
  entries <- Directory.getDirectoryContents root
  directories <- newIORef []
  files <- newIORef []
  
  forM_ entries $ \entry -> do
    let fullPath = combine root entry
    isFile <- Directory.doesFileExist fullPath
    if isFile then
      modifyIORef files (entry:)
     else do
      when (entry /= "." && entry /= "..") $ do
        isDirectory <- Directory.doesDirectoryExist fullPath
        when isDirectory $ modifyIORef directories (entry:)

  ds <- readIORef directories
  fs <- readIORef files
  action (root, sort ds, sort fs)

  forM_ ds $ \dir ->
    walk (combine root dir) action