import Control.Monad.Trans.List (ListT(..))
import Control.Monad.Trans (lift)
import System.Directory (getDirectoryContents, doesDirectoryExist)

listDir :: FilePath -> ListT IO FilePath
listDir path = do
    isDirectory <- lift $ doesDirectoryExist path
    if not isDirectory
      then ListT $ return [path]
      else do
        contents <- lift $ getDirectoryContents path
        let files = map ((path ++ "/") ++) . filter (`notElem` [".", ".."]) $ contents
        (ListT $ return files) >>= listDir