import Control.Monad ((>=>))
import Data.Maybe (listToMaybe)
import System.Directory (doesDirectoryExist, listDirectory)
import System.Environment (getArgs)
import System.FilePath ((</>))

ll :: FilePath -> IO [FilePath]
ll p = do
  b <- doesDirectoryExist p
  if b
    then do
      l <- listDirectory p
      concat <$> mapM (ll . (p </>)) (p : l)
    else pure [p]

main :: IO ()
main =
  getArgs
    >>= maybe (pure ()) (ll >=> mapM_ putStrLn) . listToMaybe