list.hs

import Data.Traversable
import Data.Foldable
import Data.List
import System.IO.Unsafe
import System.Directory

main = do
 xs <- foo ["/Users/qnikst/"]
 for_ xs $ putStrLn

foo [] = pure []
foo (d:ds) = do
  current <- listDirectory d
  xt <- for current $ \fn -> do
    isDir <- doesDirectoryExist fn
    pure (isDir, fn)
  let (dirs, files) = partition fst xt
  next <- unsafeInterleaveIO $ foo ((map snd dirs)++ds)
  pure $ (map ((\f->d++"/"++f).snd) files)++next