Quicksort with reading from stdin

qsstdin.hs

main :: IO ()
main = do
  putStrLn "Please enter a sequence of numbers to be sorted"
  putStrLn "Press ENTER twice to start sorting"
  nums <- getnums
  nums_s <- return $ sort nums
  putStrLn $ "Well, the sorted list is " ++ show nums_s
  return ()

sort :: (Ord a) => [a] -> [a]
sort []    = []
sort (p:n) =
  sort smaller ++ [p] ++ sort larger
  where
    smaller = do
      x <- n
      True <- return (x < p)
      return x
    larger = do
      x <- n
      True <- return (x >= p)
      return x

getnums :: (Num a, Read a) => IO [a]
getnums = readnum >>= \mnum ->
  case mnum of
    Nothing      -> return []
    Just num     -> getnums >>= \nums -> return (num:nums)


readnum :: (Num a, Read a) => IO (Maybe a)
readnum = readword >>= \word ->
  case word of
    []           -> return Nothing
    word         -> return $ Just (read word)

readword :: IO (String)
readword = getChar >>= \char ->
  case char of
    '\n'         -> return []
    ' '          -> return []
    chr          -> readword >>= \chrs -> return (chr:chrs)