OrderingのMonoid実装を使って書いたじゃんけんです＞ω＜　unsafeInterleaveIOを使ってどうにか動くようにしてみました＞＜　もっとうまい方法を知っていたら教えていただけると幸いですm(_ _)m

janken.hs

import Control.Monad
import Data.Monoid
import System.IO
import System.Random

data Janken = Guu|Choki|Paa deriving (Show, Read, Eq, Enum)

instance Ord Janken where
  Guu   `compare` Choki = LT
  Choki `compare` Paa   = LT
  Paa   `compare` Guu   = LT
  l     `compare` r
    |l == r    = EQ
    |otherwise = GT

janken :: IO Ordering
janken = do
  putStrLn "Janken Pon!"
  you <- prompt "you > " >>= return . read :: IO Janken
  cpu <- getStdRandom (randomR (0, 2)) >>= return . (!!) [Guu .. Paa]
  putStrLn $ "cpu > " ++ show cpu
  (return $ you `compare` cpu) `mappendM` (unsafeInterleaveIO $ putStrLn "Draw" >> janken)
  where
    prompt xs = putStr xs >> hFlush stdout >> getLine
    mappendM = liftM2 mappend

main = do
  res <- janken
  case res of
    LT -> putStrLn "Win!"
    GT -> putStrLn "Loss!"