import Data.List
import System.Random

match :: [Int] -> [Int] -> String
match code guess = concat $ zipWith replicate [bs, ws, ds] "BW."
    where bs = length . filter id $ zipWith (==) code guess
          ds = length (foldr delete code guess)
          ws = length code - bs - ds

go :: [Int] -> IO ()
go code = do hits <- fmap (match code) readLn
             if all (== 'B') hits then putStrLn "You Win!"
                else putStr (hits ++ "\nTry again: ") >> go code

play :: Maybe [Int] -> IO ()
play code = do putStr "Enter your guess as a list: "
               rnd <- fmap (take 4 . randomRs (1, 8)) getStdGen
               go $ maybe rnd id code

It was an interesting exercise.