bullsandcows.hs

import System.Random

{-
    A little more generic version of the Bulls and Cows 
    guessing game.

    To reproduce the original game simply call:
        bullandcows 4 "123456789"

    For a detailed description of the game look at: 
        https://en.wikipedia.org/wiki/Bulls_and_Cows
-}
delete :: Int -> [a] -> [a]
delete n xs =
    let (ys, zs) = splitAt n xs
    in ys ++ (tail zs)

randompick :: Int -> String -> IO String
randompick n digits = randompick' n digits []
  where
    randompick' :: Int -> String -> String -> IO String
    randompick' n' digits' secret = do
        index <- randomRIO (0, length digits' - 1)
        let digit = digits' !! index
        let digits'' = delete index digits'
        if n' > 0
            then randompick' (n' - 1) digits'' (digit : secret)
            else return (secret)

check :: String -> String -> (Int, Int)
check secret guess =
    if length secret /= length guess
        then (0, 0)
        else let pairs = zip secret guess
                 bulls = length (filter (\(x, y) -> x == y) pairs)
                 cows = length (filter (\x -> x `elem` secret) guess) - bulls
             in (bulls, cows)

turn :: String -> String -> IO ()
turn secret guess = do
    if secret == guess
        then putStrLn "You win!"
        else mkguess secret

mkguess :: String -> IO ()
mkguess secret = do
    putStr "  Enter your guess: "
    guess <- getLine
    let (bulls, cows) = check secret guess
    putStrLn $ "Bulls: " ++ show bulls ++ " Cows: " ++ show cows
    turn secret guess
    
bullsandcows :: Int -> String -> IO ()
bullsandcows secretlen symbols = do
    secret <- randompick secretlen symbols
    turn secret ""