Simple RWST use
RWST on IO example. Guess the number between 1 and 10 within a certain number of tries. Guesses are entered via stdin.
Reader: Env is max tries Writer: Guesses are logged to a String. State: Number of guesses in session. IO: Get next guess from stdin
import System.Random import Control.Monad.RWS main = do answer <- getStdRandom (randomR (1,10)) -- think of a number let maxTries = 5 putStrLn "I'm thinking of a number between 1 and 10, can you guess it?" putStrLn $ "You've got " ++ show maxTries ++ " tries!" (guesses,log) <- execRWST (guessSession answer) maxTries 0 putStrLn $ "Finished in " ++ (show guesses) ++ " tries." putStrLn $ "Guesses: " ++ show log guessSession :: Int -> RWST Int [Int] Int IO () guessSession answer = do maxTries <- ask -- Get the maxTries from Reader Env gs <- lift getLine -- get guess from user let g = read gs -- convert to number tell $ [g] -- Log guess to Writer modify (+1) -- increment number of guesses in State tries <- get -- Get current number of guess from State case (compare g answer, compare tries maxTries) of (EQ, _) -> do lift $ putStrLn "Got it!" (LT,EQ) -> do lift $ putStrLn "Too low" lift $ putStrLn "Game over!" (GT,EQ) -> do lift $ putStrLn "Too high" lift $ putStrLn "Game over!" (LT, _) -> do lift $ putStrLn "Too low" guessSession answer (GT, _) -> do lift $ putStrLn "Too high" guessSession answer