# Difference between revisions of "Simple RWST use"

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− | RWST on IO example.<br\> |
+ | RWST on IO example. Inspired by [[Simple_StateT_use]].<br\> |

Guess the number between 1 and 10 within a certain number of tries.<br\> |
Guess the number between 1 and 10 within a certain number of tries.<br\> |
||

Guesses are entered via stdin. |
Guesses are entered via stdin. |

## Latest revision as of 19:47, 9 January 2018

RWST on IO example. Inspired by Simple_StateT_use.<br\> Guess the number between 1 and 10 within a certain number of tries.<br\> Guesses are entered via stdin.

Reader: Env is max tries.<br\> Writer: Guesses are logged to a String.<br\> State: Number of guesses in session.<br\> IO: Get next guess from stdin.<br\>

```
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
```