# Simple StateT use

### From HaskellWiki

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io :: IO a -> StateT [Integer] IO a | io :: IO a -> StateT [Integer] IO a | ||

io = liftIO | io = liftIO | ||

+ | |||

+ | -- | ||

+ | -- another example: a guessing game | ||

+ | -- (from http://scsibug.com/2006/11/28/a-simple-game-with-statet/) | ||

+ | -- | ||

+ | |||

+ | module Main where | ||

+ | import System.Random | ||

+ | import Control.Monad.State | ||

+ | |||

+ | main = do answer <- getStdRandom (randomR (1,100)) -- think of a number | ||

+ | putStrLn "I'm thinking of a number between 1 and 100, can you guess it?" | ||

+ | guesses <- execStateT (guessingSession answer) 0 | ||

+ | putStrLn $ "Success in " ++ (show guesses) ++ " tries." | ||

+ | |||

+ | guessSession :: Int -> StateT Int IO () | ||

+ | guessSession answer = | ||

+ | do gs <- lift getLine -- get guess from user | ||

+ | let g = read gs -- convert to number | ||

+ | modify (+1) -- increment number of guesses | ||

+ | case compare g answer of | ||

+ | LT -> do lift $ putStrLn “Too low” | ||

+ | guessSession answer | ||

+ | GT -> do lift $ putStrLn “Too high” | ||

+ | guessSession answer | ||

+ | EQ -> lift $ putStrLn “Got it!” | ||

</haskell> | </haskell> | ||

[[Category:Code]] | [[Category:Code]] | ||

[[Category:Monad]] | [[Category:Monad]] |

## Revision as of 08:39, 4 April 2007

A small example showing how to combine a State monad (in this case a unique supply), with the IO monad, via a monad transformer.

No need to resort to nasty mutable variables or globals!

import Control.Monad.State main :: IO () main = runStateT code [1..] >> return () -- -- layer an infinite list of uniques over the IO monad -- code :: StateT [Integer] IO () code = do x <- pop io $ print x y <- pop io $ print y return () -- -- pop the next unique off the stack -- pop :: StateT [Integer] IO Integer pop = do (x:xs) <- get put xs return x io :: IO a -> StateT [Integer] IO a io = liftIO -- -- another example: a guessing game -- (from http://scsibug.com/2006/11/28/a-simple-game-with-statet/) -- module Main where import System.Random import Control.Monad.State main = do answer <- getStdRandom (randomR (1,100)) -- think of a number putStrLn "I'm thinking of a number between 1 and 100, can you guess it?" guesses <- execStateT (guessingSession answer) 0 putStrLn $ "Success in " ++ (show guesses) ++ " tries." guessSession :: Int -> StateT Int IO () guessSession answer = do gs <- lift getLine -- get guess from user let g = read gs -- convert to number modify (+1) -- increment number of guesses case compare g answer of LT -> do lift $ putStrLn “Too low” guessSession answer GT -> do lift $ putStrLn “Too high” guessSession answer EQ -> lift $ putStrLn “Got it!”