# Simple StateT use

### From HaskellWiki

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io :: IO a -> StateT [Integer] IO a |
io :: IO a -> StateT [Integer] IO a |
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io = liftIO |
io = liftIO |
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+ | |||

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

+ | module Main where |
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+ | import System.Random |
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+ | import Control.Monad.State |
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+ | |||

+ | main = do answer <- getStdRandom (randomR (1,100)) -- think of a number |
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+ | putStrLn "I'm thinking of a number between 1 and 100, can you guess it?" |
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+ | guesses <- execStateT (guessingSession answer) 0 |
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+ | putStrLn $ "Success in " ++ (show guesses) ++ " tries." |
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+ | |||

+ | guessSession :: Int -> StateT Int IO () |
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+ | guessSession answer = |
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+ | do gs <- lift getLine -- get guess from user |
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+ | let g = read gs -- convert to number |
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+ | modify (+1) -- increment number of guesses |
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+ | case compare g answer of |
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+ | LT -> do lift $ putStrLn “Too low” |
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+ | guessSession answer |
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+ | GT -> do lift $ putStrLn “Too high” |
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+ | guessSession answer |
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+ | EQ -> lift $ putStrLn “Got it!” |
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</haskell> |
</haskell> |
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## 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!”