# Simple StateT use

### From HaskellWiki

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 (guessSession 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!" -- -- another example: a global state -- (corrected from http://www.haskell.org/hawiki/MonadState?action=show) -- import Control.Monad.State data Vars = Vars { var1 :: Int, var2 :: Float } type MyState a = StateT Vars IO a type Selector a = (MyState a, a -> MyState ()) s1 :: Selector Int s1 = (gets var1, \x -> modify (\vs -> vs {var1 = x})) s2 :: Selector Float s2 = (gets var2, \x -> modify (\vs -> vs {var2 = x})) sel :: Selector a -> MyState a sel = fst mods :: Selector a -> (a -> a) -> MyState () mods (gf,uf) mfun = do st <- gf uf (mfun st) sample :: MyState () sample = do a <- sel s1 mods s2 (\x -> x * (fromIntegral a)) b <- sel s2 liftIO $ print b main = runStateT sample (Vars 2 1.3)