# Simple StateT use

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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)
```