# New monads/MonadSTO

From HaskellWiki

From New monads, copied from old wiki.

Here's an extension of the ST monad in which the references are ordered and showable (they list their creation index).

```
{-# OPTIONS_GHC -fglasgow-exts #-}
module STO (
STO,
runSTO,
newSTORef,
readSTORef,
writeSTORef,
modifySTORef
) where
import Control.Monad.State
import Control.Monad.ST
import Data.STRef
import Data.Ord (comparing)
newtype STO s a = STO { unSTO :: StateT Integer (ST s) a }
deriving (Functor, Monad)
runSTO :: (forall s. STO s a) -> a
runSTO x = runST (evalStateT (unSTO x) 0)
data STORef s a = STORef { idx :: Integer, ref :: STRef s a }
deriving Eq
instance Show (STORef s a) where
show ref = "<STORef: " ++ (show (idx ref)) ++ ">"
instance Ord (STORef s a) where
compare = comparing idx
newSTORef :: a -> STO s (STORef s a)
newSTORef x = STO $ do
n <- get
put (n+1)
r <- lift $ newSTRef x
return $ STORef { idx = n, ref = r }
readSTORef :: STORef s a -> STO s a
readSTORef (STORef { ref = r }) = STO $ lift $ readSTRef r
writeSTORef :: STORef s a -> a -> STO s ()
writeSTORef (STORef { ref = r }) x = STO $ lift $ writeSTRef r x
modifySTORef :: STORef s a -> (a -> a) -> STO s ()
modifySTORef (STORef { ref = r }) f = STO $ lift $ modifySTRef r f
```