# 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