# New monads/MonadSTO

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From New monads, copied from [http://haskell.org/hawiki/STOMonad old wiki]. | From New monads, copied from [http://haskell.org/hawiki/STOMonad old wiki]. | ||

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

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<haskell> | <haskell> | ||

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import Control.Monad.ST | import Control.Monad.ST | ||

import Data.STRef | import Data.STRef | ||

+ | import Data.Ord (comparing) | ||

newtype STO s a = STO { unSTO :: StateT Integer (ST s) a } | newtype STO s a = STO { unSTO :: StateT Integer (ST s) a } | ||

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instance Ord (STORef s a) where | instance Ord (STORef s a) where | ||

− | compare | + | compare = comparing idx |

newSTORef :: a -> STO s (STORef s a) | newSTORef :: a -> STO s (STORef s a) |

## Latest revision as of 18:30, 21 February 2010

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