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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].
   

Revision as of 18:09, 26 October 2006

From New monads, copied from old wiki.

MonadSTO

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

"CaleGibbard/BSDLicense"

{-# OPTIONS_GHC -fglasgow-exts #-}
module STO (
        STO,
        runSTO,
        newSTORef,
        readSTORef,
        writeSTORef,
        modifySTORef
    ) where
 
import Control.Monad.State
import Control.Monad.ST
import Data.STRef
 
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 x y = compare (idx x) (idx y) 
 
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