Difference between revisions of "New monads/MonadSTO"

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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).
 
[http://haskell.org/hawiki/CaleGibbard_2fBSDLicense "CaleGibbard/BSDLicense"]
 
   
 
<haskell>
 
<haskell>

Revision as of 23:10, 13 November 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).

{-# 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