# New monads/MonadSTO

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< New monads(Difference between revisions)

BrettGiles (Talk | contribs) m (Remove NewMonads link) |
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+ | [[Category:Code]] |
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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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− | = 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). |
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− | [http://haskell.org/hawiki/CaleGibbard_2fBSDLicense "CaleGibbard/BSDLicense"] |
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<haskell> |
<haskell> |
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import Control.Monad.ST |
import Control.Monad.ST |
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import Data.STRef |
import Data.STRef |
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+ | import Data.Ord (comparing) |
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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 |
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− | compare x y = compare (idx x) (idx y) |
+ | 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