New monads/MonadUnique
From HaskellWiki
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evalUniqueT (UniqueT s) = evalStateT s 0 | evalUniqueT (UniqueT s) = evalStateT s 0 | ||
evalUnique (Unique s) = runIdentity (evalUniqueT s) | evalUnique (Unique s) = runIdentity (evalUniqueT s) | ||
| + | </haskell> | ||
| + | |||
| + | == STSupply == | ||
| + | |||
| + | There is also a simple way to get the same functionality in the ST Monad. Here's a quick module to construct infinite supplies of unique values in the ST monad: | ||
| + | |||
| + | [http://haskell.org/hawiki/CaleGibbard_2fBSDLicense "CaleGibbard/BSDLicense"] | ||
| + | |||
| + | <haskell> | ||
| + | module STSupply (Unique, createSupply) where | ||
| + | |||
| + | import Control.Monad.ST | ||
| + | import Data.STRef | ||
| + | |||
| + | newtype Unique = Unique Integer deriving (Eq, Ord) | ||
| + | |||
| + | createSupply :: ST s (ST s Unique) | ||
| + | createSupply = do | ||
| + | v <- newSTRef $ Unique 0 | ||
| + | return $ do | ||
| + | Unique x <- readSTRef v | ||
| + | writeSTRef v $ Unique (x+1) | ||
| + | return $ Unique x | ||
| + | </haskell> | ||
| + | |||
| + | A test example: | ||
| + | |||
| + | <haskell> | ||
| + | import Control.Monad.ST | ||
| + | import STSupply | ||
| + | |||
| + | main = print test1 | ||
| + | |||
| + | test1 = runST supplyTest | ||
| + | where supplyTest = do | ||
| + | fresh <- createSupply | ||
| + | x <- fresh | ||
| + | y <- fresh | ||
| + | return (x == x, x == y, x < y) | ||
</haskell> | </haskell> | ||
Revision as of 23:36, 24 August 2006
From NewMonads, copied from old wiki.
1 MonadUnique
This is a simple (trivial) monad transformer for supplying unique integer values to an algorithm.
{-# OPTIONS_GHC -fglasgow-exts #-} module MonadUnique ( UniqueT, Unique, MonadUnique, fresh, evalUniqueT, evalUnique ) where import Control.Monad import Control.Monad.State import Control.Monad.Identity newtype UniqueT m a = UniqueT (StateT Integer m a) deriving (Functor, Monad, MonadTrans, MonadIO) newtype Unique a = Unique (UniqueT Identity a) deriving (Functor, Monad, MonadUnique) class Monad m => MonadUnique m where fresh :: m Integer instance (Monad m) => MonadUnique (UniqueT m) where fresh = UniqueT $ do n <- get put (succ n) return n evalUniqueT (UniqueT s) = evalStateT s 0 evalUnique (Unique s) = runIdentity (evalUniqueT s)
1.1 STSupply
There is also a simple way to get the same functionality in the ST Monad. Here's a quick module to construct infinite supplies of unique values in the ST monad:
module STSupply (Unique, createSupply) where import Control.Monad.ST import Data.STRef newtype Unique = Unique Integer deriving (Eq, Ord) createSupply :: ST s (ST s Unique) createSupply = do v <- newSTRef $ Unique 0 return $ do Unique x <- readSTRef v writeSTRef v $ Unique (x+1) return $ Unique x
A test example:
import Control.Monad.ST import STSupply main = print test1 test1 = runST supplyTest where supplyTest = do fresh <- createSupply x <- fresh y <- fresh return (x == x, x == y, x < y)
