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Mutable variable

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Although the functional programming paradigm emphasises the virtues of immutable variables, sometimes you need mutable variables nonetheless.

You can either simulate mutable variables using the state monad provided for instance by
in the transformers package or you can use really mutable variables as provided by
from the stm package. In either case you need a monad in order to cope with mutability, while staying purely functional.

There are several packages that provide a single interface to these different implementations of mutable variables (in alphabetical order):

1 Haskell 98 solution

If the type of the reference depends only on the monad then there is an elegant Haskell 98 solution:

module Data.Ref where
import Data.IORef (newIORef, readIORef, writeIORef, )
import Data.STRef (newSTRef, readSTRef, writeSTRef, )
import Control.Concurrent.STM.TVar (newTVar, readTVar, writeTVar, )
import Control.Concurrent.STM (STM, )
import Control.Monad.ST (ST)
import Prelude hiding (read)
data T m a = Cons { write :: a -> m (), read :: m a }
modify :: Monad m => T m a -> (a -> a) -> m ()
modify ref f = write ref . f =<< read ref
class C m where
   new :: a -> m (T m a)
instance C IO where
   new = fmap (\r -> Cons (writeIORef r) (readIORef r)) . newIORef
instance C (ST s) where
   new = fmap (\r -> Cons (writeSTRef r) (readSTRef r)) . newSTRef
instance C STM where
   new = fmap (\r -> Cons (writeTVar r) (readTVar r)) . newTVar

2 See also