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--         The President and Fellows of Harvard College.
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-- 1. Redistributions of source code must retain the above copyright
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--------------------------------------------------------------------------------
-- |
-- Module      :  Control.Monad.Ref
-- Copyright   :  (c) Harvard University 2006-2010
-- License     :  BSD-style
-- Maintainer  :  mainland@eecs.harvard.edu
--
-- Stability   :  experimental
-- Portability :  non-portable (uses Control.Monad.ST)
--
-- Mutable references in the (strict) ST monad.
--
--------------------------------------------------------------------------------

{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses #-}

module Control.Monad.Ref (
    MonadRef(..)
  ) where

import Control.Monad.ST
import Data.IORef
import Data.STRef

-- | The 'MonadRef' type class abstracts over the details of manipulating
-- references, allowing one to write code that uses references and can operate
-- in either the ST monad or the IO monad.

class (Monad m) => MonadRef r m | m -> r where
    -- |Create a new reference
    newRef    :: a -> m (r a)
    -- |Read the value of a reference
    readRef   :: r a -> m a
    -- |Write a new value to a reference
    writeRef  :: r a -> a -> m ()
    -- |Mutate the contents of a reference
    modifyRef :: r a -> (a -> a) -> m ()
    modifyRef r f = readRef r >>= writeRef r . f

instance MonadRef (STRef s) (ST s) where
    newRef    = newSTRef
    readRef   = readSTRef
    writeRef  = writeSTRef
    modifyRef = modifySTRef

instance MonadRef IORef IO where
    newRef    = newIORef
    readRef   = readIORef
    writeRef  = writeIORef
    modifyRef = modifyIORef