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-- Copyright (c) 2006-2010
-- The President and Fellows of Harvard College.
--
-- Redistribution and use in source and binary forms, with or without
-- modification, are permitted provided that the following conditions
-- are met:
-- 1. Redistributions of source code must retain the above copyright
-- notice, this list of conditions and the following disclaimer.
-- 2. Redistributions in binary form must reproduce the above copyright
-- notice, this list of conditions and the following disclaimer in the
-- documentation and/or other materials provided with the distribution.
-- 3. Neither the name of the University nor the names of its contributors
-- may be used to endorse or promote products derived from this software
-- without specific prior written permission.
--
-- THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY AND CONTRIBUTORS ``AS IS'' AND
-- ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
-- IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
-- ARE DISCLAIMED. IN NO EVENT SHALL THE UNIVERSITY OR CONTRIBUTORS BE LIABLE
-- FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
-- DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
-- OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
-- HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
-- LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
-- OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
-- SUCH DAMAGE.
--------------------------------------------------------------------------------
-- |
-- 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 TypeFamilies #-}
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 m where
type Ref m :: * -> *
-- |Create a new reference
newRef :: a -> m (Ref m a)
-- |Read the value of a reference
readRef :: Ref m a -> m a
-- |Write a new value to a reference
writeRef :: Ref m a -> a -> m ()
-- |Mutate the contents of a reference
modifyRef :: Ref m a -> (a -> a) -> m ()
modifyRef r f = readRef r >>= writeRef r . f
instance MonadRef (ST s) where
type Ref (ST s) = STRef s
newRef = newSTRef
readRef = readSTRef
writeRef = writeSTRef
modifyRef = modifySTRef
instance MonadRef IO where
type Ref IO = IORef
newRef = newIORef
readRef = readIORef
writeRef = writeIORef
modifyRef = modifyIORef
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