Data/STRef.hs

{-# LANGUAGE Trustworthy #-}

-----------------------------------------------------------------------------
-- |
-- Module : Data.STRef
-- Copyright : (c) The University of Glasgow 2001
-- License : BSD-style (see the file libraries/base/LICENSE)
-- 
-- Maintainer : libraries@haskell.org
-- Stability : experimental
-- Portability : non-portable (uses Control.Monad.ST)
--
-- Mutable references in the (strict) ST monad.
--
-----------------------------------------------------------------------------

module Data.STRef (
 -- * STRefs
 STRef, -- abstract
 newSTRef,
 readSTRef,
 writeSTRef,
 modifySTRef,
 modifySTRef'
 ) where

import GHC.ST
import GHC.STRef

-- | Mutate the contents of an 'STRef'.
--
-- Be warned that 'modifySTRef' does not apply the function strictly. This
-- means if the program calls 'modifySTRef' many times, but seldomly uses the
-- value, thunks will pile up in memory resulting in a space leak. This is a
-- common mistake made when using an STRef as a counter. For example, the
-- following will leak memory and likely produce a stack overflow:
--
-- >print $ runST $ do
-- > ref <- newSTRef 0
-- > replicateM_ 1000000 $ modifySTRef ref (+1)
-- > readSTRef ref
--
-- To avoid this problem, use 'modifySTRef'' instead.
modifySTRef :: STRef s a -> (a -> a) -> ST s ()
modifySTRef ref f = writeSTRef ref . f =<< readSTRef ref

-- | Strict version of 'modifySTRef'
--
-- @since 4.6.0.0
modifySTRef' :: STRef s a -> (a -> a) -> ST s ()
modifySTRef' ref f = do
 x <- readSTRef ref
 let x' = f x
 x' `seq` writeSTRef ref x'