{-# 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 Prelude 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'