{-# 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.-------------------------------------------------------------------------------moduleData.STRef(-- * STRefsSTRef ,-- abstractnewSTRef ,readSTRef ,writeSTRef ,modifySTRef ,modifySTRef' )whereimportGHC.ST importGHC.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 4.6.0.0modifySTRef'::STRef s a ->(a ->a )->ST s ()modifySTRef' ref f =dox <-readSTRef ref letx' =f x x' `seq`writeSTRef ref x'