{-# 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'.---- >>> :{-- runST (do-- ref <- newSTRef ""-- modifySTRef ref (const "world")-- modifySTRef ref (++ "!")-- modifySTRef ref ("Hello, " ++)-- readSTRef ref )-- :}-- "Hello, world!"---- 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 may produce a stack overflow:---- >>> import Control.Monad (replicateM_)-- >>> :{-- print (runST (do-- ref <- newSTRef 0-- replicateM_ 1000 $ modifySTRef ref (+1)-- readSTRef ref ))-- :}-- 1000---- To avoid this problem, use 'modifySTRef'' instead.modifySTRef ::STRef s a ->(a ->a )->ST s ()modifySTRef :: STRef s a -> (a -> a) -> ST s ()
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