Movatterモバイル変換

{-# 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.STimportGHC.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::STRefsa->(a->a)->STs()modifySTRefreff=writeSTRefref.f=<<readSTRefref-- | Strict version of 'modifySTRef'---- @since 4.6.0.0modifySTRef'::STRefsa->(a->a)->STs()modifySTRef'reff=dox<-readSTRefrefletx'=fxx'`seq`writeSTRefrefx'