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|
{-# 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
-- $setup
-- >>> import Prelude
-- >>> import Control.Monad.ST
-- | 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 seldom 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 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'
|
