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|
{-# LANGUAGE Unsafe #-}
{-# LANGUAGE NoImplicitPrelude, MagicHash, UnboxedTuples #-}
{-# OPTIONS_HADDOCK not-home #-}
-----------------------------------------------------------------------------
-- |
-- Module : GHC.STRef
-- Copyright : (c) The University of Glasgow, 1994-2002
-- License : see libraries/base/LICENSE
--
-- Maintainer : cvs-ghc@haskell.org
-- Stability : internal
-- Portability : non-portable (GHC Extensions)
--
-- References in the 'ST' monad.
--
-----------------------------------------------------------------------------
module GHC.STRef (
STRef(..),
newSTRef, readSTRef, writeSTRef
) where
import GHC.ST
import GHC.Base
-- $setup
-- >>> import Prelude
-- >>> import Control.Monad.ST
data STRef s a = STRef (MutVar# s a)
-- ^ a value of type @STRef s a@ is a mutable variable in state thread @s@,
-- containing a value of type @a@
--
-- >>> :{
-- runST (do
-- ref <- newSTRef "hello"
-- x <- readSTRef ref
-- writeSTRef ref (x ++ "world")
-- readSTRef ref )
-- :}
-- "helloworld"
-- |Build a new 'STRef' in the current state thread
newSTRef :: a -> ST s (STRef s a)
newSTRef init = ST $ \s1# ->
case newMutVar# init s1# of { (# s2#, var# #) ->
(# s2#, STRef var# #) }
-- |Read the value of an 'STRef'
readSTRef :: STRef s a -> ST s a
readSTRef (STRef var#) = ST $ \s1# -> readMutVar# var# s1#
-- |Write a new value into an 'STRef'
writeSTRef :: STRef s a -> a -> ST s ()
writeSTRef (STRef var#) val = ST $ \s1# ->
case writeMutVar# var# val s1# of { s2# ->
(# s2#, () #) }
-- | Pointer equality.
--
-- @since 2.01
instance Eq (STRef s a) where
STRef v1# == STRef v2# = isTrue# (sameMutVar# v1# v2#)
|
