    {-# LANGUAGE Unsafe #-}
    {-# LANGUAGE NoImplicitPrelude, MagicHash, UnboxedTuples #-}
    {-# OPTIONS_HADDOCK hide #-}

    -----------------------------------------------------------------------------
    -- |
    -- 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

    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#)