    \begin{code}
    {-# OPTIONS_GHC -fno-implicit-prelude #-}
    -----------------------------------------------------------------------------
    -- |
    -- 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.
    --
    -----------------------------------------------------------------------------

    -- #hide
    module GHC.STRef where

    import GHC.ST
    import GHC.Base

    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@

    -- |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#, () #) }

    -- Just pointer equality on mutable references:
    instance Eq (STRef s a) where
        STRef v1# == STRef v2# = sameMutVar# v1# v2#
    \end{code}