content: {-# LANGUAGE Trustworthy #-}
  {-# LANGUAGE CPP #-}
  
  -----------------------------------------------------------------------------
  -- |
  -- 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, instance Eq
          newSTRef,       -- :: a -> ST s (STRef s a)
          readSTRef,      -- :: STRef s a -> ST s a
          writeSTRef,     -- :: STRef s a -> a -> ST s ()
          modifySTRef     -- :: STRef s a -> (a -> a) -> ST s ()
   ) where
  
  import Prelude
  
  #ifdef __GLASGOW_HASKELL__
  import GHC.ST
  import GHC.STRef
  #endif
  
  #ifdef __HUGS__
  import Hugs.ST
  import Data.Typeable
  
  #include "Typeable.h"
  INSTANCE_TYPEABLE2(STRef,stRefTc,"STRef")
  #endif
  
  -- |Mutate the contents of an 'STRef'
  modifySTRef :: STRef s a -> (a -> a) -> ST s ()
  modifySTRef ref f = writeSTRef ref . f =<< readSTRef ref
  
  