    {-# LANGUAGE CPP, BangPatterns, MagicHash, UnboxedTuples #-}
    --
    -- (c) The University of Glasgow 2002
    --
    -- Unboxed mutable Ints

    module FastMutInt(
            FastMutInt, newFastMutInt,
            readFastMutInt, writeFastMutInt
      ) where

    #define SIZEOF_HSINT  4

    import GHC.Exts
    # if __GLASGOW_HASKELL__>=612
    import GHC.IO     (IO(IO))
    #else
    import GHC.IOBase (IO(IO))
    #endif

    data FastMutInt = FastMutInt (MutableByteArray# RealWorld)

    newFastMutInt :: IO FastMutInt
    newFastMutInt = IO $ \s ->
      case newByteArray# size s of { (# s, arr #) ->
      (# s, FastMutInt arr #) }
      where !(I# size) = SIZEOF_HSINT

    readFastMutInt :: FastMutInt -> IO Int
    readFastMutInt (FastMutInt arr) = IO $ \s ->
      case readIntArray# arr 0# s of { (# s, i #) ->
      (# s, I# i #) }

    writeFastMutInt :: FastMutInt -> Int -> IO ()
    writeFastMutInt (FastMutInt arr) (I# i) = IO $ \s ->
      case writeIntArray# arr 0# i s of { s ->
      (# s, () #) }