    \begin{code}
    {-# OPTIONS_GHC -XNoImplicitPrelude #-}
    {-# OPTIONS_HADDOCK hide #-}
    -----------------------------------------------------------------------------
    -- |
    -- Module      :  GHC.Pack
    -- Copyright   :  (c) The University of Glasgow 1997-2002
    -- License     :  see libraries/base/LICENSE
    --
    -- Maintainer  :  cvs-ghc@haskell.org
    -- Stability   :  internal
    -- Portability :  non-portable (GHC Extensions)
    --
    -- This module provides a small set of low-level functions for packing
    -- and unpacking a chunk of bytes. Used by code emitted by the compiler
    -- plus the prelude libraries.
    --
    -- The programmer level view of packed strings is provided by a GHC
    -- system library PackedString.
    --
    -----------------------------------------------------------------------------

    -- #hide
    module GHC.Pack
           (
            -- (**) - emitted by compiler.

            packCString#,      -- :: [Char] -> ByteArray#    (**)
            unpackCString,
            unpackCString#,    -- :: Addr# -> [Char]         (**)
            unpackNBytes#,     -- :: Addr# -> Int# -> [Char] (**)
            unpackFoldrCString#,  -- (**)
            unpackAppendCString#,  -- (**)
           )
            where

    import GHC.Base
    import GHC.Err ( error )
    import GHC.List ( length )
    import GHC.ST
    import GHC.Num
    import GHC.Ptr

    data ByteArray ix              = ByteArray        ix ix ByteArray#
    data MutableByteArray s ix     = MutableByteArray ix ix (MutableByteArray# s)

    unpackCString :: Ptr a -> [Char]
    unpackCString a@(Ptr addr)
      | a == nullPtr  = []
      | otherwise      = unpackCString# addr

    packCString#         :: [Char]          -> ByteArray#
    packCString# str = case (packString str) of { ByteArray _ _ bytes -> bytes }

    packString :: [Char] -> ByteArray Int
    packString str = runST (packStringST str)

    packStringST :: [Char] -> ST s (ByteArray Int)
    packStringST str =
      let len = length str  in
      packNBytesST len str

    packNBytesST :: Int -> [Char] -> ST s (ByteArray Int)
    packNBytesST (I# length#) str =
      {-
       allocate an array that will hold the string
       (not forgetting the NUL byte at the end)
      -}
     new_ps_array (length# +# 1#) >>= \ ch_array ->
       -- fill in packed string from "str"
     fill_in ch_array 0# str   >>
       -- freeze the puppy:
     freeze_ps_array ch_array length#
     where
      fill_in :: MutableByteArray s Int -> Int# -> [Char] -> ST s ()
      fill_in arr_in# idx [] =
       write_ps_array arr_in# idx (chr# 0#) >>
       return ()

      fill_in arr_in# idx (C# c : cs) =
       write_ps_array arr_in# idx c  >>
       fill_in arr_in# (idx +# 1#) cs

    -- (Very :-) ``Specialised'' versions of some CharArray things...

    new_ps_array    :: Int# -> ST s (MutableByteArray s Int)
    write_ps_array  :: MutableByteArray s Int -> Int# -> Char# -> ST s ()
    freeze_ps_array :: MutableByteArray s Int -> Int# -> ST s (ByteArray Int)

    new_ps_array size = ST $ \ s ->
        case (newByteArray# size s)   of { (# s2#, barr# #) ->
        (# s2#, MutableByteArray bot bot barr# #) }
      where
        bot = error "new_ps_array"

    write_ps_array (MutableByteArray _ _ barr#) n ch = ST $ \ s# ->
        case writeCharArray# barr# n ch s#  of { s2#   ->
        (# s2#, () #) }

    -- same as unsafeFreezeByteArray
    freeze_ps_array (MutableByteArray _ _ arr#) len# = ST $ \ s# ->
        case unsafeFreezeByteArray# arr# s# of { (# s2#, frozen# #) ->
        (# s2#, ByteArray 0 (I# len#) frozen# #) }
    \end{code}