{-# LANGUAGE Safe #-}
{-# LANGUAGE CPP, NoImplicitPrelude #-}

-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Word
-- Copyright   :  (c) The University of Glasgow 2001
-- License     :  BSD-style (see the file libraries/base/LICENSE)
-- 
-- Maintainer  :  libraries@haskell.org
-- Stability   :  experimental
-- Portability :  portable
--
-- Unsigned integer types.
--
-----------------------------------------------------------------------------

module Data.Word
  (
        -- * Unsigned integral types

        Word,
        Word8, Word16, Word32, Word64,

        -- * Notes

        -- $notes
        ) where

#ifdef __GLASGOW_HASKELL__
import GHC.Word
#endif

#ifdef __HUGS__
import Hugs.Word
#endif

#ifdef __NHC__
import NHC.FFI (Word8, Word16, Word32, Word64)
import NHC.SizedTypes (Word8, Word16, Word32, Word64)   -- instances of Bits
type Word = Word32
#endif

{- $notes

* All arithmetic is performed modulo 2^n, where n is the number of
  bits in the type.  One non-obvious consequence of this is that 'Prelude.negate'
  should /not/ raise an error on negative arguments.

* For coercing between any two integer types, use
  'Prelude.fromIntegral', which is specialized for all the
  common cases so should be fast enough.  Coercing word types to and
  from integer types preserves representation, not sign.

* It would be very natural to add a type @Natural@ providing an unbounded 
  size unsigned integer, just as 'Prelude.Integer' provides unbounded
  size signed integers.  We do not do that yet since there is no demand
  for it.

* The rules that hold for 'Prelude.Enum' instances over a bounded type
  such as 'Prelude.Int' (see the section of the Haskell report dealing
  with arithmetic sequences) also hold for the 'Prelude.Enum' instances
  over the various 'Word' types defined here.

* Right and left shifts by amounts greater than or equal to the width
  of the type result in a zero result.  This is contrary to the
  behaviour in C, which is undefined; a common interpretation is to
  truncate the shift count to the width of the type, for example @1 \<\<
  32 == 1@ in some C implementations. 
-}

