expand advice on importing these modules
[packages/old-time.git] / Data / Word.hs
1 {-# OPTIONS_GHC -fno-implicit-prelude #-}
2 -----------------------------------------------------------------------------
3 -- |
4 -- Module : Data.Word
5 -- Copyright : (c) The University of Glasgow 2001
6 -- License : BSD-style (see the file libraries/base/LICENSE)
7 --
8 -- Maintainer : libraries@haskell.org
9 -- Stability : experimental
10 -- Portability : portable
11 --
12 -- Unsigned integer types.
13 --
14 -----------------------------------------------------------------------------
15
16 module Data.Word
17 (
18 -- * Unsigned integral types
19
20 Word,
21 Word8, Word16, Word32, Word64,
22
23 -- * Notes
24
25 -- $notes
26 ) where
27
28 #ifdef __GLASGOW_HASKELL__
29 import GHC.Word
30 #endif
31
32 #ifdef __HUGS__
33 import Hugs.Word
34 #endif
35
36 #ifdef __NHC__
37 import NHC.FFI (Word8, Word16, Word32, Word64)
38 import NHC.SizedTypes (Word8, Word16, Word32, Word64) -- instances of Bits
39 type Word = Word32
40 #endif
41
42 {- $notes
43
44 * All arithmetic is performed modulo 2^n, where n is the number of
45 bits in the type. One non-obvious consequence of this is that 'Prelude.negate'
46 should /not/ raise an error on negative arguments.
47
48 * For coercing between any two integer types, use
49 'Prelude.fromIntegral', which is specialized for all the
50 common cases so should be fast enough. Coercing word types to and
51 from integer types preserves representation, not sign.
52
53 * It would be very natural to add a type @Natural@ providing an unbounded
54 size unsigned integer, just as 'Prelude.Integer' provides unbounded
55 size signed integers. We do not do that yet since there is no demand
56 for it.
57
58 * The rules that hold for 'Prelude.Enum' instances over a bounded type
59 such as 'Prelude.Int' (see the section of the Haskell report dealing
60 with arithmetic sequences) also hold for the 'Prelude.Enum' instances
61 over the various 'Word' types defined here.
62
63 * Right and left shifts by amounts greater than or equal to the width
64 of the type result in a zero result. This is contrary to the
65 behaviour in C, which is undefined; a common interpretation is to
66 truncate the shift count to the width of the type, for example @1 \<\<
67 32 == 1@ in some C implementations.
68 -}