# Straightforward conversion from Int <-> Word

Simon Marlow simonmar@microsoft.com
Tue, 26 Feb 2002 09:56:31 -0000

```> -----Original Message-----
> From: Jan-Willem Maessen [mailto:jmaessen@alum.mit.edu]=20
> Sent: 25 February 2002 16:46
> Subject: Straightforward conversion from Int <-> Word
>=20
>=20
> I've cast about a bit and haven't been able to find the appropriate
> functionality, so I'm asking here.
>=20
> I'd like to convert Int32 to and from Word32 in the good old-fashioned
> bitwise fashion (perserving modular arithmetic).  I have the following
> code to show what I mean:
>=20
>   intToNat i
>     | i >=3D 0    =3D fromIntegral i
>     | otherwise =3D fromIntegral (i - minBound) + negBound
>     where negBound =3D fromInteger . negate . toInteger \$=20
> (minBound :: Int)
>=20
> One might think that "intToWord32" and "word32ToInt" in lang/Word
> would do what I want---but no!  These are depracated, and I must use
> fromIntegral, which will only perform the conversion correctly for the
> portions of the numeric range which overlap.
>=20
> How do I do the straightforward conversion module 2^32?  I'm sure it's
> buried in there somewhere...  I can't even turn up an appropriate type
> signature, though.

But fromIntegral *does* do the right thing, doesn't it?

> Numeric.showHex (fromIntegral (-1 :: Int32) :: Word32) ""
"0xffffffff"

it does it by a kind of roundabout route, but this is the defined =
behaviour (at least in GHC).  And if you turn optimisation on, the =
fromIntegral should reduce to a simple cast.

Cheers,
Simon

```