Bits Problem

[Dominic Steinitz]

Is this a recent change? I've downloaded Ian Lynagh's DES module and it
doesn't compile because of this. I assume it used to. I don't see the
rationale for this. I can see Bool can be an instance of Bit but why does it
need to be an instance of Num?

Dominic Steinitz
----- Original Message -----
From: "Glynn Clements" <glynn.clements@virgin.net>
To: "Dominic Steinitz" <dominic.steinitz@blueyonder.co.uk>
Cc: <haskell@haskell.org>
Sent: Sunday, April 06, 2003 2:35 PM
Subject: Re: Bits Problem


>
> Dominic Steinitz wrote:
>
> > Can anyone explain this? Hugs doesn't complain.
> >
> > Prelude> :set --version
> > The Glorious Glasgow Haskell Compilation System, version 5.04.1
> >
> > test.hs:5:
> >     No instance for (Num Bool)
> >     arising from the instance declaration at test.hs:5
> >     In the instance declaration for `Bits Bool'
> >
> > module Main(main) where
> >
> > import Bits
> >
> > instance Bits Bool where
> >    complement False = True
> >    complement True  = False
>
> GHC's definition of Bits requires that instances of Bits are also
> instances of Num.
>
> This constraint is required for the default implementations of bit and
> testBit:
>
>     bit i               = 1 `shift` i
>     x `testBit` i       = (x .&. bit i) /= 0
>
> --
> Glynn Clements <glynn.clements@virgin.net>