Sun, 6 Apr 2003 14:07:48 +0100
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?
----- Original Message -----
From: "Glynn Clements" <email@example.com>
To: "Dominic Steinitz" <firstname.lastname@example.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
> bit i = 1 `shift` i
> x `testBit` i = (x .&. bit i) /= 0
> Glynn Clements <email@example.com>