Proposal: Remove Num superclass of Bits

[Bas van Dijk]

On 15 October 2011 19:23, Johan Tibell <johan.tibell at gmail.com> wrote:
> On Sat, Oct 15, 2011 at 3:56 AM, Roman Leshchinskiy <rl at cse.unsw.edu.au>
> wrote:
>>
>> On 15/10/2011, at 11:11, Ian Lynagh wrote:
>>
>> > On Sat, Oct 15, 2011 at 11:56:27AM +0200, Bas van Dijk wrote:
>> >> On 15 October 2011 00:01, Ian Lynagh <igloo at earth.li> wrote:
>> >>> Removing the Num superclass of Bits was also mentioned, but that would
>> >>> need its own proposal.
>> >>
>> >> Ok, I would like to propose removing the Num superclass of Bits.
>> >
>> > Would we just remove default methods like
>> >    bit i = 1 `shiftL` i
>> >    x `testBit` i = (x .&. bit i) /= 0
>> > ?
>>
>> The Num superclass is only needed to be able to say 0 and 1. Perhaps we
>> should just add the methods zero and one to Bits?
>
> That sounds reasonable.
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We can also combine Gábor's and Roman's solutions to get both
portability and convenience:

    zero, one :: a

#ifdef __GLASGOW_HASKELL__
    default zero, one :: Num a => a
    zero = 0
    one  = 1
#endif

This way, not existing instances need to be adapted.

Another problem is that the default implementation of popCount uses a '-':

    popCount          :: a -> Int
    popCount = go 0
      where
        go !c 0 = c
        go c w = go (c+1) (w .&. w - 1)

Bas