Proposal: Explicitly require "Data.Bits.bit (-1) == 0" property

[Edward Kmett]

Nothing forbids you from allowing negative bit positions in a data type,
for instance for fractional bits in a fixed position numeric type.

Consequently, I'm -1 on this proposal.

You can currently construct 0 several ways, e.g. clearBit (bit 0) 0 could
be use to supply a default for any such zeroBits member of the class.

-Edward


On Sun, Feb 16, 2014 at 5:14 AM, Herbert Valerio Riedel <hvr at gnu.org> wrote:

> Hello *,
>
> Right now, there seems to be no "defined" way to create a zero
> 'Bits'-value (w/o requiring also a 'Num' instance), that has all bits
> cleared without involving at least two operations from the 'Bits' class
> (e.g. `clearBit (bit 0) 0` or `let x = bit 0 in xor x x`).
>
> OTOH, introducing a new method 'class Bits a where bitZero :: a' seems
> overkill to me.
>
> However, "bit (-1)"[1] seems to result in just such a zero-value for all
> 'Bits' instances from base, so I'd hereby propose to simply document
> this as an expected property of 'bit', as well as the recommended way to
> introduce a zero-value (for when 'Num' is not available).
>
> Discussion period: 2 weeks
>
>  [1]: ...or more generally 'bit n == 0' for n<0, as it's usually
>       implemented as 'bit n = 1 shift n'
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