[swift-evolution] protocol-oriented integers (take 2)

[Brent Royal-Gordon]

> On Jan 30, 2017, at 11:25 AM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
> 
>> I mean that `OptionSet.RawValue` currently has to conform to
>> `BitwiseOperations`, 
> 
> Actually it doesn't.  You just have to implement these yourself in that
> case:
> 
>  extension OptionSet where Self.RawValue : BitwiseOperations {

Oh, I didn't realize it was implemented that way (and was going to stay that way). Thanks for the correction.

>> but would now need to conform to `BinaryInteger` (or a sub-protocol).
> 
> Does that limit you in some useful way?

Well, a type like `Data` could be usefully conformed to `BitwiseOperations`, which would permit its use as a variable-sized bit buffer, but conforming it to `BinaryInteger` would make no sense and might cause mis-conformances. (For instance, `BinaryInteger.Type.+` and the `+` operator that works on `RangeReplaceableCollection`s like `Data` are incompatible). You would instead have to use a big-int type, but it's apparently common for those to be implemented in ways that make bitwise operations slow.

However, unless I'm mistaken, I believe a `BitwiseOperations` protocol could be extracted from `BinaryInteger` later (right? Resilience permits you to add a new super-protocol and move some of the sub-protocol's requirements up into it?), so we can pick that up later.

-- 
Brent Royal-Gordon
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