[swift-dev] Rationalizing FloatingPoint conformance to Equatable

[Jonathan Hull]

As someone mentioned earlier, we are trying to square a circle here. We can’t have everything at once… we will have to prioritize.  I feel like the precedent in Swift is to prioritize safety/correctness with an option ignore safety and regain speed.

I think the 3 point solution I proposed is a good compromise that follows that precedent.  It does mean that there is, by default, a small performance hit for floats in generic contexts, but in exchange for that, we get increased correctness and safety.  This is the exact same tradeoff that Swift makes for optionals!  Any speed lost can be regained by providing a specific override for FloatingPoint that uses ‘&==‘.

For example, if someone wants to write a generic function that works both on Integer and FloatingPoint, then they would have to use the new protocol which would force them to correctly handle cases involving NaN.  If speed is super important in that particular case, then they can write overrides for the FloatingPoint case which uses &==, and for Equatable which uses ==.

Because Float’s Equatable conformance is just being depreciated (with a warning/fixit), authors have at least a version to decide whether speed or correctness (or hopefully both) is most important to them.

Thanks,
Jon

P.S. We really should not be comparing against the speed of algorithms which don’t correctly handle NaN. Let’s compare Apples to Apples.


> On Oct 25, 2017, at 6:36 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> On Wed, Oct 25, 2017 at 8:26 PM, Jonathan Hull <jhull at gbis.com <mailto:jhull at gbis.com>> wrote:
> > On Oct 25, 2017, at 9:01 AM, David Sweeris via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
> >
> > That said, I fully acknowledge that this is all above my pay grade (also I hadn't realized that the issue was as settled as it apparently is). If splitting the protocols is a no-go from the get go, I'll go back to trying to figure out a better way to handle it without doing that.
> 
> 
> I don’t think it is settled.  The issue that Xiaodi mentioned was a PartiallyEq protocol which still had a signature of (T,T)->Bool.  People just used that protocol instead of Equatable without taking into account the difference in behavior.  The signature of (T,T)->Bool? changes things because people are forced to deal with the optional.
> 
> Currently, I think we should do 3 things:
> 
> 1) Create a new protocol with a partial equivalence relation with signature of (T, T)->Bool? and automatically conform Equatable things to it
> 2) Depreciate Float, etc’s… Equatable conformance with a warning that it will eventually be removed (and conform Float, etc… to the partial equivalence protocol)
> 3) Provide an '&==‘ relation on Float, etc… (without a protocol) with the native Float IEEE comparison
> 
> I think this provides several benefits.  #3 allows pure speed when needed, but not in a generic context (and is appropriately scary to cause some thought).  #1 forces correct handling in generic contexts.  #2 gives people time to make the adjustment, but also eventually requires them to switch to using #1 or #3.
> 
> I think it will cause A LOT of currently incorrect code to be fixed.
> 
> One issue which occurred to me only recently, which I hadn't considered, renders my `&==` idea and all similar schemes untenable:
> 
> Useful algorithms can and are written which operate on both floating-point and integer numeric types. In fact, the whole point of laboriously designing `Numeric` as part of SE-0104 was to make it possible to do so. If IEEE comparison is relegated to `FloatingPoint` and the only operator remaining on `Numeric` is `==`, then not only will there be a mandatory performance hit, but currently correct algorithms can be broken with absolutely no way to express a fix.

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