[swift-evolution] SE-0170: NSNumber bridging and Numeric types

[Stephen Canon]

> On Apr 18, 2017, at 12:17 PM, Joe Groff <jgroff at apple.com> wrote:
> 
> 
>> On Apr 17, 2017, at 5:56 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
>> 
>> It seems Float.init(exactly: NSNumber) has not been updated to behave similarly?
>> 
>> I would have to say, I would naively expect "exactly" to behave exactly as it says, exactly. I don't think it should be a synonym for Float(Double(exactly:)).
>> On Mon, Apr 17, 2017 at 19:24 Philippe Hausler via swift-evolution <swift-evolution at swift.org> wrote:
>> I posted my branch and fixed up the Double case to account for your concerns (with a few inspired unit tests to validate)
>> 
>> https://github.com/phausler/swift/tree/safe_nsnumber
>> 
>> There is a builtin assumption here though: it does presume that the swift’s representation of Double and Float are IEEE compliant. However that is a fairly reasonable assumption in the tests.
> 
> (+Steve Canon) What is the behavior of Float.init(exactly: Double)? NSNumber's behavior would ideally be consistent with that.

The implementation is essentially just:

	self.init(other)
	guard Double(self) == other else {
		return nil
	}

i.e. if the result is not equal to the source when round-tripped back to double (which is always exact), the result is nil.

– Steve