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

[Martin R]

> On 19. Apr 2017, at 01:48, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> So, as I understand it, `Float.init(exactly: Double.pi) == nil`. I would expect NSNumber to behave similarly (a notion with which Martin disagrees, I guess). I don't see a test that shows whether NSNumber behaves or does not behave in that way.

At present they behave differently: 

    print(Float(exactly: Double.pi) as Any)
    // nil
    print(Float(exactly: NSNumber(value: Double.pi)) as Any)
    // Optional(3.14159274)

I realize that identical behavior would be logical and least surprising. My only concern was about cases like

    let num = ... // some NSNumber from a JSON deserialization
    let fval = Float(exactly: num)

where one cannot know how the number is represented internally and what precision it needs. But then one could use the truncating conversion or `.floatValue` instead.

> 
> 
> On Tue, Apr 18, 2017 at 11:43 AM, Philippe Hausler <phausler at apple.com <mailto:phausler at apple.com>> wrote:
> 
>> On Apr 18, 2017, at 9:22 AM, Stephen Canon <scanon at apple.com <mailto:scanon at apple.com>> wrote:
>> 
>>> 
>>> On Apr 18, 2017, at 12:17 PM, Joe Groff <jgroff at apple.com <mailto:jgroff at apple.com>> wrote:
>>> 
>>> 
>>>> On Apr 17, 2017, at 5:56 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org <mailto: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 <mailto: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 <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.
>>> 


Even with the updated code at https://github.com/phausler/swift/tree/safe_nsnumber

    print(Double(exactly: NSNumber(value: Int64(9000000000000000001))) as Any)
    // Optional(9e+18)

still succeeds, however the reason seems to be an error in the `init(exactly value: someIntegerType)` inititializers of Float/Double, I have submitted a bug report: https://bugs.swift.org/browse/SR-4634 <https://bugs.swift.org/browse/SR-4634>.


>>> (+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
> 
> Pretty much the same trick inside of CFNumber/NSNumber
> 

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