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

[Philippe Hausler]

> On Apr 19, 2017, at 3:23 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
> 
> On Wed, Apr 19, 2017 at 3:19 PM, Martin R <martinr448 at gmail.com <mailto:martinr448 at gmail.com>> wrote:
>> On 19. Apr 2017, at 01:48, Xiaodi Wu <xiaodi.wu at gmail.com <mailto: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.
> 
> JSON numbers are double-precision floating point, unless I'm misunderstanding something. If someone writes `Float(exactly: valueParsedFromJSON)`, surely, that can only mean that they *really, really* prefer nil over an imprecise value. I can see no other reason to insist on using both Float and .init(exactly:).

JSON does not claim 32 or 64 bit floating point, or for that matter 128 or infinite bit floating point :(

After thinking about it more; it seems reasonable to restrict it to the behavior of Float(exactly: Double(…)). I am certain this will probably in the end cause more bugs for me to have to address and mark as “behaves correctly” and confuse a few new developers - but in the end they chose Swift and the consistent story would be the current behavior of Float(exactly: Double).

> 
>> 
>> 
>> 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 <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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