> On 19. Apr 2017, at 01:48, Xiaodi Wu <[email protected]> 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 <[email protected]
> <mailto:[email protected]>> wrote:
>
>> On Apr 18, 2017, at 9:22 AM, Stephen Canon <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>>>
>>> On Apr 18, 2017, at 12:17 PM, Joe Groff <[email protected]
>>> <mailto:[email protected]>> wrote:
>>>
>>>
>>>> On Apr 17, 2017, at 5:56 PM, Xiaodi Wu via swift-evolution
>>>> <[email protected] <mailto:[email protected]>> 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
>>>> <[email protected] <mailto:[email protected]>> 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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