[swift-users] Decimal imported as NSDecimal not NSDecimalNumber in Swift 3 to Objective C

Mon Nov 14 17:02:42 CST 2016

Literals in Swift do not inherently have a type (at least conceptually).
They are simply literals.

The compiler will interpret them as whatever ExpressibleBy_____Literal type
is required to make the expression sensible.

There are default types which literals will become if no type information
is available: a string literal becomes String, a floating point literal
becomes Double, and so forth. But the literals themselves do not have a
type, and will become whatever type makes sense in context.

Nevin

On Mon, Nov 14, 2016 at 4:59 PM, Chris Anderson via swift-users <
swift-users at swift.org> wrote:

> Thanks for that follow up, I’m still a little confused at why one
> direction works and the other does not, but I’m getting there.
>
> I’ve found another issue I’ll bug report, but it’s along the same lines
> and wanted to run it by this thread. If I have an NSDecimalNumber, in
> Swift, and perform math on a literal value, (product) the code compiles. If
> I assign that value to a variable, or use any of the other Decimal/Double
> types, I cannot compile. I would expect a Double to not work, but I would
> expect ‘Decimal’ to work, in this case, as I’m not crossing the Objective C
> border. And, I’m confused how using the literal ‘2.0’ is interpreted as an
> NSDecimalNumber, and works in the ‘product’ stop, but I would expect the
> compiler to try and make it into a Double, as it does on the ‘test’
> variable.
>
>         let value = NSDecimalNumber(value: 2)
>         let test = 2.0                                               //
> double
>         let product = value.multiplying(by: 2.0)      // compiles
>         let x = value.multiplying(by: Decimal(2.0)) // does not compile
>         let y = value.multiplying(by: Double(2.0))   // does not compile
>         let z = value.multiplying(by: test)                // does not
> compile
>
> Best,
> Chris Anderson
>
> On Nov 11, 2016, at 6:07 PM, Philippe Hausler <phausler at apple.com> wrote:
>
>
> NSDecimal is not toll free bridged, but it does have a bridge to
> NSDecimalNumber.
>
> So take this for example:
>
> @objc class Exam: NSObject {
>         var grade: Double = 90.0
> }
>
> It would be reasonable to expect that is exposed in objc as:
>
> @interface Exam : NSObject
> @property double grade;
> @end
>
> and not:
>
> @interface Exam : NSObject
> @property NSNumber *grade;
> @end
>
> As it stands this is exposing as the structural type since that structural
> type comes from objective-c. Unlike String or Dictionary that have direct
> counterparts - NSDecimal and NSDecimalNumber both are sourced from the
> objective-c headers. That being said an API exposed in objc as returning a
> NSDecimalNumber should be exposed into swift as returning a Decimal (the
> struct NSDecimal). So if Exam was implemented in objc as such:
>
> @interface Exam : NSObject
> @property NSDecimalNumber *grade;
> @end
>
> that should be imported into swift as:
>
> class Exam : NSObject {
> var grade : Decimal
> }
>
> On Nov 11, 2016, at 2:58 PM, Adam C. Lickel via swift-users <
> swift-users at swift.org> wrote:
>
> NSDecimal has toll-free bridging with NSDecimalNumber so you can still do
> as casting when talking to an Objective-C API.
>
> On Nov 11, 2016, at 2:56 PM, Chris Anderson via swift-users <
> swift-users at swift.org> wrote:
>
> Sure thing. Yeah, ideally the bridging would be fixed, but at the least,
> correcting the documentation will be a good start. Will file, thanks.
>
> Best,
> Chris Anderson
>
> On Nov 11, 2016, at 5:55 PM, Tony Parker <anthony.parker at apple.com> wrote:
>
> Hi Chris,
>
> Can you file a radar or JIRA for us on this? It looks like something
> should be fixed in the documentation at least, or perhaps in the bridging.
>
> - Tony
>
> On Nov 11, 2016, at 1:46 PM, Chris Anderson via swift-users <
> swift-users at swift.org> wrote:
>
> I'm having problems with the type conversion between a Swift `Decimal` and
> an Objective C `NSDecimalNumber`.
>
> If I have the Swift class:
>
>     @objc class Exam: NSObject {
>         var grade: Decimal = 90.0
>     }
>
> And try to use that Swift class in Objective C,
>
>     Exam *exam = [[Exam alloc] init];
>     NSDecimalNumber *result = [[NSDecimalNumber zero]
> decimalNumberByAdding:grade.value];
>
> I get the error:
>
> Sending 'NSDecimal' to parameter of incompatible type 'NSDecimalNumber *
> _Nonnull'
>
> as it seems like `grade` is being treated as an `NSDecimal` not an
> `NSDecimalNumber`. This seems incorrect as per
> https://developer.apple.com/reference/foundation/nsdecimalnumber it says
>
> "The Swift overlay to the Foundation framework provides the Decimal
> structure, which bridges to the NSDecimalNumber class. The Decimal value
> type offers the same functionality as the NSDecimalNumber reference type,
> and the two can be used interchangeably in Swift code that interacts with
> Objective-C APIs. This behavior is similar to how Swift bridges standard
> string, numeric, and collection types to their corresponding Foundation
> classes."
>
> So I'm not sure if 1) I'm doing something wrong. 2) there's an error in
> the documentation or 3) this is a Swift bug. Number 1 on that list is
> definitely the most likely, but I wanted to see what I’m missing here.
>
> I don't want to explicitly make the values in my Swift class
> `NSDecimalNumber` because then I cannot do simple arithmetic operations
> such as `+` without doing the whole ugly `decimalNumberByAdding` dance.
>
> Thanks for the help!
>
> Best,
> Chris Anderson
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