如何在 Bound 是通用的地方扩展 ClosedRange

Question

For some additional functionality I'd like to extend ClosedRange where the Bounds are Measurement .

extension ClosedRange where Bound == Measurement

…obviously isn't working as Measurement as a generic type requires an <> argument.

extension ClosedRange where Bound == Measurement<UnitType> // Cannot find type 'UnitType' in scope

Concatenating where s doesn't seem to work either.

I am out of ideas.

The only thing I found to be working is:

extension ClosedRange where Bound == Measurement<UnitLength> {

    func unit() -> UnitLength {
        return lowerBound.unit
    }

}

let range = Measurement(value: 5, unit: UnitLength.meters)...Measurement(value: 15, unit: UnitLength.meters)
range.unit().symbol // m

But I don't want to copy and paste this for all unit types. Isn't this what generics are there for after all?

This comes close to what I want, doesn't work though:

extension ClosedRange where Bound == Measurement<UnitType> { // Cannot find type 'UnitType' in scope

    func unit() -> UnitType { // Cannot find type 'UnitType' in scope
        return lowerBound.unit
    }

}

let range = Measurement(value: 5, unit: UnitLength.meters)...Measurement(value: 15, unit: UnitLength.meters)
range.unit

EDIT:

Thanks Dávid Pásztor for the answer. I would like to extend my question:

How can I add a function for Measurement s with UnitType s that inherit from Dimension ?

extension ClosedRange {

    func convertedLowerBound<UnitType>(to unit: UnitType) -> Double where Bound == Measurement<UnitType> {
        lowerBound.converted(to: unit).value // Referencing instance method 'converted(to:)' on 'Measurement' requires that 'UnitType' inherit from 'Dimension'
    }

}

let range = Measurement(value: 5, unit: UnitLength.meters)...Measurement(value: 15, unit: UnitLength.meters)
range.convertedLowerBound(to: .centimeters)

Referencing instance method 'converted(to:)' on 'Measurement' requires that 'UnitType' inherit from 'Dimension'

Yes, I could just call range.lowerBound.converted(to: .centimeters) . The exact functionality I plan to implement doesn't work like this though.

Answer 1

You need to make the extension itself generic, however that's not yet supported by Swift. You can make the function itself generic though.

extension ClosedRange {
    func unit<UnitType>() -> UnitType where Bound == Measurement<UnitType> {
        lowerBound.unit
    }

    func convertedLowerBound<UnitType: Dimension>(to unit: UnitType) -> Double where Bound == Measurement<UnitType> {
        lowerBound.converted(to: unit).value
    }
}