当数组是另一个数组的元素时，如何约束数组的元素类型

Question

我有一个阵列，在数组情况下，我想约束内部Array的元素类型。

// this doesn't work but should illustrate what I want. Is there
// any way to constrain the Element type for an Array when it's
// already declared as the type for an Array 
extension Array where Element == Array<Element2: SomeProtocol> { 
}

最终，我认为解决这个问题将解决以下编译错误

protocol Thingy {
    associatedtype MeasurementType: Measurement

    var topMeasurements: [[MeasurementType]] { get }
}

extension Thingy {
    func doStuff() {

        // COMPILE ERROR'[[Self.MeasurementType]]' is not convertible to 'Array<Array<Measurement>>'
        let compileError = self.topMeasurements.measurements(forUUIDs: ["A"])

        // Hack workaround
        // The fact that this works leads me to believe that the constraint in the
        // extension is inadequate. The extension wants [[Measurement]] and 
        // event though MeasurementType is a kind of Measurement the compiler won't 
        // allow it. The hope is that if we could write something like 
        // [[Element]] where Element: Measurement
        let measurements: [[Measurement]] = self.topMeasurements
        let thisWorks = measurements.doSomething(ids: ["A"])
        print(thisWorks)
    }
}

// I'm hoping that if I constrain the following that it'll fix my issue
// constrain this type ----------------------
//                                          |
//                                          \/
extension Array where Element == Array<Measurement> {
    func doSomething(ids: [String]) -> [Measurement] {
        // do something return some elements, 
        // I've removed any code that could cause confusion
        return []
    }
}

Answer 1

这更容易实现作为所有集合的扩展，而不仅仅是在数组上。我相信你想要的扩展是这样的：

extension Collection where Element: Collection, Element.Element: Measurement {
    func measurements(forUUIDs: [String]) -> [Measurement] {
        return []
    }
}

基本问题是Swift缺少更高级的类型，所以你不能扩展Array（因为那不是一个合适的类型）。但你可以扩展Collection（因为这是一个PAT）。