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

我有一个数组，在数组的情况下，我想约束内部数组的元素类型。

// 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，我认为解决此问题将修复以下编译错误

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

这更容易实现为所有集合上的扩展，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 缺少高级类型，所以你不能扩展Array(因为这不是一个合适的类型(。但是您可以扩展Collection(因为这是 PAT(。