swift - 对集合扩展功能的歧义调用

编译器告诉我

Expression type '([String]) -> Bool' is ambiguous without more context

我不知道如何在语句中添加更多上下文。我以为扩展声明会告诉函数它需要的关于集合中元素的所有信息。我在代码片段中留下了更多的想法。寻找标有“！!! 编译错误！!!"
您应该能够将代码复制并粘贴到空项目中，以查看编译错误。

protocol Measurement {
    var id: String { get }
    var displayPriority: Int { get }
}

extension Collection where Element: Collection, Element.Element: Measurement {
    func contains(ids: [String]) -> Bool {
        return self.filter{ measurementCollection -> Bool in
            // !!! COMPILE ERROR !!!
            // Error: Expression type '([String]) -> Bool' is ambiguous without more context
            //
            // The compiler doesn't under stand that this is a collection of Measurement,
            // Do I somehow need to tell the compiler that the elements to the collection
            // or collections with elements with a type that is a kind of Measurement again?
            // I thought the extension declaration already specified everything it needed.
            return measurementCollection.isEquals(ids: ids)
        }.count > 0
    }
}


extension Collection where Element == Measurement {
    func isEquals(ids: [String]) -> Bool {
        // get all the IDs
        let allIDs = self.map{ $0.id }

        // convert it to a set to make comparisons easier
        return Set(allIDs) == Set(ids)
    }
}

func whatIwantToDo() {
    let measurements = [[ConcreteMeasurement("A")],
                        [ConcreteMeasurement("B")],
                        [ConcreteMeasurement("C"), ConcreteMeasurement("D")]
    ]

    let ids = ["C", "D"]
    // Ultimately, I want to fix my compile error to run the following line of code, this should output to True
    print("Does measurements contain: \(ids) -> \(measurements.contains(ids: ids))" )
}

最佳答案

第一个扩展名具有约束条件Element.Element: Measurement，
这意味着在闭包中，measurementCollection是
元素类型采用Measurement协议。
因此，为了从这个集合的第二个扩展调用isEquals()方法，它必须定义为

extension Collection where Element: Measurement { // Not: Element == Measurement
    func isEquals(ids: [String]) -> Bool { ... }
}

还要注意，一般来说，测试

filter { ... }.count > 0

更有效的方法是

contains(where: { ... })