对 Collection 扩展函数的调用不明确

Question

编译器告诉我

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

我不确定如何为声明添加更多上下文。我认为扩展声明会告诉函数它需要的关于集合中元素的一切。我在代码片段中留下了更多想法。查找标记为“!!! COMPILE ERROR !!!”的区域

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{ [=11=].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))" )
}

Answer 1

第一个扩展具有约束 Element.Element: Measurement，这意味着在闭包中，measurementCollection 是一个集合，其元素类型采用 Measurement 协议。

因此，为了从该集合的第二个扩展中调用 isEquals() 方法，必须将其定义为

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

另请注意，通常，测试

filter { ... }.count > 0

比

更有效率

contains(where: { ... })

对 Collection 扩展函数的调用不明确

Ambiguous call to Collection extension function

collections

associated-types

swift