为什么这段代码有歧义？

Question

我已将此扩展写入 SequenceType 以模仿 Python's collections.Counter。

let input = [
    "a", "a", "a", "a", "a",
    "b", "b", "b", "b",
    "c", "c", "c",
    "d", "d",
    "e"
]


let counts = input.countRepetitions()

print(counts) //expected result: ["a": 5 , "b" : 4, "c" : 3, "d" : 2, "e" : 1]

代码如下：

extension SequenceType where Self.Generator.Element : Hashable {
    func countRepetitions() -> [Self.Generator.Element : Int] {
        return self.reduce([Self.Generator.Element : Int]()) { dict, element in
            dict[key: element] = (dict[element] ?? 0) + 1
        }
    }
}

我收到以下错误：

Playground execution failed: OS X.playground:26:22: error: type of expression is ambiguous without more context
                return self.reduce([Self.Generator.Element : Int]()) { dict, element in
                                   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Answer 1

这似乎是用嵌套定义变量时的限制类型。虽然

let foo1: [Self.Generator.Element : Int] = [:]

在您的方法中编译，这不会：

let foo2 = [Self.Generator.Element : Int]()
//  error: type of expression is ambiguous without more context

作为解决方法，您可以定义类型别名：

typealias E = Self.Generator.Element
let foo3 = [E : Int]()

适用于您的情况：

extension SequenceType where Self.Generator.Element : Hashable {
    func countRepetitions() -> [Self.Generator.Element : Int] {
        typealias E = Self.Generator.Element
        return self.reduce([E : Int]()) { (dict, element) ->  [E : Int] in
            var dict = dict
            dict[element] = (dict[element] ?? 0) + 1
            return dict
        }
    }
}

(注意闭包参数是常量，所以你必须首先是可变副本。另外闭包必须 return 一个值。）

但实际上你可以避免问题并让编译器推断类型:

extension SequenceType where Self.Generator.Element : Hashable {
    func countRepetitions() -> [Self.Generator.Element : Int] {
        return self.reduce([:]) { (dict, element) in
            var dict = dict
            dict[element] = (dict[element] ?? 0) + 1
            return dict
        }
    }
}

另请注意，reduce 在每次迭代中都会创建一个新字典步。一个更有效的解决方案是

extension SequenceType where Generator.Element : Hashable {
    func countRepetitions() -> [Generator.Element : Int] {
        var dict: [Generator.Element: Int] = [:]
        self.forEach {
            dict[[=15=]] = (dict[[=15=]] ?? 0) + 1
        }
        return dict
    }
}

我也省略了（多余的）Self.。

为什么这段代码有歧义？

Why is this code ambiguous?

generics

ambiguous

swift

swift2