当数组在Swift中是Equatable时,为什么数组不符合Equatable？

Question

更新:

从包含Swift 4.1的Xcode 9.3开始,数组相等性按预期工作,原始问题中的代码编译时没有错误.

但是,请参阅接受的答案,因为它提供了更好,更现代的解决方案.

原始问题如下:

---

当我尝试用类型声明一个通用枚举的实例时[Post],我得到一个错误说

类型'[Post]'不符合协议'Equatable'

这是无稽之谈,因为Post符合Equatable并且我实际上可以比较两个[Post]没有编译错误的实例？

---

在下面的例子,我延长Post和Result<T>类型的Equatable,然后我做了一些测试:

1. 测试我可以比较两种Post类型:好的
2. 测试我可以比较两种[Post]类型:好的
3. 测试我可以比较两种Result<Post>类型:好的
4. 测试我可以比较两种Result<[Post]>类型:ERROR

import Foundation

struct Post {
    let text: String
}
extension Post: Equatable {}
func ==(lhs: Post, rhs: Post) -> Bool {
    return lhs.text == rhs.text
}


enum Result<T: Equatable> {
    case success(result: T)
    case error
}
extension Result: Equatable {
    static func ==(lhs: Result<T>, rhs: Result<T>) -> Bool {
        switch (lhs, rhs) {
        case let (.success(lhsVal), .success(rhsVal)):
            return lhsVal == rhsVal
        case (.error, .error):
            return true
        default:
            return false
    }
}

func test() {

    // Test 1: Check Post type for equality: OK
    let post1: Post = Post(text: "post 1")
    let post2: Post = Post(text: "post 2")

    if post1 == post2 {
        print("equal posts")
    }

    // Test 2: Check [Post] type for equality: OK
    let arrayOfPosts1: [Post] = [ post1, post2 ]
    let arrayOfPosts2: [Post] = [ post1, post2 ]

    if arrayOfPosts1 == arrayOfPosts2 {
        print("equal arrays of post")
    }

    // Test 3: Check Result<Post> type for equality: OK
    let result1: Result<Post> = Result<Post>.success(result: post1)
    let result2: Result<Post> = Result<Post>.success(result: post2)

    if result1 == result2 {
        print("equal results of post")
    }

    // Test 4: Check Result<[Post]> type for equality: ERROR
    // Compiler error: "Type '[Post]' does not conform to protocol 'Equatable'"
    let arrayResult1: Result<[Post]> = Result<[Post]>.success(result: arrayOfPosts1)
    let arrayResult2: Result<[Post]> = Result<[Post]>.success(result: arrayOfPosts2)

    if arrayResult1 == arrayResult2 {
        print("equal results of array of posts")
    }
}

Answer 1

Swift 4.1更新：

随着Swift 4.1中的条件一致性的引入，Array现在符合Equatable，因此无需使用任何变通办法即可解决问题。

而且，只要通过将一致性声明为原始类型定义（而不是扩展名）的一部分，而无需实现其任何要求，Swift现在允许类型自动合成Equatable一致性，前提是其所有成员均为。如果提供的关联值是，则此方法适用于枚举。EquatableEquatableEquatable

现在，可以更简洁地编写此问题的代码，如下所示：

import Foundation

struct Post: Equatable {
    let text: String
}

enum Result<T>: Equatable where T: Equatable {
    case success(result: T)
    case error
}

此代码将通过问题中指定的所有测试：

func test() {

    // Test 1: Check Post type for equality: OK
    let post1 = Post(text: "post")
    let post2 = Post(text: "post")

    if post1 == post2 {
        print("equal posts")
    }

    // Test 2: Check [Post] type for equality: OK
    let arrayOfPosts1 = [post1, post2]
    let arrayOfPosts2 = [post1, post2]

    if arrayOfPosts1 == arrayOfPosts2 {
        print("equal arrays of post")
    }

    // Test 3: Check Result<Post> type for equality: OK
    let result1 = Result<Post>.success(result: post1)
    let result2 = Result<Post>.success(result: post2)

    if result1 == result2 {
        print("equal results of post")
    }

    // Test 4: Check Result<[Post]> type for equality: OK
    let arrayResult1: Result<[Post]> = Result<[Post]>.success(result: arrayOfPosts1)
    let arrayResult2: Result<[Post]> = Result<[Post]>.success(result: arrayOfPosts2)

    if arrayResult1 == arrayResult2 {
        print("equal results of array of posts")
    }
}

这是输出：

test()
/*
 prints:
 equal posts
 equal arrays of post
 equal results of post
 equal results of array of posts
*/