Test enum for equality in for … where clause

问题

How can I test for enum equality when a case has an associated value? A contrived example:

enum Status : Equatable {
    case success
    case failed(error: String)

    static func == (lhs: Status, rhs: Status) -> Bool {
        switch (lhs, rhs) {
        case (.success, .success), (.failed, .failed):
            return true
        default:
            return false
        }
    }
}

let statuses = [
    Status.success,
    .failed(error: "error 1"),
    .failed(error: "error 2"),
    .success
]

// Failed: Binary operator '==' cannot be applied to operands of type 'Status' and '_'
for s in statuses where s == .failed {
    print(s)
}

(I know I can test for s != .success but the actual enum has more cases so they are a hassle)

回答1:

You can use if case:

for status in statuses {
    if case .failed = status {
        ...
    }
}

But, unfortunately, you can't use case with the where clause of for loop.

---

In this case, though, because you've defined .failed to be equal to another regardless of what the error associated value was, you theoretically could do:

for status in statuses where status == .failed(error: "") {
    show("\(status)")
}

I'm not crazy about that pattern because (a) it's contingent upon the fact that .failed values are equal even if they have different error associated values; and (b) it results in code that is easily misunderstood.

来源：https://stackoverflow.com/questions/44099325/test-enum-for-equality-in-for-where-clause