Converting ErrorType to NSError loses associated objects

Question

In Swift 2.0 NSError conforms to the ErrorType protocol.

For a customly defined error, we can specify the associating object(s) for some cases, like below.

enum LifeError: ErrorType {
    case BeBorn
    case LostJob(job: String)
    case GetCaughtByWife(wife: String)
    ...
}

We can comfortably do the following:

do {
    try haveAffairWith(otherPerson)
} catch LifeError.GetCaughtByWife(let wife) {
    ...
}

However if we want it to pass into other places as an NSError, it loses its associating object information.

println("\(LifeError.GetCaughtByWife("Name") as NSError)")

prints:

Error Domain=... Code=1 "The operation couldn't be completed". (... error 1)

and its userInfo is nil.

Where is my wife associated with the ErrorType?

Answer 1

New in Xcode 8: CustomNSError protocol.

enum LifeError: CustomNSError {
    case beBorn
    case lostJob(job: String)
    case getCaughtByWife(wife: String)

    static var errorDomain: String {
        return "LifeError"
    }

    var errorCode: Int {
        switch self {
        case .beBorn:
            return 0
        case .lostJob(_):
            return 1
        case .getCaughtByWife(_):
            return 2
        }
    }

    var errorUserInfo: [String : AnyObject] {
        switch self {
        case .beBorn:
            return [:]
        case .lostJob(let job):
            return ["Job": job]
        case .getCaughtByWife(let wife):
            return ["Wife": wife]
        }
    }
}