[swift-users] Confusing Error localizedDescription output

Jordan Rose jordan_rose at apple.com
Tue Feb 28 15:10:56 CST 2017

'localizedDescription' isn't a requirement of the Error protocol, which means it's not a customization point. You need to actually conform to LocalizedError and provide a valid 'errorDescription' instead.

(It's too bad that your second test works at all, since MyError doesn't conform to LocalizedError. Unfortunately NSError does conform to LocalizedError, and all Errors can be bridged to NSError.)

If you're interested, you can find more information about the design in its proposal, SE-0112 <https://github.com/apple/swift-evolution/blob/master/proposals/0112-nserror-bridging.md>.

Hope this helps,

> On Feb 28, 2017, at 12:54, Jon Shier via swift-users <swift-users at swift.org> wrote:
> Swifters:
> 	I have a custom error enum:
> enum MyError: Error {
>    case network(error: Error)
>    case json(error: Error)
>    case value(error: Error)
>    var localizedDescription: String {
>        switch self {
>        case let .network(error): return error.localizedDescription
>        case let .json(error): return error.localizedDescription
>        case let .value(error): return error.localizedDescription
>        }
>    }
> }
> However, the localizedDescription output is odd to me. When the value is treated as:
> Error:  po error.localizedDescription
> "The operation couldn’t be completed. (MyApp.MyError error 1.)”
> LocalizedError:  po (error as! LocalizedError).localizedDescription
> "The operation couldn’t be completed. (MyApp.MyError error 1.)”
> MyError: Error:  po (error as! MyError).localizedDescription
> "JSON could not be serialized because of error:\nThe data couldn’t be read because it isn’t in the correct format."
> Shouldn’t my implementation of localizedDescription take precedence in all cases here? (This is Xcode 8.3b3).
> Jon
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