[swift-users] Confusing Error localizedDescription output

[Ole Begemann]

It's how protocols work in Swift.

errorDescription is a protocol requirement of LocalizedError (that is, 
it's part of the protocol definition, not just added in an extension). 
Only protocol requirements are dynamically dispatched.

Take a look at the implementation of Error.localizedDescription in the 
stdlib [1]:

public extension Error {
   /// Retrieve the localized description for this error.
   var localizedDescription: String {
     return (self as NSError).localizedDescription
   }
}

So Error.localizedDescription casts the value to NSError. And NSError's 
localizedDescription implementation presumably calls errorDescription 
(via dynamic dispatch), which ends up calling your code.

In your second example, Error.localizedDescription is _not_ a protocol 
requirement and therefore only statically dispatched. One (_the_?) 
reason for this discrepancy is that only methods defined in the original 
protocol definition get an entry in the protocol's witness table — since 
anybody can add extensions later, there's no good way for the compiler 
to reserve space for methods defined in extensions.

This time, NSError.localizedDescription doesn't dispatch to your 
implementation because it can't be reached via dynamic dispatch.

It's a confusing interaction between Swift's protocol dispatch rules and 
the automatic casting of Error to NSError, which then ends up calling 
LocalizedError.errorDescription.

[1]: 
https://github.com/apple/swift/blob/master/stdlib/public/SDK/Foundation/NSError.swift#L203-L208

On 28/02/2017 22:37, Jon Shier via swift-users wrote:
> Guh, second version should be this (Discourse save me).
>
> enum MyError: Error {
>
>     case network
>     case json
>     case value
>
>     var localizedDescription: String {
>         switch self {
>         case .network: return "first"
>         case .json: return "second"
>         case .value: return "third"
>         }
>     }
>
> }
>
> extension MyError: LocalizedError { }
>
> print((MyError.value as Error).localizedDescription)
>
>
>> On Feb 28, 2017, at 4:36 PM, Jon Shier <jon at jonshier.com
>> <mailto:jon at jonshier.com>> wrote:
>>
>> Something strange I just observed. This version of MyError properly
>> prints its localized description (in a Playground).
>>
>> enum MyError: Error {
>>
>>     case network
>>     case json
>>     case value
>>
>>     var localizedDescription: String {
>>         switch self {
>>         case .network: return "first"
>>         case .json: return "second"
>>         case .value: return "third"
>>         }
>>     }
>>
>>     var errorDescription: String? { return localizedDescription }
>>
>> }
>>
>> extension MyError: LocalizedError { }
>>
>> print((MyError.value as Error).localizedDescription): third
>>
>> However, this version does not:
>>
>> enum MyError: Error {
>>
>>     case network
>>     case json
>>     case value
>>
>>     var localizedDescription: String {
>>         switch self {
>>         case .network: return "first"
>>         case .json: return "second"
>>         case .value: return "third"
>>         }
>>     }
>>
>>     var errorDescription: String? { return localizedDescription }
>>
>> }
>>
>> extension MyError: LocalizedError { }
>>
>> print((MyError.value as Error).localizedDescription): The operation
>> couldn’t be completed. (__lldb_expr_103.MyError error 2.)
>>
>> Surely that’s a bug?
>>
>>
>>
>> Jon
>>
>>> On Feb 28, 2017, at 4:32 PM, Jon Shier <jon at jonshier.com
>>> <mailto:jon at jonshier.com>> wrote:
>>>
>>> Sorry, I forgot that part; MyError does conform to LocalizedError.
>>> However, even if it didn’t, Error has a localizedDescription
>>> property, so my question was why my implementation of that property
>>> wasn’t used in favor of the generated one. Or is that not the case
>>> for properties that aren’t required by the protocol? Even with the
>>> conformance, treating MyError as Error or LocalizedError doesn’t
>>> return the correct value for the property. localizedDescription seems
>>> to work fine when talking to Error instances backed by NSError
>>> though. This behavior doesn’t seem to be correct, otherwise there’s
>>> no way to get a useful description out of a custom Error without also
>>> implementing LocalizedError and treating the value as that type instead.
>>>
>>>
>>>
>>> Jon
>>>
>>>
>>>> On Feb 28, 2017, at 4:10 PM, Jordan Rose <jordan_rose at apple.com
>>>> <mailto:jordan_rose at apple.com>> wrote:
>>>>
>>>> '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,
>>>> Jordan
>>>>
>>>>
>>>>> On Feb 28, 2017, at 12:54, Jon Shier via swift-users
>>>>> <swift-users at swift.org <mailto: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
>>>>> _______________________________________________
>>>>> swift-users mailing list
>>>>> swift-users at swift.org <mailto:swift-users at swift.org>
>>>>> https://lists.swift.org/mailman/listinfo/swift-users