[swift-evolution] [Accepted] SE-0112: Improved NSError Bridging

[Kevin Ballard]

> On Aug 5, 2016, at 7:36 PM, Erica Sadun <erica at ericasadun.com> wrote:
> 
> On Aug 5, 2016, at 8:10 PM, Kevin Ballard <kevin at sb.org <mailto:kevin at sb.org>> wrote:
>> 
>>> 
>>> On Aug 5, 2016, at 5:16 PM, Erica Sadun <erica at ericasadun.com <mailto:erica at ericasadun.com>> wrote:
>>> 
>>> 
>>>> On Aug 5, 2016, at 4:19 PM, Douglas Gregor via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>>> 
>>>>> On Aug 5, 2016, at 12:59 PM, Kevin Ballard via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>>> 
>>>>> If all you want to do is get the localized description, then you can just say `(error as NSError).localizedDescription`.
>>>> 
>>>> Just ‘error.localizedDescription’ works now. That was part of SE-0112.
>>>> 
>>>> 	- Doug
>>> 
>>> Would it kill to allow:
>>> 
>>> let err = NSError()
>>> err.localizedDescription = "bad things happen"
>>> throw err
>>> 
>>> or even
>>> 
>>> throw NSError("Bad things happen")
>>> 
>>> for lightweight use? I ended up refactoring entirely to enum : Error because Swift yelled at me for using NSError(): "this results in an invalid NSError instance. It will raise an exception in a future release. Please call errorWithDomain:code:userInfo: or initWithDomain:code:userInfo:. This message shown only once."
>>> 
>>> enum Errors: Error {case bad}
>>> Errors.bad._code // 0
>>> Errors.bad._domain // "Errors"
>>> Errors.bad._userInfo // Optional({})
>>> Errors.bad.localizedDescription // "The operation couldn’t be completed. (Errors error 0.)"
>>> 
>>> Bleh.
>> 
>> NSErrors need a domain/code. It doesn’t make much sense to throw one without it. And besides, there’s a fairly trivial solution for doing what you want to do:
>> 
>> struct GenericError: LocalizedError {
>>     let message: String
>>     init(_ message: String) {
>>         self.message = message
>>     }
>>     var errorDescription: String? {
>>         return message
>>     }
>> }
>> 
>> Now you can just say `throw GenericError(“Bad things happen”)`.
>> 
>> -Kevin Ballard
> 
> I know I can build workarounds but if we're going to have the error.localizedDescription, making it an initializable/assignable property just seems like a nice thing™. Why can't we have nice things™?

I don’t actually think it’s a nice thing™ to have it be assignable like you ask, because we should be encouraging people to use typed errors. You may as well just ask for String to conform to Error (in fact, you could just add that conformance yourself and skip the GenericError wrapper entirely).

-Kevin
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