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

Jon Shier jon at jonshier.com
Sun Aug 14 19:04:43 CDT 2016

	Yes, if you return a single error type it works fine. However, since Alamofire wraps the underlying networking frameworks there’s a need to return the errors returned from them as well, which are now returned as Error. In order to let these errors be returned alongside Alamofire’s errors we’d either have to get rid of the generic error and just use Error or wrap all errors in our error type. So far the least painful method seems to be just getting rid of the generic error and returning Error for everything. That way users will use the same logic they would have had to use anyway and can also provide their own error wrapper if they want. Thankfully (or unfortunately, depending on your point of view), the new pattern is to cast out the various error types, so the previous concern about having to do that is no longer an issue.


> On Aug 14, 2016, at 4:18 AM, Charles Srstka <cocoadev at charlessoft.com> wrote:
>> On Aug 14, 2016, at 2:34 AM, Jon Shier <jon at jonshier.com <mailto:jon at jonshier.com>> wrote:
>> 	Sorry Charles, I should’ve been more specific. This isn’t about some other type named Error being usable, but about our doubly generic Result and other types being usable with Error as the generic error type. If we have Result defined as:
>> public enum Result<Value, ErrorType: Error> {
>>     case success(Value)
>>     case failure(ErrorType)
>> }
>> then attempting to create the type Result<Whatever, Error> results in the error message I posted. For methods that previously returned Result<Whatever, NSError>, where the NSError is either a system error or an Alamofire one, this is something of a problem. Of course, removing the generic error type fixes this but may have an undesirable usability impact, as making Result double generic was the primary reason behind the quick upgrade between Alamofire 2 and 3. I think it’s less of a problem now, as having to enumerate all of the different expected error types is how user’s are supposed to interact with Error in the first place, but at the time it wasn’t desirable to try and force everything through ErrorProtocol for that reason. Perhaps this is a good compromise, where instead of returning all NSErrors from the framework and allowing consumers to add their own to the mix we’ll just create our own AFError type (whether enum or struct) and then anything coming back will have to be checked for that type in addition to the system types. From my testing it looks like users could still wrap everything coming in by capturing the underlying Error.
> Still seems to be working well, unless I’m misunderstanding what you’re trying to do:
> import Foundation
> enum Result<Value, ErrorType: Error> {
>     case success(Value)
>     case failure(ErrorType)
> }
> struct MyThing {
>     enum Error: Swift.Error, LocalizedError {
>         case doesNotCompute
>         case imSorryDave
>         case mustSterilize
>         case irrelevant
>         case endOfLine
>         var failureReason: String? {
>             switch self {
>             case .doesNotCompute:
>                 return "Does Not Compute! Does Not Compute! Does Not Compute!"
>             case .imSorryDave:
>                 return "I'm sorry Dave, I'm afraid I can't do that."
>             case .mustSterilize:
>                 return "Error! Must Sterilize! Must Steeerrrrilllliiiiiiizzzzzzeeeeeee"
>             case .irrelevant:
>                 return "Irrelevant. Resistance is futile."
>             case .endOfLine:
>                 return "End of Line!"
>             }
>         }
>     }
>     func trySomething(shouldWork: Bool, completionHandler: (Result<String, Error>) -> ()) {
>         if shouldWork {
>             completionHandler(.success("It worked!"))
>         } else {
>             completionHandler(.failure(Error.imSorryDave))
>         }
>     }
> }
> let thing = MyThing()
> let completionHandler = { (result: Result<String, MyThing.Error>) in
>     switch result {
>     case let .success(value):
>         print("returned '\(value)'")
>     case let .failure(error):
>         print("error: \(error.localizedDescription)")
>     }
> }
> thing.trySomething(shouldWork: true, completionHandler: completionHandler)
> thing.trySomething(shouldWork: false, completionHandler: completionHandler)
> returned 'It worked!'
> error: The operation couldn’t be completed. I'm sorry Dave, I'm afraid I can't do that.
> Program ended with exit code: 0
> Charles

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