> On Aug 5, 2016, at 4:32 PM, Charles Srstka via swift-evolution
> <[email protected]> wrote:
>
> MOTIVATION:
>
> SE-0112 includes the CustomNSError protocol, which includes the properties
> errorDomain, errorCode, and errorUserInfo. These properties can be used to
> tell Swift how to convert an error to an NSError. However, there are no
> default implementations for errorDomain and errorCode, and there is no way to
> access the default values for _domain and _code that Error enums get in
> Swift. Thus, even if all one wanted to do was to provide a value for
> NSURLErrorKey, one has to do all this:
>
> enum MyError: CustomNSError {
> case foo(URL)
> case bar(URL)
> case baz(URL)
>
> static var errorDomain: String {
> return “com.MyCompany.MyApp.MyError”
> }
>
> var errorCode: Int {
> switch self {
> case .foo(_):
> return 1
> case .bar(_):
> return 2
> case .baz(_):
> return 3
> }
> }
>
> var errorUserInfo: [String : NSObject] {
> // construct the actual user info
> }
> }
>
> Notice how far down you have to read before you finally get to the part that
> constructs the interesting information.
>
> PROPROSED SOLUTION:
>
> Add default implementations for all the properties in CustomNSError.
>
> DETAILED DESIGN:
>
> The implementations for errorCode and errorDomain will simply provide the
> default values of _code and _domain already provided by Swift enums. The
> default implementation for errorUserInfo will simply return an empty
> dictionary.
>
> This would allow the above enum to be written simply as:
>
> enum MyError: CustomNSError {
> case foo(URL)
> case bar(URL)
> case baz(URL)
>
> var errorUserInfo: [String : NSObject] {
> // construct the actual user info
> }
> }
>
> and the frameworks would provide something appropriate for the domain and
> code.
The core team discussed this and will take this as an amendment to SE-0112.
Charles, can you turn this into a pull request to update the SE-0112 document?
- Doug
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