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.
Charles
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