> Unfortunately, the Cocoa APIs needed to display an error object to the user
> all take NSErrors, and the conversion from other ErrorTypes to NSErrors is
> not very good; using “as NSError” to convert a MyError will result in
> something that has all of the associated values removed, and the message
> displayed to the user will be a cryptic and not particularly user-friendly
> “MyError error 1” rather than something more descriptive.
I too badly want to be able to control the `userInfo` of an NSError, or at
least have casting to NSError and back round-trip nicely.
> One can define an “toNSError()” method on one’s own error type that will
> properly propagate the NSError’s userInfo dictionary such that it will be
> displayed in a more meaningful way:
This, however, seems like a recipe for boilerplate.
What I would like to see is this:
* `var userInfo: [String: AnyObject] { get }` is added to `ErrorType`, or
perhaps to a sub-protocol specifically for this purpose. There's a default
implementation which simply returns `[:]`.
* A type like this exists in the standard library, or perhaps the Foundation
overlay:
private class _NSSwiftError: NSError {
var swiftError: ErrorType
init(swiftError: ErrorType) {
self.swiftError = swiftError
super.init(domain: swiftError.domain, code:
swiftError.code, userInfo: swiftError.userInfo)
}
}
I don't know if it can actually be private, but it should be invisible to
developers, and considered an implementation detail. It might also make sense
to retrieve the `userInfo` from `swiftError` on demand—the exact implementation
isn't very important here.
* Casting a native Swift error enum to `NSError` actually constructs an
`_NSSwiftError`. (Bridged error code enums like `NSCocoaError` use `NSError` as
usual.)
* Casting an `_NSSwiftError` back to `ErrorType` or a native Swift `ErrorType`
enum pulls the original instance back out of `swiftError`, so all associated
objects are preserved. (Bridged error code enums just construct a new instance.)
This way, the only code you have to write is a `userInfo` implementation, and
even that only if you want to do something custom. There's no need to write a
bunch of code to construct an entire `NSError` instance, or to unpack the
`userInfo` back into a Swift enum.
--
Brent Royal-Gordon
Architechies
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