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

[Douglas Gregor]

> On Jul 5, 2016, at 5:00 PM, Ben Rimmington <me at benrimmington.com> wrote:
> 
> <https://github.com/apple/swift-evolution/blob/master/proposals/0112-nserror-bridging.md>
> 
> The new protocols could be combined into a single CustomNSError protocol.
> This would mirror the NSError class APIs, which are being customized.

Why is that good? The two primary protocols—LocalizedError and RecoverableError—provide a more focused, easy-to-understand experience for opting in to specific behavior. CustomNSError is a fallback for “I want to do something special with the generated NSError”.

> 
> Instead of using NSError.setUserInfoValueProvider(forDomain:provider:) 
> could you wrap the CustomNSError value inside an NSError subclass?
> 
> 	class _CustomNSError: NSError {
> 
> 	    private let _error: CustomNSError
> 
> 	    init(_error: CustomNSError) {
> 	        self._error = _error
> 	        super.init(
> 	            domain:   _error.dynamicType.errorDomain,
> 	            code:     _error.errorCode,
> 	            userInfo: _error.errorUserInfo)
> 	    }
> 
> 	    override var localizedDescription: String {
> 	        return _error.errorDescription ?? super.localizedDescription
> 	    }
> 
> 	    override var localizedFailureReason: String? {
> 	        return _error.failureReason ?? super.localizedFailureReason
> 	    }
> 
> 	    override var localizedRecoverySuggestion: String? {
> 	        return _error.recoverySuggestion ?? super.localizedRecoverySuggestion
> 	    }
> 
> 	    override var localizedRecoveryOptions: [String]? {
> 	        return _error.recoveryOptions ?? super.localizedRecoveryOptions
> 	    }
> 
> 	    override var recoveryAttempter: AnyObject? {
> 	        if _error.recoveryOptions != nil {
> 	            return _NSErrorRecoveryAttempter(error: _error)
> 	        } else {
> 	            return super.recoveryAttempter
> 	        }
> 	    }
> 
> 	    override var helpAnchor: String? {
> 	        return _error.helpAnchor ?? super.helpAnchor
> 	    }
> 	}

We could, but why? This is precisely what user-info value providers were designed for.

	- Doug