[swift-evolution] [Proposal] Foundation Swift Encoders

Itai Ferber iferber at apple.com
Thu Mar 16 15:18:31 CDT 2017

Foundation’s common currency type for errors is unequivocally 
`NSError`, and we’re not looking to change that as part of this 
Reusing errors from the Cocoa domain has always been a reasonable choice 
for 3rd-party developers, and we’d like to keep doing this.

We are looking to improve the experience of throwing such errors (with 
some trivial extensions) by allowing you to do something like `throw 
CocoaError.error(.coderInvalidValue)`. (Of course, don’t quote me on 
the exact syntax, as this is totally subject to change.)
This is pretty simple, and nets you free user-facing localization.

On 15 Mar 2017, at 22:46, Zach Waldowski wrote:

> Just a little one that came up with when thinking over the companion 
> proposal… see inline.
>> On Mar 15, 2017, at 6:43 PM, Itai Ferber via swift-evolution 
>> <swift-evolution at swift.org> wrote:
>> Foundation-Provided Errors
>> Along with providing the above encoders and decoders, we would like 
>> to promote the use of a common set of error codes and messages across 
>> all new encoders and decoders. A common vocabulary of expected errors 
>> allows end-users to write code agnostic about the specific 
>> encoder/decoder implementation they are working with, whether 
>> first-party or third-party:
>> extension CocoaError.Code {
>>     public static var coderInvalidValue: CocoaError.Code
>>     public static var coderTypeMismatch: CocoaError.Code
>>     public static var coderReadCorrupt: CocoaError.Code
>>     public static var coderValueNotFound: CocoaError.Code
>> }
> I find all the reasons compelling, but I don’t with the overall 
> choice. A common vocabulary, side table info, and localization can be 
> achieved with a dedicated error type for coders. I understand ABI 
> (sorta) concerns are involved here, that older Foundations will 
> continue to return the same NSError codes. However, it's never felt 
> appropriate to create my own error instances with NSCocoaErrorDomain, 
> and this feels very similar to that. What are our options here?
> Zach Waldowski
> zach at waldowski.me

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