[swift-users] Decode a JSON object of unknown format into a Dictionary with Decodable in Swift 4

[Itai Ferber]

Hi Kevin,

> On Jun 29, 2017, at 12:30 PM, Kevin Wooten via swift-users <swift-users at swift.org> wrote:
> 
>> Hi Jon,
>> 
>> I just joined this mailing list and have tried to catch up on the 
>> history of this thread, so please excuse me if I’ve missed something.
>> 
>> I’m sorry the Codable API at the moment does not answer your needs — 
>> you’re clearly not the only one who’s run into this, so let’s see 
>> how we can work together to make the API better for everyone.
>> For one thing, in the case of grabbing a subtree of JSON as 
>> "unevaluated" or "unmapped" (as it appears to be in the metadata case), 
>> it should be fairly simple to add a `JSONDecoder.UnevaluatedJSON` type 
>> that will allow you to essentially decode that part of the tree as an 
>> `Any`. `JSONDecoder` would have knowledge of this type and would be able 
>> to return the subtree inside of it — you’d decode a property as 
>> `JSONDecoder.UnevaluatedJSON.self` and access the contents through `var 
>> value: Any?`, or something similar. This would be simple additive API, 
>> which although might not make it in the upcoming betas, should be fairly 
>> simple introduce. Would this solve that use case?
>> 
>> We’re also working on improving `NSISO8601DateFormatter`. I don’t 
>> think I saw it in any of your emails — what specific use case are you 
>> looking for that it doesn’t at the moment support?
>> 
>> — Itai
> 
> 
> Itai, 
> 
> Is this a formal solution that is going to be implemented? This would solve just about every issue I currently have with Decodable.
I can’t make any promises at the moment — we’ve got a lot of high-priority things to fix before the Swift 4.0 release. However, this is something I’d certainly like to put through API review and eventually release, since this is clearly something that would be beneficial to a lot of our users.

> Two points…
> 
> 1) Putting it on `JSONDecoder` seems dubious since you’d only have access to `Decoder` (although conditional casting could solve that). It seems adding the method to `Decoder` and using `Decoder.Unevaluated.self` as the requested type, would be more useful. A user could then conditionally cast that value to things like `[String: Any]` and possibly use its contents generically.
Putting that on Decoder would require all Decoders to have an "unevaluated type" representation, which may not be appropriate for all formats.
Since this is very often a request when working with 3rd-party APIs which you don’t control (and are rarely offered in more than one format, if that), putting this directly on JSONDecoder seems reasonable — you’d only really expect this representation if you’re decoding from JSON; if you’re encoding to/from a different format, you’re likely in control of the data in those formats.

> 2) Matching it with an equivalent on `Encoder` would be great as well.  We take in JSON that has “metaData” like one aforementioned exampled. We then have to send back the equivalent metadata during a subsequent update; without ever inspecting or altering the unevaluated data. Being able encode a `Decoder.Unevaluated` would solve that problem as well.
Yes, we’d add an equivalent type on encode as well.

> 
> 
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