Hi Kevin,
You’re right — this is one of the limitations of the box design
here. One thing we can do is expose the underlying boxed value as a
`KeyedDecodingContainerProtocol` existential value using an accessor on
the box so you can down-cast.
However, it shouldn’t be necessary to add any methods to
`_JSONEncoder` or `_JSONDecoder` to support this. What are you trying to
do?
— Itai
On 1 Jul 2017, at 9:07, Kevin Wooten wrote:
Itai,
I tried copying JSONEncoder.swift from the swift repo and implementing
the `Unevaluated` type in it. It doesn’t appear to be a workable
solution due to the fact that the `KeyedEncodingContainer` type
erasure box is the only value returned from `container(keyedBy:)`.
This means that any extra methods implemented inside `JSONDecoder` (or
`_JSONDecoder` in this case) can ever be publicly accessed (even with
casting) unless the type erasure box also includes an equivalent
method.
How do you propose getting around this? Seems like the design of this
might be a bit limiting when considering extension/enhancement in user
specific code.
On Jun 29, 2017, at 12:47 PM, Itai Ferber <ifer...@apple.com> wrote:
Hi Kevin,
On Jun 29, 2017, at 12:30 PM, Kevin Wooten via swift-users
<swift-users@swift.org <mailto:swift-users@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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