[swift-users] dealing with heterogenous lists/dictionary with Codable

[Kevin Nattinger]

So, what I see as the problem here is contradictory compiler statements:

MyPlayground.playground:5:9: note: cannot automatically synthesize 'Encodable' because '[String : Any]' does not conform to 'Encodable'
    let someDict: [String : Any]
        ^

warning: MyPlayground.playground:8:18: warning: 'is' test is always true
[String : Any]() is Codable
                 ^

So is it codable or isn't it? You can use a [String: Any] in encode() and decode() directly, so why doesn't it get compiler support? At the least, the message needs to be updated to something that's actually true. 

> On Oct 19, 2017, at 1:38 PM, David Sweeris via swift-users <swift-users at swift.org> wrote:
> 
> Oh! Yeah, my bad... You are correct; I'd started thinking like I was on -evolution instead of -users.
> 
> 
>> On Oct 19, 2017, at 1:29 PM, Itai Ferber <iferber at apple.com <mailto:iferber at apple.com>> wrote:
>> 
>> Mm, the point I’m trying to get at here is that JSON is inherently untyped while Swift is strongly typed, and the two don’t line up.
>> It doesn’t really make sense to ask to decode an existential because there’s not enough type information to influence what you get back.
>> 
>> On 19 Oct 2017, at 13:20, David Sweeris wrote:
>> 
>> I think if you can figure that out, you’re halfway to letting protocols conform to themselves.
>> 
>> (Syntactically, I would probably say that something like “Codable.Self” would read well, but I think that already means something. Maybe the answer will become clearer when we rework the reflection APIs?)
>> 
>> Sent from my iPhone
>> 
>> On Oct 19, 2017, at 13:13, Itai Ferber <iferber at apple.com <mailto:iferber at apple.com>> wrote:
>> 
>>> Even then, that wouldn’t necessarily help in the general case. If you decode {"key" : 1} as [String : Codable], what concrete type would 1 have? Int? Double? Int8? (Arguments can be made for any one of these, but the key here is that it is inherently ambiguous and there isn’t necessarily a good answer.)
>>> 
>>> On 19 Oct 2017, at 12:57, David Sweeris wrote:
>>> 
>>> On Oct 19, 2017, at 12:50 PM, David Baraff via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>>> 
>>> Yes; this is a case where anywhere in the code base I want to just say
>>> struct MyNewType : Codable {
>>> // add codable datatypes
>>> }
>>> 
>>> and don’t want/can’t always go to the centralized place to add it in.
>>> Is there some extension-like trick I can pull off that lets me spread the implementation out over different files/libraries?
>>> 
>>> Ah, ok.
>>> 
>>> No, I don't think you'll be able to do that until/unless Swift gets more macro/metaprogramming features. Maybe if protocols ever get to conform to themselves? That's a common request, but implementing it is apparently beyond tricky. I'm pretty sure somebody's working on it, but "bigger fish" and all that...
>>> 
>>> - Dave Sweeris
>>> 
>> 
> 
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