[swift-evolution] [Pitch] Add Null struct to Foundation

[michael.peternell at gmx.at]

Just one question: If I have functions

json_encode(j: JSON) -> String
and
json_decode(j: String) -> JSON throws

what should be the type of JSON? What should '{"a":2,"b":null}' decode to? What should the type of the JSON null value be in Swift? I think String and String? and String??? are wrong in this case.

I'm not saying that I'm convinced that NSNull() is the best way to represent null in this case. I just want to explain the use case that I was thinking of.

-Michael

> Am 26.06.2016 um 19:53 schrieb David Rönnqvist via swift-evolution <swift-evolution at swift.org>:
> 
> I'm not convinced that Swift needs more than on way of representing the lack of a value. As far as I've understood (and remember), NSNull main reason to exist is that it's an actual object and won't for example terminate array literals. From what I've observed of people who are new to Objective-C, NSNull is a big surprise, both its general existence but also when to expect it (read check for NSNull to make sure one doesn't crash) and when not to.
> 
> The way I've imagined that the same problem would be solved in Swift is with an optional, optional value. That is: if a field in a response can either be set or not, that's an optional. If that field can either contain a value or the explicit lack of a value that's another optional:
> 
> let nickname: String?? = "Little Bobby Tables"
> 
> As I see it, this is both safer (requiring that the inner nil value is dealt with), serves as a documentation of when an explicit missing value is expected and when it's not, and is more consistent. 
> 
> I would still expect a newcomer to wonder why there is two question marks in some places, but I'd imagine that that explanation would feel more logical.
> 
> And it's (still) possible (at least in the latest Swift Playground) to safely unwrap both levels:
> 
> if case let name?? = nickname { }
> 
> - David
> 
> Sent from my iPad
> 
> On 24 Jun 2016, at 11:32, Brent Royal-Gordon via swift-evolution <swift-evolution at swift.org> wrote:
> 
>>> Not really. What is the type of Optional.none? `let empty = Optional.none` does not compile, it says "Generic parameter 'Wrapped' could not be inferred". NSNull() is a unique concrete value, and it's compatible with Objective C, NSObject and AnyObject. We could of course use `Optional<Int16>.none`, but someone else may use `Optional<AnyObject>.none` instead. The extra type information is just misleading in this case.
>> 
>> If you want a single, unique value, use `()`.
>> 
>> But I'm not sure why you wouldn't just make this member an Optional<Any> in the first place. Is there some reason that wouldn't be suitable?
>> 
>> -- 
>> Brent Royal-Gordon
>> Architechies
>> 
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