[swift-evolution] [Pitch] Enum with generic cases

[Austin Zheng]

I once put together a sketch for a proposal for GADTs. I wouldn't recommend
it be submitted for review today (or ever, really), but it's there if
anyone is interested.

https://github.com/austinzheng/swift-evolution/blob/az-gadt/proposals/XXXX-enums-as-gadts.md

Best,
Austin

On Mon, Apr 24, 2017 at 3:24 PM, Jaden Geller via swift-evolution <
swift-evolution at swift.org> wrote:

>
> On Apr 24, 2017, at 3:21 PM, Kevin Nattinger <swift at nattinger.net> wrote:
>
>
> On Apr 24, 2017, at 3:16 PM, Jaden Geller <jaden.geller at gmail.com> wrote:
>
>
> On Apr 24, 2017, at 2:38 PM, Kevin Nattinger <swift at nattinger.net> wrote:
>
>
> How can I improve your understanding?
>
>
> Given the enum I was using earlier:
>
> enum Thing {
>     case thingOne<T>(T)
>     case thingTwo<T>(T)
> }
>
> - Write a function that takes a thingOne<String> or thingTwo<Int> but
> nothing else.
>
>
> This isn’t possible since generic types introduced on cases are erased in
> the type of `Thing`.
>
> We can actually already achieve what you want by moving the generics onto
> the type itself, and this is already possible in Swift! No new features are
> necessary.
>
> ```
> enum Thing<T1, T2> {
>     case thingOne(T1)
>     case thingTwo(T2)
> }
>
> func test(_ x: Thing<String, Int>) {
>     switch x {
>     case .thingOne(let s):
>         print("The string has value \(s)!")
>     case .thingTwo(let i):
>         print("The int has value \(i)!")
>     }
> }
> ```
>
>
> Yes, that was almost exactly my original example. My understanding of the
> proposal is that it will remove that capability, which I find completely
> unacceptable.
>
>
> Hi Kevin,
>
> If that is what is being proposed, I agree that that is entirely
> unacceptable. I however did not understand the proposal to be removing that
> capability, but instead I understood that it was simply adding another.
> That is, `Thing<T1, T2>` would *still* be accepted. This feature is
> entirely additive.
>
> It would be great if the original proposal author could confirm this.
>
> Thanks,
> Jaden Geller
>
>
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>
>
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