Hi Brent, I understand that SubC.cc as C does not work, that’s the case in my bikeshedding example I noticed that because the enum case is always known at compile time, there will be an error with informs you to downgrade SubC to one of its super-types case. Plus I forget about the RawRepresentable for enums while making this example completely - my bad.
I might not fully understand how sub-typing on value types should work, that’s way I’m looking forward to the document Matthew will write one day. :) Sub-typing enums to remove cases sounds also really interesting to me. :) Thank you for your feedback. -- Adrian Zubarev Sent with Airmail Am 12. Februar 2017 um 09:57:39, Brent Royal-Gordon ([email protected]) schrieb: > On Feb 11, 2017, at 2:25 AM, Adrian Zubarev via swift-evolution > <[email protected]> wrote: > > // Allowed because `C` is open, and open allows sub-typing, conforming > // and overriding to the client > enum SubC : C { > case cc > } There's a subtle mistake here. For a sum type, subtyping actually means *removing* cases, not *adding* cases. (Or it can mean changing some of the associated types into their subtypes, but that's a different story.) To understand why, think about casting between `C` and `SubC`. Specifically, imagine this upcast: SubC.cc as C What is this supposed to do? There is no `cc` case in `C`, so `SubC.cc` cannot be represented as a `C`. That means `SubC` is not a subtype of `C`. In fact, `SubC` would be a *supertype* of `C`—you would need to say `SubC.cc as? C`, but you could always say `C.c as SubC`. (The ultimate expression of this is that `Never`—which can be thought of as the subtype of all types—is an empty enum.) That's not to say that this is a feature we should add; rather, it's to say that the concept of inheritance doesn't really make any sense for `enum`s. Just something I noticed when I was reading this thread. I'll have more to say on the main topic, hopefully soon. -- Brent Royal-Gordon Architechies
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