[swift-evolution] ValueEnumerable protocol with derived implementation for enums

[John McCall]

> On Apr 22, 2016, at 11:48 PM, Jacob Bandes-Storch <jtbandes at gmail.com> wrote:
> On Fri, Apr 22, 2016 at 11:34 PM, John McCall <rjmccall at apple.com <mailto:rjmccall at apple.com>> wrote:
>> On Apr 22, 2016, at 11:11 PM, Jacob Bandes-Storch <jtbandes at gmail.com <mailto:jtbandes at gmail.com>> wrote:
>> On Fri, Apr 22, 2016 at 10:50 PM, John McCall <rjmccall at apple.com <mailto:rjmccall at apple.com>> wrote:
>> 
>> I have not been following this discussion, but I would be extremely antsy about guaranteeing any particular representation for the set of values.  Guaranteeing a contiguous array implementation seems like a really bad idea, especially if that's taken to mean that we're going to actually provide a static global array.  But there's no way to avoid providing a public API, because a public conformance itself implies a public API with some level of corresponding overhead.
>> 
>> A "compiler magic" version of the feature, like #allValues(MyEnum), could generate a static array *internal* to the caller's module, but I'm sure there are implications or details of this which I'm not aware of.
> 
> That cannot be reconciled with resilience.  The caller cannot reliably know the set of stored cases; only the defining module can.
> 
> For similar reasons, only the defining module can be allowed to magically derive a conformance to your protocol, if indeed it's done with a protocol.
> 
> What should/can this mean for enums imported from Obj-C, via other modules, or bridging headers / custom module maps? In these cases, the defining module has no Swift code. Is there a way to do it with pre-existing metadata in the module?

Imported declarations are always something of a special case for this sort of thing.

It is possible to import an enum as "open", meaning that the library reserves the right to add new enumerators; it would never be possible to make such a type ValuesEnumerable, at least via the built-in magic.  But otherwise we could certainly give imported enums this conformance by default if we wanted to.

John.
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