[swift-evolution] [Pitch] consistent public access modifiers

Jordan Rose jordan_rose at apple.com
Wed Feb 15 20:13:24 CST 2017

> On Feb 13, 2017, at 09:28, James Froggatt via swift-evolution <swift-evolution at swift.org> wrote:
> Having loosely followed this discussion, the way I'm thinking of ‘closed’ is as a modifier which would let you switch over something from outside the module in which it is declared.
> From outside the declaring module:
> • A closed enum's cases can be exhaustively switched.
> • A closed protocol's conforming types can be exhaustively switched.
> • A closed class's subclasses can be exhaustively switched.
> If this is correct, I can't help but think ‘closed’ is describing something subtly different in each case - picking and choosing the ‘important’ relationship for each type, while protocols already have a subtyping relationship, and it sounds like there's possibility for enum subtyping in the future.
> I'd rather keep ‘open’ (and a potential future ‘closed’) purely describing the subtyping relationship, and have some other means of labelling conformance and cases as switchable.

This is definitely not correct. A "closed" enum's cases can be exhaustively switched from any module, but a non-open protocol's conforming types or non-open class's subclasses cannot be exhaustively switched from anywhere except the defining module (because there may be internal subclasses).


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