[swift-evolution] [Pitch] consistent public access modifiers (value subtyping digression)

Jordan Rose jordan_rose at apple.com
Wed Feb 15 20:17:03 CST 2017

> On Feb 13, 2017, at 09:33, Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
>> On Feb 13, 2017, at 11:28 AM, 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.
> I am drafting a manifesto-style document regarding value subtyping which will make it clear how value subtypes fit into the picture.  This document covers the relationship of enum cases with value subtyping and will show clearly how enum cases are analogous to subclasses.

I'm not sure how it fits your document, but I suspect value subtyping is pretty much not at all a source-compatibility or binary-compatibility concern. I think the only reasonable implementation here would be to perform conversions (unidirectional or bidirectional?), which means that the subtyping is almost entirely a client-side feature and the only potential dynamic operation would be using 'as?' with a generic type. This is very different from protocols and classes, which perform dynamic dispatch to method implementations present on their subtypes.

That is, the only effect of making a type "non-open" with respect to value subtyping would be to cut down on potential costs of 'as?'.

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