This is the thread that I mentioned before: Should we rename “class” when referring to protocol conformance?
-- Adrian Zubarev Sent with Airmail Am 21. Oktober 2016 um 17:12:37, Mike Kasianowicz via swift-evolution ([email protected]) schrieb: Just from an outside perspective, the class restriction seems to be there as a kludge for technical reasons... but that's neither here nor there. It is not so much to enforce a lack of identity - in the struct case, it would be to enforce copy-by-value semantics. I think the strongest argument I've got is, say, a serialization or caching framework where you want to enforce that something is entirely writeable via memory pointer or copyable. A value-type restriction would get us mostly there, albeit there would still be ways to break the contract. However, as noted in my previous email, I see a lot of possibilities for enums too - in that case the protocol somewhat acts as 'base type' without adding the complexity of a base type. I listed some of my examples in my previous email - I could elaborate if it helps. On Fri, Oct 21, 2016 at 9:51 AM, Karl Wagner <[email protected]> wrote: IIRC, the reason we have "class" there is for the optimiser, so it can optimise for the protocol being satisfied by a reference-counted type. Classes are semantically unique from values because they have identity, which is also something a protocol might want to codify. There may be some optimisation gains by requiring all conformers to be values, but I struggle to think of why you might want to codify that a conformer should not have identity. Personally I don't really like this asymmetry in the language either, and would support changes to make these two elements more explicit. For example, a magic "hasIdentity" protocol which is automatically satisfied only by classes, and moving the optimisation guides to usage site (e.g. when declaring a variable of type MyProto, I could declare it of type AnyClass<MyProto> or AnyValue<MyProto> instead, to annotate this specific instance as being refcountable or not, without making such optimisation hints part of the MyProto definition) - Karl On Oct 21, 2016 at 8:39 am, <Mike Kasianowicz via swift-evolution> wrote: Currently protocols can have the class constraint: protocol MyProtocol : class {} It would be (a) intuitive and (b) useful to allow such things as: protocol Model : struct {} or protocol Event : enum {} These types of restrictions can help prevent accidental anti-patterns or misuse of APIs. Seems simple and non-controversial... right? [Note: I'd like to see even more heavy-handed protocol restrictions in the future. For example, a protocol describing an enum with a common case, or a struct with no reference members. Great stuff for defensively coding APIs.] _______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution _______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
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