[swift-evolution] [Pitch] Non-class type requirements on protocols (eg : struct, : enum)

Adrian Zubarev adrian.zubarev at devandartist.com
Fri Oct 21 11:01:51 CDT 2016

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 (swift-evolution at swift.org) 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 <razielim at gmail.com> 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 swift-evolution at swift.org https://lists.swift.org/mailman/listinfo/swift-evolution

swift-evolution mailing list
swift-evolution at swift.org
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20161021/30cff506/attachment.html>

More information about the swift-evolution mailing list