> On May 4, 2016, at 5:50 PM, Dave Abrahams via swift-evolution > <[email protected]> wrote: > > > on Wed May 04 2016, Matthew Johnson <[email protected] > <mailto:[email protected]>> wrote: > >>> On May 4, 2016, at 1:29 PM, Dave Abrahams via swift-evolution >>> <[email protected]> wrote: >>> >>> >>> on Wed May 04 2016, Adrian Zubarev <[email protected]> wrote: >>> >> >>>> Not sure what to think about the enum cases inside a protocol (if AnyEnum >>>> would >>>> even exist), it could be a nice addition to the language, but this is an >>>> own >>>> proposal I guess. >>>> >>>> We should start by adding AnyValue protocol to which all value types >>>> conforms. >>> >>> Having a way to constrain conformance to things with value semantics is >>> something I've long wanted. *However*, the approach described is too >>> simplistic. It's possible to build classes whose instances have value >>> semantics (just make them immutable) and it's possible to build structs >>> whose instances have reference semantics (just put the struct's storage >>> in a mutable class instance that it holds as a property, and don't do >>> copy-on-write). >>> >>> In order for something like AnyValue to have meaning, we need to impose >>> greater order. After thinking through many approaches over the years, I >>> have arrived at the (admittedly rather drastic) opinion that the >>> language should effectively outlaw the creation of structs and enums >>> that don't have value semantics. (I have no problem with the idea that >>> immutable classes that want to act as values should be wrapped in a >>> struct). The language could then do lots of things much more >>> intelligently, such as correctly generating implementations of equality. >> >> That is a drastic solution indeed! How would this impact things like >> Array<UIView>? While Array itself has value semantics, the aggregate >> obviously does not as it contains references which usually be mutated >> underneath us. > > Value semantics and mutation can only be measured with respect to > equality. The definition of == for all class types would be equivalent > to ===. Problem solved. > >> Similar considerations apply to simpler wrapper structs such as Weak. > > Same answer.
Hmm. If those qualify as “value semantic” then what kind of structs and enums would not? A struct wrapping a mutable reference type certainly doesn’t “feel” value semantic to me and certainly doesn’t have the guarantees usually associated with value semantics (won’t mutate behind your back, thread safe, etc). > >> My expectation is a generic aggregate such as Array would have to >> conditionally conform to AnyValue only when Element also conforms to >> AnyValue. >> >> I’m also wondering how such a rule would be implemented while still >> allowing for CoW structs that *do* implement value semantics, but do >> so while using references internally. > > I am not talking about any kind of statically-enforceable rule, although > we could probably make warnings sophisticated enough to help with this. You said the you have arrived at the opinion that the language should “effectively outlaw” structs and enums that do not have value semantics. That sounded like static enforcement to me. Maybe you meant we should allow the compiler to assume value semantics for structs and enums despite the fact that it doesn’t statically enforce this? >> >> If the compiler can be sophisticated enough to verify value semantics >> statically maybe it would be better to have that mechanism be >> triggered by conformance to AnyValue rather than for all structs and >> enums. Types that conform to AnyValue would receive the benefits of >> the compiler knowing they have value semantics, while other uses of >> structs and enums would remain valid. Best practice would be to >> conform structs and enums to AnyValue whenever possible. >> >> Another possible advantage of this approach would be allowing >> immutable reference types to conform to AnyValue and receive the >> associated benefits such as the generated implementation of equality, >> etc. >> >> -Matthew >> >>> >>> >>> -- >>> Dave >>> >>> _______________________________________________ >>> swift-evolution mailing list >>> [email protected] >>> https://lists.swift.org/mailman/listinfo/swift-evolution >> >> _______________________________________________ >> swift-evolution mailing list >> [email protected] <mailto:[email protected]> >> https://lists.swift.org/mailman/listinfo/swift-evolution >> <https://lists.swift.org/mailman/listinfo/swift-evolution> > > -- > Dave > > _______________________________________________ > swift-evolution mailing list > [email protected] <mailto:[email protected]> > https://lists.swift.org/mailman/listinfo/swift-evolution > <https://lists.swift.org/mailman/listinfo/swift-evolution>
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