> On Aug 19, 2017, at 7:41 AM, Matthew Johnson <matt...@anandabits.com> wrote: > > Regardless of which approach we take, it feels like something that needs to > be implicit for structs and enums where value semantics is trivially provable > by way of transitivity. When that is not the case we could require an > explicit `value` or `nonvalue` annotation (specific keywords subject to > bikeshedding of course).
There is no such thing as "trivially provable by way of transitivity". This type is comprised of only value types, and yet it has reference semantics: struct EntryRef { private var index: Int var entry: Entry { get { return entries[index] } set { entries[index] = newValue } } } This type is comprised of only reference types, and yet it has value semantics: struct OpaqueToken: Equatable { class Token {} private let token: Token static func == (lhs: OpaqueToken, rhs: OpaqueToken) -> Bool { return lhs.token === rhs.token } } I think it's better to have types explicitly declare that they have value semantics if they want to make that promise, and otherwise not have the compiler make any assumptions either way. Safety features should not be *guessing* that your code is safe. If you can somehow *prove* it safe, go ahead—but I don't see how that can work without a lot of manual annotations on bridged code. -- Brent Royal-Gordon Architechies
_______________________________________________ swift-evolution mailing list swift-evolution@swift.org https://lists.swift.org/mailman/listinfo/swift-evolution