> On Mar 6, 2017, at 3:46 PM, Karl Wagner <[email protected]> wrote:
> Thanks very much for writing this up, it’s very interesting.
>
> The one thing which I think needs improvement is the Copyable protocol. I
> think that this is actually part of a larger problem in Swift, which is that
> we don’t expose enough information to generic code for it to operate safely.
> This goes back to people asking for a value-equivalent to the “class” or
> “AnyObject” magic protocols.
>
> For example, often you would like to wrap a Collection and keep some
> information about what it contains. In order to do that generically, you need
> to ensure a client can hand you an exclusive view of a collection’s contents,
> so you know they will not mutate underneath your feet.
>
> Currently, that means you need a RangeReplaceableCollection because it
> includes an empty initialiser which allows you to create a unique copy of the
> Collection’s contents. It’s a workaround, but it means we have this
> unnecessary protocol requirement in the standard library, and in the
> implementation, which is making copies that may not be required. If my
> CollectionWrapper is initialised with something which has ValueSemantics, I
> don’t need to create a whole new instance with equal contents to ensure
> exclusivity of those contents. If this was an Array, for example, I could
> simply assign it and it would ensure the underlying contiguous buffer is
> never mutated.
>
> struct CollectionWrapper<C: Collection> {
> var _underlying: C
>
> init(contentsOf other: C) where C: RangeReplaceableCollection {
> _underlying = C();
>
> _underlying.replaceSubrange(_underlying.startIndex..<_underlying.endIndex,
> with: other)
> }
> }
>
> // Would love to do this…
>
> extension CollectionWrapper {
> init(contentsOf other: C) where C: ValueSemantics {
> _underlying = other
> }
> }
>
> As the proposal notes (with the File example), these are semantic
> considerations which go beyond simple struct/class distinctions: structs may
> have reference semantics, and classes may have value semantics. Would it be
> possible to model Copyable/move-only in terms of two new protocols,
> ValueSemantics and ReferenceSemantics, with trivial types/non-trivial types
> given appropriate defaults (overridable by the programmer, such as for
> “File”)?
Class types have reference semantics and are still copyable; ownership is not
going to change that. More generally, I don't see how anything we can do in
ownership could ever do something like eliminate the possibility of a
reference-semantics collection from the language.
John.
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