[swift-evolution] [Oversight] Reference types allow mutating methods through generics

Matthew Johnson matthew at anandabits.com
Wed May 4 13:32:48 CDT 2016

> On May 4, 2016, at 1:21 PM, David Sweeris via swift-evolution <swift-evolution at swift.org> wrote:
>> On May 4, 2016, at 11:12, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
>> I can see value in there being some kind of PureValue protocol, for types that represent fully self-contained values, but conforming to that protocol requires a bit more thought than just being a struct or enum, since there are structs that have reference semantics (such as UnsafePointer), and there are hybrid value types that contain references to data that isn't part of the value (an Array<Class>, for instance).
>> -Joe
> +1
> I'd think that both "PureValue" and "PureReference" would be useful. Isn't it the mixed types that make for tricky mutation rules & serialization?

I also like Joe’s idea.  It fits with the direction in Swift of attaching semantics, not just syntax, to protocols.

I was thinking of something pretty similar to your PureReference idea, but slightly different.  Pure / immutable references have value semantics.  With that in mind I was thinking of an ImmutableObject protocol which would inherit from both AnyObject and PureValue.

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