[Proposal] add is identical methods to concrete types

[vanvoorden]

Many types in Foundation are “copy-on-write” data structures. These types present as value types, but can leverage a reference to some shared state to optimize for performance. When we copy this value we copy a reference to shared storage. If we perform a mutation on a copy we can preserve value semantics by copying the storage reference to a unique value before we write our mutation: we “copy” on “write”.

This means that many types in Foundation already have some private reference that can be checked in constant-time to determine if two values are identical. Because these types copy before writing, two values that are identical by their shared storage must be equal by value.

[vanvoorden]

@parkera Would you have any opinions about running this proposal concurrently with our proposal to add these methods to concrete types from stdlib?¹ Can you think of examples in the past where one evolution proposal opened on stdlib and another proposal on Foundation covered similar designs but just focused on different concrete types?

I'm thinking there might be interesting feedback in one proposal review that we might want to feed back and influence the other proposal review. Or it might be easier to start with stdlib and then consider that completed design the "prior art" for a later proposal on Foundation.

Hmm… any ideas about that?

Footnotes

1. https://github.com/swiftlang/swift-evolution/pull/2875 ↩

[parkera]

If we run a proposal through swift-evolution and it includes extensions to Foundation API, and the Foundation workgroup doesn't object for some reason, then we can consider it accepted.

[parkera]

Maybe we just give interested parties a heads up by creating a thread in the Foundation section, in case they don't follow the other evolution threads on the forums.