[Pitch #4] Actors

[Douglas_Gregor]

Hey all, I've made a bunch of changes on the road to a "pitch #5", based on feedback here. You can preview the changes at https://github.com/DougGregor/swift-evolution/pull/62, and here's the summary:

• Changes in the fifth pitch:
  • Drop the prohibition on having multiple isolated parameters. We don't need to ban it.
  • Replace Sendable with Sendable to better track SE-0302.
  • Add the Actor protocol back, as an empty protocol whose details will be filled in with a subsequent proposal for [custom executors][customexecs].
  • Replace ConcurrentValue with Sendable and @concurrent with @sendable to track the evolution of [SE-0302][se302].
  • Clarify the presentation of actor isolation checking.
  • Add more examples for non-isolated declarations.
  • Added a section on isolated or "sync" actor types.

@Zhu_Shengqi

@Douglas_Gregor

So, I think we fundamentally disagree about what kinds of protocols are important for actors to conform to. For me, most interactions with actors are naturally asynchronous---they are coordinators and controllers and owners of data models that need to be consistent.

Can we make a simpler and more succinct proposal based on what has been commonly agreed on? This pitch is already in round 4, which may be an indicator that there is too much things to discuss and settle in a single proposal. By putting things that causing arguments and disagreement into a separate pitch we can discuss in a more clear and narrow scope.

From my perspective, the proposal is not complete without nonisolated, and my read of the situation is that there is no actual disagreement about whether nonisolated is a good idea, or it's semantics, or even it's spelling (now). Yes, the proposal would be somewhat smaller without it, but at the cost of having yet another concurrency proposal that scatters the knowledge further. It fits well alongside isolated parameters to have the modifier that disables the implicit isolated on self. If we rewound the clock to the pre-Swift 1.0 days, could we imagine introducing inout without also introducing mutating?

Global actors was large, so we split it out of the actors proposal. async let was both controversial and confusing the discussion of structured concurrency, so we split it out of that proposal. Both were separable. For me, nonisolated doesn't meet that bar.

The actual disagreement, as I understand it, is whether another feature, called isolated conformances in the future directions of this proposal, is also a critical part of the model. We have neither a set of agreed-upon examples nor a complete design to evaluate that feature. My own experience is that I have not needed this feature. We should keep exploring it further, but we shouldn't hold up progress on actors themselves waiting for it.

Doug