Assign MainActor function to not async/MainActor closure

carlhung · 2024-07-20T22:05:41.547Z

  struct Box {
      let action: () -> Void
  }
  
  @MainActor
  func f() -> Void {}
  
  let b = Box(action: f)
  b.action()

It doesn't seem legit to me. as action isn't an async or MainActor clsoure.

vns · 2024-07-21T08:24:15.420Z

That's behaviour introduced by swift-evolution/proposals/0434-global-actor-isolated-types-usability.md at main · swiftlang/swift-evolution · GitHub

carlhung · 2024-07-21T09:36:18.023Z

I read the proposal. It doesn’t seem like the same thing.

carlhung · 2024-07-21T10:49:41.404Z

carlhung:

struct Box {
      let action: () -> Void
  }
  
  @MainActor
  func f() -> Void {}
  
  let b = Box(action: f)
  b.action()

I know the reason. thanks.

vns · 2024-07-21T10:55:14.390Z

Yeah, I forgot detail that top-level code implicitly run on main actor. That's why that's legal. If you wrap that in a nonisolated function, it will raise an error:

nonisolated func run() {
    let b = Box(action: f)
    b.action()
}

But in Swift 5 it actually warns about loosing main actor isolation, while in Swift 6 the top-level version don't, because of the inference.