Code that is generic over actor isolation

Using Swift
1

I have a protocol which looks something like the following:

protocol MyProtocol {
  var foo: Foo { get set }
}

And a method which uses it inout:

func useMyProtocol<T: MyProtocol>(_ value: inout T) async { … }

Inside the body, I need to do value.foo = someFoo()
This works for simple structs but in more complex scenarios I need the type conforming to MyProtocol to be an actor.
I could do this by changing the protocol requirement to func updateFoo(_ newValue: Foo) async but this complicates the case when the type conforming to MyProtocol is a simple struct. Is there a way I can run a block of code that is isolated to the type conforming to MyProtocol if it is an actor, but still support having simple structs conform to MyProtocol?

1 Like
2

Hmm, this could be challenging. To start, I'm curious what the actor conformance would look like here. A synchronous setter is usually a problem for anything with static isolation.

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3

To conform your protocol using actor you would have use either @preconcurrency or nonisolated(unsafe), paired with isolated parameter in function (which you'll have to specify explicitly I guess). Both options turn off checks and might crash.

I'd say that open setters on protocols — especially if you expect it to conform to an actor — is better to be replaced with a method. This will give you much more flexibility, and will carry isolation by default, without the need to do some tricks with passing it.

BTW, I've run into a compiler crash when modified function to

func useMyProtocol<T: MyProtocol>(
    _ value: inout T, 
    isolation: isolated (any Actor)?
) async

and called

useMyProtocol(&myActor, isolation: myActor)

resulted in

error: compile command failed due to signal 11 (use -v to see invocation)

...
1.      Apple Swift version 6.0 (swiftlang-6.0.0.9.10 clang-1600.0.26.2)
2.      Compiling with the current language version
3.      While evaluating request TypeCheckSourceFileRequest
4.      While evaluating request TypeCheckFunctionBodyRequest
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4

Please file an issue about that crash. I’ve also run into many crashes when using sending and/or isolated parameters with 6.0. It makes coming up with solutions to tricky isolation problems like this one particularly hard.

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