@isolated(any)

Ahh, @isolated(any). It’s an attribute of contradictions. You might see it a lot, but it’s ok to ignore it. You don’t need to use it, but I think it should be used more. It must always take an argument, but that argument cannot vary.

Confusing? Definitely. But we’ll get to it all.

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To understand why @isolated(any) was introduced, we need to take a look at async functions.

let respondToEmergency: () async -> Void

This is about as simple a function type as we can get. But, things start to get a little more interesting when we look at how a function like this is used. A variable with this type must always be invoked with await.

await respondToEmergency()

This, of course, makes sense. All async functions have been called with await. But! Consider this:

let sendAmbulance: @MainActor () -> Void = {
    print("🚑 WEE-OOO WEE-OOO!")
}

let respondToEmergency: () async -> Void = sendAmbulance

await respondToEmergency()

The explicit types are there to help make what’s going on clear. We first define a synchronous function that must run on the MainActor. And then we assign that to a plain old, non-MainActor async function. We’ve changed so much that you might find it surprising this even compiles.

Remember what await actually does. It allows the current task to suspend. That doesn’t just let the task wait for future work to complete. It also is an opportunity to change isolation. This makes async functions very flexible!

Just like a dispatcher doesn’t sit there doing nothing while waiting for the ambulance to arrive, a suspended task doesn’t block its thread. When the dispatcher puts you on hold to coordinate with the ambulance team, that’s the isolation switch - they’re transferring your request to a different department that specializes in that type of work.

But change to where, exactly?

Ok, so we know that async functions, because they must always be awaited, gain a lot of flexibility. We are close, but have to go just a little further to find the motivation for this attribute.

func dispatchResponder(_ responder: () async -> Void) async {
    await responder()
}

await dispatchResponder {
    // no explicit isolation => nonisolated
    print("🚒 HONK HOOOOONK!")
    await airSupport()
    print("🚁 SOI SOI SOI SOI SOI!")
}

await dispatchResponder { @MainActor in
    print("🚑 WEE-OOO WEE-OOO!")
}

We now have a function that accepts other functions as arguments. It’s possible to pass in lots of different kinds of functions to dispatchResponder. They could be async functions themselves, or even be synchronous. And they can be isolated to any actor. All thanks to the power of await.

Except there’s a little problem now. Have a look at dispatchResponder on its own:

func dispatchResponder(_ responder: () async -> Void) async {
    await responder()
}

The type of responder fully describes everything about this function, except for one thing. We have no way to know its isolation. That information is only available at callsites. The isolation is still present, so the right thing happens at runtime. It’s just not possible to inspect it statically or even programmatically. If you’ve encountered type erasure before, this should seem familiar. The flexibility of async has come with a price - a loss of information.

This is where @isolated(any) comes in.

Using @isolated(any)

We can change the definition of dispatchResponder to fix this.

func dispatchResponder(_ responder: @isolated(any) () async -> Void) async {
    print("responder isolation:", responder.isolation)

    await responder()
}

When you apply @isolated(any) to a function type, it does two things. Most importantly, it gives you access to a special isolation property. You can use this property to inspect the isolation of the function. The isolation could be an actor. Or it could be non-isolated. This is expressible in the type system with (any Actor)?.

Functions with properties felt really strange to me at first. But, after thinking for a minute, it became quite natural. Why not? It’s just a type like any other. In fact, we can simulate how this all works with another feature: callAsFunction.

struct IsolatedAnyFunction<T> {
    let isolation: (any Actor)?
    let body: () async -> T

    func callAsFunction() async -> T {
        await body()
    }
}

let value = IsolatedAnyFunction(isolation: MainActor.shared, body: {
    // isolated work goes here
})

await value()

This analogy is certainly not perfect, but it’s close enough that it might help.

There is one other subtle change that @isolated(any) makes to a function that you should be aware of. Its whole purpose is to capture the isolation of a function. Since that could be anything, callsites need an opportunity to switch. And that means an @isolated(any) function must be called with an await — even if it isn’t itself explicitly async.

func dispatchResponder(_ responder: @isolated(any) () -> Void) async {
    await responder() // note the function is synchronous
}

This makes synchronous functions marked with @isolated(any) a little strange. They still must be called with await, yet they aren’t allowed to suspend internally?

As it turns out, there are some valid (if rare) situations where such an arrangement can make sense. But adding this kind of constraint to your API should at least merit some extra documentation.

How @isolated(any) Affects Callers

All of the task creation APIs — Task initializers and TaskGroup — make use of @isolated(any). These are used a lot and are usually encountered very early on when learning about concurrency. So, it’s pretty natural to run into this attribute and think:

“Ugh another thing to understand!”

This is pretty reasonable because the components of a function type dictate how it can be used. They are all essential qualities for API consumers. They are the interface.

• Parameters
• Return value
• Does it throw?
• Is is async?

This is not an exhaustive list, but what’s important is all of these are things callers must care about. Except for @isolated(any), which is the opposite. It doesn’t affect callers at all.

This, I think, is the root of a lot of confusion around @isolated(any). Unlike other qualities of a function, this attribute is used to capture information for the API producer.

I’m so close to saying “you can and should just ignore @isolated(any)“. But I just cannot quite go that far, because there is one situation you should be aware of.

Scheduling

To help understand when you should be thinking about using @isolated(any), I’m going to quote the proposal:

This allows the API to make more intelligent scheduling decisions about the function.

I’ve highlighted “intelligent scheduling”, because this is the key component of @isolated(any). The attribute gives you access to the isolation of a function argument. But what would you use that for?

Did you know that, before Swift 6, the ordering of the following code was undefined?

@MainActor
func threeAlarmFire() {
    Task { print("🚒 Truck A reporting!") }
    Task { print("🚒 Truck B checking in!") }
    Task { print("🚒 Truck C on the case!") }
}

Ordering turns out to be a very tricky topic when working with unstructured tasks. And while it will always require care, Swift 6.0 did improve the situation. We now have some stronger guarantees about scheduling work on the MainActor, and @isolated(any) was needed to make that possible.

Take a look at this:

@MainActor
func sendAmbulance() {
    print("🚑 WEE-OOO WEE-OOO!")
}

nonisolated func dispatchResponders() {
    // synchronously enqueued
    Task { @MainActor in
        sendAmbulance()
    }

    // synchronously enqueued
    Task(operation: sendAmbulance)

    // not synchronously enqueued!
    Task {
        await sendAmbulance()
    }
}

These are three ways to achieve the same goal. But, there is a subtle difference is how the last form is scheduled. Task takes an @isolated(any) function so it can look at its isolation and synchronously submit it to an actor. This is how ordering can be preserved! But, it cannot do that in the last case. That closure passed into Task isn’t actually itself MainActor — it has inherited nonisolated from the enclosing function.

I think it might help to translate this into GCD.

func dispatchResponders() {
    // synchronously enqueued
    DispatchQueue.main.async {
        sendAmbulance()
    }

    // synchronously enqueued
    DispatchQueue.main.async(execute: sendAmbulance)

    // not synchronously enqueued!
    DispatchQueue.global().async {
        DispatchQueue.main.async {
            sendAmbulance()
        }
    }
}

Look really closely at that last one! What we are doing there is introducing a new async closure that then calls our MainActor function. There are two steps. This doesn’t always matter, but it certainly could. And if you need to precisely schedule asynchronous work, @isolated(any) can help.

isolated(all)

All this talk about @isolated(any) got me thinking…

It’s kinda strange that only some functions get to have this isolation property. It would certainly feel more consistent to me if all functions had it. In fact, I think we can go further. I can imagine a future where an explicit @isolated(any) isn’t even necessary for async functions. As far as I can tell, there is no downside.

And a little less syntactic noise would be most welcome. Perhaps one day!

isolated(some)

We do have to talk about that any. It’s surprising that this attribute requires an argument, yet permits only one possible value. The reason here comes down to future considerations.

The concrete actor type that this isolation property returns is always (any Actor)?. This is the most generic type for isolation and matches the #isolation macro. Today, there is no way to constrain a function to only specific actor types, such as @isolated(MyActor). The any keyword here was chosen to mirror how protocols handle this. But accepting an argument leaves the door open to more sophisticated features in the future.

And that really fits the spirit of @isolated(any). Putting some work now in exchange for flexibility down the road.

Because you’ll see it in many foundational concurrency APIs, it’s very natural to feel like you must understand @isolated(any). I’m 100% behind technical curiosity! But, in this case, it is not required. For the most part, you can just ignore this attribute. You will rarely, if ever, need to use it yourself.

But if you ever find yourself capturing isolated functions and passing them along to other APIs that use @isolated(any), you should consider adopting it. It could prove useful. It’s even a source-compatible change to add or remove this attribute from an async function.

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So there you have it.

As with many parts of the concurrency system, there’s a surprising depth to @isolated(any). Thankfully, from a practical perspective, we can enjoy the ordering guarantees of task creation that it enables without needing to master it. And one less thing on this journey is most welcome.

Isolated maybe, but never alone.