Using async let with synchronous functions

Using Swift
1

Hi folks,

I’m looking for clarification around async let usage with synchronous functions. The snippet below compiles, and at runtime it does seem to run f and g concurrently (they are executing on different off-main threads).

My confusion is that f and g are not annotated as async, which I thought was a requirement for using async let.

I did a quick grep through the swift book and didn’t see anything documenting this behavior. Is this a blessed usage of async let?

I’m using the following settings:

  • Approachable concurrency: off
  • Default actor isolation: nonisolated
  • Strict concurrency checking: complete
  • Swift language version: 6

@MainActor
struct ContentView: View {
    var body: some View {
        EmptyView()
            .task {
                print(await expensive())
            }
    }
    nonisolated func expensive() async -> Int {
        whichThread()
        async let f = f()
        async let g = g()
        return await f + g
    }
}

func whichThread() {
    print("Expensive is on thread: \(Thread.current)")
}

nonisolated func f() -> Int {
    print("f is on thread: \(Thread.current)")
    return 1
}

nonisolated func g() -> Int {
    print("g is on thread: \(Thread.current)")
    return 2
}

Thank you,
Lou

2

In SE-0317, the only warning about synchronous usage is that “It is not allowed to declare async let as top-level code, in synchronous functions or closures.”

So, while you can only use async let from within an asynchronous context, it says nothing about prohibiting async let to call a function that is synchronous. In fact, SE-0317 has some examples (on different topics), where they do precisely that.

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