https://www.v2ex.com/t/1081497

sequentialize.swift

func sequentialize<T: Sendable, each P: Sendable>(
    fn: @Sendable @escaping (repeat each P) async -> (T)
) -> @Sendable (repeat each P) async -> (T) {
    let (s, c) = AsyncStream.makeStream(of: Void.self)
    c.yield(())

    return { (x: repeat each P) in
        var i = s.makeAsyncIterator()
        await i.next()

        let result = await fn(repeat each x)
        c.yield(())

        return result
    }
}

let clock = ContinuousClock()

let sequentializedPlus = sequentialize { (a: Int, b: Int) in
    try? await Task.sleep(for: .seconds(1))
    print(clock.now, a, b)
    return a + b
}

await withTaskGroup(of: Void.self) { group in
    print(clock.now)
    group.addTask {
        print(await sequentializedPlus(1, 2))
    }
    group.addTask {
        print(await sequentializedPlus(3, 4))
    }
    await group.waitForAll()
}

/*
 Instant(_value: 1214028.911775166 seconds)
 Instant(_value: 1214029.9298497501 seconds) 1 2
 3
 Instant(_value: 1214030.983612708 seconds) 3 4
 7
 */