Async/let producing error in playground? Xcode 13.0 beta

Question

Created Jun ’21

Replies 12

Boosts 0

Views 3.7k

Participants 7

I'm seeing an error trying to test out async let. It seems like this should work, based on the async/let and structured concurrency session videos.

Here's the code:

func doIt() async -> String {
    let t = TimeInterval.random(in: 0.25 ... 2.0)
    Thread.sleep(forTimeInterval: t)
    return String("\(Double.random(in: 0...1000))")
}

async {
    async let a = doIt()
    async let b = doIt()
    async let c = doIt()
    async let d = doIt()

    let results = await [a, b, c, d]
    for result in results {
        print("  \(result)")
    }
}

But, I get this error for every "async let" statement:

error: AsyncLetSwift55WWDC21.playground:12:15: error: expression is 'async' but is not marked with 'await'
    async let a = doIt()
              ^
              await

Am I missing something key, or is this a bug?

I'm running this in the Xcode 13.0 beta, in a Playground.

Thanks!!

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Answer 1

DTS Engineer OP

Apple

Jun ’21

Are you running Xcode 13 on macOS 12 beta? Or macOS 11?

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

0

Answer 2

DTS Engineer OP

Apple

Jun ’21

If you put the same code in a command-line tool project (created from the macOS > Command Line Tool template) do you get the error there?

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—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

0

Answer 3

DTS Engineer OP

Apple

Jun ’21

So maybe playground-specific?

Sounds like it.

When you did your playground testing, were you using an iOS playground? If so, try a macOS one.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

0

Answer 4

LionKing OP

Jun ’21

i build a iOS project on Xcode 13.0 beta, and this code works on simulator

0

Answer 5

new_bee OP

Jul ’21

@eskimo Do we have to file a bug for playgrounds? I am still on macOS BigSur, so can't run it on a command line project as it would require macOS 12 (Beta). I was preparing some tutorials to demonstrate async let so thought playgrounds was ideal as opposed to an iOS project

0

Answer 6

ashrafi OP

Aug ’21

When running this code from command line it works fine.System Version: macOS 12.0 (21A5294g)

Tested Playgrounds both iOS and Mac platform. Both failed but the code is fine. It is just a Playground issue.

Could someone make sure this bug was filed with the XCode Playgrounds team.

0

Answer 7

DTS Engineer OP

Apple

Aug ’21

I am still on macOS BigSur, so can't run it on a command line project

If you’re on macOS 11 then you can’t run async/await code natively because macOS 11 does not include the necessary runtime bits. The solution is to target iOS:

You can create an iOS app and run it on an iOS 15 beta device
Or on an iOS 15 beta simulator.
Or, if you want to work in a playground, create an iOS playground which will run on the iOS 15 beta simulator.

With regards that last point, here’s how I tested this today:

On macOS 11.4, I ran Xcode 13.0b4.
I chose File > New > Playground and then selected iOS > Blank.

I changed the playground’s code to this:

import UIKit
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

print("will start task")
Task.detached {
    print("task will sleep")
    try! await Task.sleep(nanoseconds: 1_000_000_000)
    print("task did sleep")
}
print("did start task")

I ran the playground. It printed this:
```
will start task
task will sleep
did start task
```
and then, one second later, printed this:
```
task did sleep
```

Step 4 can take a while the first time around as the playground spins up a simulator instance in the background.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

0

Answer 8

ashrafi OP

Aug ’21

@eskimo we know

async {
    async let a = doIt()
    async let b = doIt()
    async let c = doIt()
    async let d = doIt()

    let results = await [a, b, c, d]
    for result in results {
        print("  \(result)")
    }
}

will not run in a Playground. We need the bug fixed :-)

0

Answer 9

DTS Engineer OP

Apple

Aug ’21

We need the bug fixed

Which bug specifically? What’s the bug number?

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

0

Answer 10

DTS Engineer OP

Apple

Aug ’21

Can eskimo open a bug for it?

It’s better if you file your own bug report; that way you can monitor its state.

Please post your bug number, just for the record.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

0

Answer 11

newwbee OP

Aug ’21

@eskimo

Feedback

I have file a feedback FB9546874

Versions:

The issue happens with Xcode Playground (iOS)
Xcode: Version 13.0 beta 5 (13A5212g)
macOS: 11.5.2 (20G95)

Note:

The same code runs fine in a SwiftUI project

Code:

import UIKit

func f1() async -> Int {
    return 10
}

Task {
    async let a = f1()
    async let b = f1()
    
    let c = await a + b
    print("c = \(c)")
}

Error:

error: AsyncLet.playground:8:15: error: expression is 'async' but is not marked with 'await'
    async let a = f1()
              ^
              await 

error: AsyncLet.playground:9:15: error: expression is 'async' but is not marked with 'await'
    async let b = f1()
              ^
              await

1

Answer 12

DTS Engineer OP

Apple

Aug ’21

I have file a feedback FB9546874

Thank you!

I had a look at your bug and it seems to have landed in the right place but, as you only filed it yesterday, I have no other info to share right now.

I’m hoping that it will get closed as a dup of an earlier bug that’s been investigated already but we’ll have to see how that pans out. In the meantime, watch out for future Xcode beta seeds and retry your test and post back here with the results.

Share and Enjoy
—
Quinn “The Eskimo!” @ Developer Technical Support @ Apple
let myEmail = "eskimo" + "1" + "@" + "apple.com"

1