Async let implicit cancellation confusion

[ovmarchenko]

Async/await basics lesson Calling async functions in parallel using async let says that:

By awaiting the results in a joined array, we group the requests together, which will also make them fail together if one of the requests fails.

But the comment in the code example says :
// data1 and data3 are **implicitly canceled** when exiting scope

So does grouping results together have anything to do with cancellation? Or do they get cancelled because the error causes premature scope exit?

[dkahhori]

As far as I understand, grouping results together does not cause cancellation. Cancellation happens because the scope exits prematurely due to an error, which cancels remaining tasks that haven't completed.

Example: let result = try await [data1, data2, data3]
If data2 throws, we do not proceed with data1 & data3

[AvdLee]

@dkahhori thanks for your answer, you're right!

So does grouping results together have anything to do with cancellation? Or do they get cancelled because the error causes premature scope exit?

The wording of the lesson was a bit unfortunate. This is the correct wording after rewriting:

By awaiting the results in a joined array, we group the requests, which will also make any running tasks in the array cancel if one of the requests fails.

In other words, in case data2 throws:

• If data1 or data3 has already finished, they are unaffected — the results are discarded.
• If they are still running, Swift sends them a cancellation signal.
• Whether they actually stop depends on whether the task’s code checks for cancellation using Task.isCancelled or handles CancellationError.