Here in guide <https://github.com/Kotlin/kotlinx.c...

# coroutines

[tyjka]

Here in guide https://github.com/Kotlin/kotlinx.coroutines/blob/master/coroutines-guide.md#cancellation-and-exceptions in the code example delay() is called in a child coroutine, and then the child coroutine is cancelled. If yield() is not called on the parent coroutine then child won’t be canceled. Why calling yield() on parent makes child cancellable, whereas delay() has to be cancellable by default?

[Vsevolod Tolstopyatov [JB]]

If yield() is not called on the parent coroutine then child won’t be canceled

It’s not true. Child will be cancelled anyway. But without yield child will be cancelled before it has a chance to start, so “Child is cancelled” will not be printed, because it will not be started (as it’s already cancelled)

[tyjka]

Thanks for the answer. Yes, it’s true, if I put delay(100) instead of yield then I still get “Child is cancelled”. I’ve read this: https://stackoverflow.com/questions/231767/what-does-the-yield-keyword-do on yield. Now I get better understanding of what yield does in generators. But can you please explain what yield does exactly in that code example?

[Vsevolod Tolstopyatov [JB]]

It gives the child chance to execute and reach delay(Long.MAX_VALUE) call.
Note that child is launched in the same context as parent, which is runBlocking, so they cannot run concurrently.

With yield:
1) Parent launches child
2) Parent invokes yield
3) Child launches, invokes delay(Long.MAX_VALUE) and suspends
4) Parent cancels child, so delay(Long.MAX_VALUE) throws an exception, finally block executes and “Child is cancelled” is printed

Without yield:
1) Parent launches child
2) Parent cancels child
3) Child immediately cancels, so launch body is not even executed
4) Parent prints “Parent is not cancelled”