with changing `launch` to `async` and `join` to `a...

# coroutines

[Evgeniy Zaharov]

with changing launch to async and join to await everything start to work as expect, I see all three catch (as described in https://github.com/Kotlin/kotlinx.coroutines/issues/61). But with launch its very strange that method-catch doesn’t call

[codyoss]

You can’t catch an exception from coroutine with launch. But you can with async/await. Examples: https://github.com/codyoss/kotlin-coroutine-examples/blob/master/src/main/kotlin/com/github/codyoss/coroutine/example/exceptions/01_LaunchAndFail.kt#L8 https://github.com/codyoss/kotlin-coroutine-examples/blob/master/src/main/kotlin/com/github/codyoss/coroutine/example/exceptions/03_Async.kt#L14

[Evgeniy Zaharov]

I know that with async/await it’s possible to do it (i have mentioned it in question). The question was about launch, about why it’s don’t catched from not suspend method

[codyoss]

when you launch something you have to think about it as it off on another world. So if an exception happens over on Mars you would never know about it on Earth. The await function provides a bridge to join the two worlds so they can interact. That analogy fell apart rather quickly…

[Vsevolod Tolstopyatov [JB]]

launch is equivalent of running a thread in coroutines world.

When you write

val thread = thread { throw Exception() }
thread.join()

You don’t expect for join to throw, do you?
Same about launch.

But async returns Deferred, which is analogue of the Future in coroutines world.
And future.get throws an exception if related computation failed with exception

[Vsevolod Tolstopyatov [JB]]

Basically launch is fire-and-forget job, while async is asynchronous computation which should return its result at some point

[Evgeniy Zaharov]

@codyoss, @Vsevolod Tolstopyatov [JB] thanks for explanation, and especially for the analogy with the thread. Now it’s much clearer.

[uli]

Does a `launch`ed child with an unhandled exception in today's implementation cancel it's parent with that exception, as it used to be the behaviour?

[Vsevolod Tolstopyatov [JB]]

@uli it cancels child with that exception.
Though this mechanism is slightly non-deterministic and being reworked under https://github.com/Kotlin/kotlinx.coroutines/issues/333 to provide consistent behaviour for users.
As soon as it’s ready I will add guide section about exceptions mental model

[uli]

Thanks @Vsevolod Tolstopyatov [JB] for the update