I'm having troubles understanding the exception ha...

# flow

[Maciek]

I'm having troubles understanding the exception handling in the flow. Could someone explain why the throwable completes the flow even with the catch { } in the chain? If I'd want to have a infinite flow I must handle the exceptions with try catch in the lambdas because flow will close after any exception "leaking" to the stream, right? Or is there a way to do some explicit recover from the exception? Example code, only 0 will be emitted, next emit 1 is lost due to flow completion after exception.

flow {
  emit(0)
  emit(1) // this emit is never reached
}
  .onEach { if (it == 0) throw Throwable() }
  .catch { println(it) }
  .collect()

[Maciek]

so in order for this code to work as expected I'd have to add try catch in the onEach

[elizarov]

catch lets you handle the flow exception, but there’s no way to recover from it the way you want. The flow is already dead.

[elizarov]

That is, in your code emit(0) had already threw an exception and emit(1) cannot be ever reached.

[Maciek]

I thought I missed something in the understanding of exception handling. Appreciate the explanation

[Maciek]

is this concept of recovery is something that conflicts with the philosophy of coroutines and will never be added to the API or is it something that could be considered? I guess there'd be some scenarios that such API could make the error handling a bit nicer or simpler to write

[Nick Allen]

It's part of the API contract. See "Exception transparency" section of Flow documentation. Flows are not allowed to catch the exceptions that come from emit.

[Nick Allen]

Exceptions are how collectors stop collecting so Flow is required to not emit anything else. This is how operators like first and take work, they throw exceptions inside collect and then catch only their own exceptions outside collect.

[Maciek]

awesome, thanks a ton for the explanation!

[expensivebelly]

We use retryWhen to recover from Exceptions, once it goes past retryWhen (if the predicate returns false) then at that point the Flow has completed with an exception (as far as I know) and there is no way to re-subscribe to it