I have a suspending function that returns a flow. It seems like it should be trivial to make it a fu...

# coroutines

[Sean Proctor]

I have a suspending function that returns a flow. It seems like it should be trivial to make it a function that returns immediately. I think flow { getSubscriptionFlow().collect { emit(it) } } would work, but it looks weird. Is there a simpler way to write that?

[Joffrey]

Are you trying to change the function so it's not suspend anymore, or are you trying to build a new function that is not suspend and uses the suspending one?

Also, why is the function suspending in the first place if it returns a Flow? By that I mean what suspending stuff does it do before returning the flow, and why is that stuff not part of the flow definition?

[Sean Proctor]

I'm trying to make a function that returns a flow immediately. It's suspending because it does a little bit of async stuff to make the flow.

[Sean Proctor]

Specifically, it gets an access token from then starts listening on a graphql subscription that returns a flow

[ephemient]

it should do that in the flow builder itself

[Joffrey]

I see. Then you could do:

fun yourFun(): Flow<X> = flow {
    val token = fetchToken()
    emitAll(getSubscriptionFlow(token))
}

[Sean Proctor]

Ah, perfect! That's exactly what I was looking for. Thank you!

[ephemient]

https://rules.sonarsource.com/kotlin/RSPEC-6309

Functions returning Flow or Channel should return the result immediately and may start a new coroutine in the background. As a consequence, such functions should not be suspending and if they launch a coroutine in the background, they should be declared as extension functions on CoroutineScope.

[Sean Proctor]

Makes sense.

[gildor]

::fetchToken.asFlow().flatMap(::getSubacriptionFlow)

[gildor]

Do not say it's a simpler from the reading point of view, but if you need more idiomatic way to create flow from a suspend function

[Joffrey]

But it's not exactly about creating a flow from a suspend function here, it's about removing suspend on a function with signature suspend fun <T> x(): Flow<T>

[Sean Proctor]

Thanks. I think I'll stick with @Joffrey’s solution, as it's much clearer what's happening.

[gildor]

Yep, removing suspend is the right in general

[Sean Proctor]

In the end, it looks basically like this:

fun observeTickets(): Flow<List<Ticket>> =
    flow {
        emitAll(
            withClient { client ->
                client.subscription().toFlow()
            }
        )
    }

I'm happy with the result. withClient has the signature suspend fun <T> withClient((Client) -> T): T

[Joffrey]

Note that the with prefix in kotlin functions usually implies that the thing is provided as a receiver rather than argument. use does provide the "thing" as argument to the lambda, on the other hand (but it implies the resource is created for the lambda and then closed after the lambda is done).

So I'd suggest either suspend fun <T> withClient(Client.() -> T): T if there is no notion of cleanup, or suspend fun <T> useClient((Client) -> T): T if you actually close some resources.

But that's nitpicking on style of course.

[Sean Proctor]

I appreciate it. I just changed the name, thanks again.