I am new to coroutines. Lets say I have a suspend function that returns a nullable type. The type ha...

# coroutines

[reik.schatz]

I am new to coroutines. Lets say I have a suspend function that returns a nullable type. The type has a List field, whose values I read from a db using r2dbc, hence they are returned as a Flow. How do I map from a Flow to a nullable instance that my suspend function returns? In Reactor there are functions to map a Flux to a Mono ..

[gildor]

Honestly I don’t understand your case, what exactly you want to convert?

[reik.schatz]

ok some code

[reik.schatz]

data class Foo(val users: List<String>)

[reik.schatz]

suspend fun find(): Foo?

[gildor]

Flow doesn’t have direct analogue to Mono, because you have suspend functions, so the closest converter of Flow to suspend function is just call flow.first() on it to get first value or wrap it to lambda to make it lazy

[gildor]

okay

[reik.schatz]

within find() I read users from a db using R2DBC. So I end up having this: val users: Flow<String> = client.execute ...

[reik.schatz]

now I need to map users to an instance of Foo

[gildor]

so, looks that first() is what you need

[reik.schatz]

that returns me the first item? I need all of them, the whole list

[gildor]

Than use flow.toList()

[gildor]

which suspend until this flow is completed

[reik.schatz]

ok thanks

[reik.schatz]

let’s say I want to create multiple Flows (using client.execute) in parallel. then I want to suspend until all of them are complete ..

[gildor]

use common parallelization approach for suspend functions (use async + awaitAll) and call toList() on each of them

[reik.schatz]

thanks, will read up on it

[gildor]

or merge flows first and use toList() after that

[reik.schatz]

flow.flatMapMerge?

[gildor]

yeah, something like:

flowOf("user1", "user2").flatMapMerge { client.execute(it) }.toList()

[gildor]

actually this approach is more simple in this case than async{}