does arrow have any integration with flows, or do ...

# arrow

[bj0]

does arrow have any integration with flows, or do i have to translate to an exception like this:

flow {
    while(true) {
        when(val r = functionThatReturnsEither()){
            is Right -> emit(r.value)
            is Left -> error("some error")
        }
    }
}

[Daniel Pitts]

Couldn't you use fold?

functionThatReturnsEither()
    .fold(
      { error("some error") },
      { emit(it) }, 
    )

[bj0]

that's just different syntax for the same thing, i'm asking if i need to translate from typed errors to exceptions

[Daniel Pitts]

It depends on what you want to happen. I'm not really sure I understand what you're hoping for, can you write an example of what you would want it to look like?

[bj0]

i want the flow to cancel with an error when functionThatReturnsEither returns a typed error. In that snippet i'm doing it manually by converting to an exception, I was wondering if there's any integration/api that does it without that. I'm basically looking for the arrow version of:

flow {
    while(true){
        // cancels flow with exception
        functionThatThrows()
    }
}
.catch {t -> /* exception here */ }
.collect()

but with a function that returns Either instead of throwing an exception

[Daniel Pitts]

Maybe something like getOrElse { cancel() }

[bj0]

that also translates the typed error to an exception, which i guess is the only option

[Daniel Pitts]

getOrElse { break } ?

[bj0]

you lose the error information

[Daniel Pitts]

Where do you want the error information to be used?

[bj0]

in the catch function

[Daniel Pitts]

Ah, so if you emit the either, use Flow.map

[bj0]

yea i probably have to emit the either and then handle it outside, or use an exception to cancel it... i guess i was just looking to see if there was already a standard way people did this type of thing

[Daniel Pitts]

It wouldn't be hard to create your own extension method:

inline fun <E, V> Flow<Either<E, V>>.onLeft(crossinline handle: (E) -> Unit) =
    transformWhile {
        it.fold(
            {
                handle(it)
                false
            },
            {
                emit(it)
                true
            },
        )
    }

[Daniel Pitts]

Not sure if arrow has something like that already though.

[bj0]

i couldn't find anything in the docs, only thing that's missing is stopping the flow

[Daniel Pitts]

That's what the false here does.

[bj0]

ah

[Daniel Pitts]

https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/transform-while.html

[bj0]

haven't used that one before

[bj0]

that's pretty good, behavior parallels the catch with exceptions.