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#coroutines
Title
# coroutines
p

pablisco

12/02/2020, 10:18 AM
Hello everyone! I was wondering if someone has tried to implement something like this:
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fun <T> loopFlow(initial: T, f: suspend (T) -> Flow<T>): Flow<T>
The idea being that the
initial
value is emitted once and then
f
is applied recursively. And then finish the flow when cancelled or when an empty flow is produced (duh). I’ll post in thread what I’ve tried so far without much luck…
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fun <T> loopFlow(initial: T, f: suspend (T) -> Flow<T>): Flow<T> = flow {
    emit(initial)
    var current: Flow<T> = f(initial)
    while (current.count() > 0) {
        emitAll(current)
        current = current.flatMapConcat(f)
    }
}
This one feels like it should work, but only the initial item is emitted. I must be running into some deadlock issue with nested suspension
g

Giorgos Neokleous

12/02/2020, 11:52 AM
Do you have a test code for this? Also, where does
count()
come from? I can't see that method in the flow interface 🤔
p

pablisco

12/02/2020, 1:30 PM
count is an extension function of
flow()
I’m not very happy with that implementation, specially because it doesn’t work as I expected
No test code yet sadly… maybe I’ll try creating something shareable. More wondering if someone has had a problem where they needed to make something recursive with flows and wanted to avoid stack hell
This is a potential candidate:
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fun <T> loopFlow(initial: T, f: suspend (T) -> Flow<T>): Flow<T> = flow {
    emit(initial)
    f(initial).collect { emitAll(loopFlow(it, f)) }
}
But it’s not stack safe 😞
This is a very simplistic example: https://pl.kotl.in/g2gb8HeSn This can be done very easily in other ways, but it was just to demostrate an infinite flow that flatmaps the result of each flatmap 🙂
I think I need to use something like what @elizarov mentions in this article, but I’m not sure how to apply it to flows 🙃 I’ll have to investigate further
g

Giorgos Neokleous

12/02/2020, 2:53 PM
Wouldn't
count()
collect everything from the flow and suspend while collecting. That means that if you've got an infinite flow then it will never "resume". I am guessing it's the same for
emitAll
. So if you are consuming/receiving a channel for example it will never proceed further. (this is just a speculation)
p

pablisco

12/02/2020, 3:01 PM
Yeah, I mean that was the example I didn’t like… the second one I posted is a bit better but but still has the problem of not being stack safe
g

Giorgos Neokleous

12/02/2020, 3:38 PM
flatMapMerge might do the trick? Playground link
p

pablisco

12/02/2020, 5:15 PM
It’s not far from what
flatMapConcat
does (main difference is that
flatMapMerge
doesn’t do the transformations in sequence). The thing with both of these functions is that they both are a one time transformation but in this case what I’m looking for is for transformation recursively like:
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flow.flatMapMerge { f(it) }.flatMapMerge { f(it) }... // and so on
🙂
Oh sorry, I just saw your playground link 😅
@Giorgos Neokleous If you try to
take(1000)
you'll find that we get a memory exception. However if we drop the list and do it like this to remove the memory constraints:
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loopFlow2(0 to 1) { (a, b) -> flowOf(b to a + b) }.take(1000).drop(999).collect { println("$it") }
With the original we get a stacktrace error… However, with the second version we get this other error (although the result is printed) 🤔
btw, we can hide de collection with an
emitAll
:
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fun <T> loopFlow2(initial: T, f: suspend (T) -> Flow<T>): Flow<T> = flow {
    emit(initial)
    emitAll(f(initial).flatMapMerge { loopFlow2(value, it) })
}
However, I like this, I think it’s going into the right direction 🙂
I’m gonna put a reminder as I’m going on holidays 😅
👍 1
g

Giorgos Neokleous

12/02/2020, 5:35 PM
Happy Holidays 🙂
l

louiscad

12/02/2020, 7:10 PM
@pablisco I think writing a proper KDoc for that function will help you (or us) to find how to implement it.
p

pablisco

12/02/2020, 7:57 PM
I'm not sure what else, other than what I put originally, would I add to a kdoc 😅
l

louiscad

12/02/2020, 7:59 PM
You would write it starting with "the idea of" ? 🙃
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