I apologise for all these dumb questions, but anyway.. Why does this work:

override fun loadData() {
        launch {
            songRepository.getSongs(SongQuery.Album(name = album.name, albumArtistName = album.albumArtist))
                .collect { songs ->
                    this@AlbumDetailPresenter.songs = songs
                    view?.setData(songs)
                }
        }
    }

But this doesn’t:

override fun loadData() {
        launch {
            songRepository.getSongs(SongQuery.Album(name = album.name, albumArtistName = album.albumArtist))
                .collect { songs ->
                    this@AlbumDetailPresenter.songs = songs
                }
            view?.setData(songs)
        }
    }

yes, I think so, not sure how it would work out coroutine-wise otherwise

Well, it seems like it doesn’t get called at all

are you sure

view

is non-null?

Yes. But in the second case,

view?.setData(songs)

is not called (breakpoint not hit in debugger) - even if

view

was null I’d still expect the line to be hit

I would verify with a logging statement, sometimes the debugger isn't perfect for Kotlin in my experience

Probably collect never finish

that is a good point

so first works just because it receives result to collect and notify, but second never passed collect function

I see, that makes sense

Check that you don’t use something like StateFlow in your getSongs method, it never completes

I am indeed using StateFlow!

it’s general rule of reactive frameworks, if you have stream it’s better do not expect that it will complete, because it’s an implementation detail, it may return 0-Infinite number of elements, and can never complete

Yeah, I’m not actually expecting it to complete - just a mistake in the braces placement - and then I forgot that it doesn’t complete

You also can use launchIn(), so you never have suspending collect, will work just as launching collection in background (exactly waht you are doing in this case by wrapping with launch), also makes it less nested:

songRepository.getSongs(...)
   .onEach { setData(it) }
   .launchIn(scope)

Oh yeah, that’s quite nice

not always mistake, but it means that you expect that you know contract of this flow, that it should complete at some point

I would usually use a suspend fun rather than a flow if I expect completion. But for some things, I want to emit elements along the way to completion

I wouldn’t call it mistake, sometimes you have a reson for this. For example recently in this Slack it was discussion about implementation of Spring Webflux API for database, which exposes select as stream of events, but not like usually you expect subsribe on data updates in table, but to make select of different rows lazy, so it completes when all rows are selected

Yeah, I see what you’re saying

But it may be a source of errors, especially when code changes, so you may be in situtation when code is changed but API of it is the same, it just exposes Flow

It would be nice if you could declare whether a Flow completes or not

But how would such type would look like?

Well, I’m not suggesting that the Coroutines API be changed, or that I have the answer. I just mean, it might be useful for this exact scenario

I suspect it would be somewhat similar to

Mono

in rx

It’s not only problem of coroutines, it’s true for any reactive-like type

Suspend functions aren’t the solution for this either - if you need intermediate emissions before completion

anyFlow..first() is the same as Mono

Yeah, sounds like Mono is not the solution to this problem then

I was suggesting a more general form of Mono, which implies a

0..N

How Mono is the solution?

it isn't, never mind me

which implies a 

0..N

This is the same what Flow or rxjava Observable or Flux is and it exactly the same problem

Tests!

Sure, tests are great. And I’m not really advocating for a ‘CompletingFlow’ type… But, tests aren’t the answer to everything. Yes, they would help to catch this error and that’s great. But just because you can write a test to catch a possible bug, doesn’t mean we couldn’t also make it harder to write the bug in the first place.

again, I just don’t understand how such type may look like. Is it just the same Flow, but with promise in name to complete?

Maybe?

I understand Mono/Single/single, it’s 1 or error, Observable/Flux/Flow it’s 0..N, even Maybe it’s 0..1, but what is completing flow?

Yeah, perhaps theres no sense in marking a Flow as ‘completable’, if that simply means it might complete in 75 years from now

that can really be true for any suspending op though

it’s true, and for any other function with Thread.sleep() or something like awaitTermination()

I am just lamenting that there’s no way for us to codify that a flow can be expected to complete. I have written some flows that scan all the directories on the device, emitting the directory name for each one it finds. I know this completes, and I do something when it completes. And then some flows monitor a database and emit new values when the database changes - and I know these never complete. There’s no way for me to indicate that this is the behaviour of these flows.

documentation comments 🙂

Now, I know you want to ask: How would one indicate this behaviour? Again, I don’t have an answer, and I’m not suggesting the API needs to change. Just discussing.

I can't think of a good way to codify it, maybe someone more experienced with language development could propose something. For now, I'm satisfied with reading documentation

Yeah, or even just the flows variable/method name

such way to represent completing event source doesn’t exist in any reactive library which I know

Right. But I’m not quite sure what your point is. Why does it matter what other libraries do?

More people who think about such problems, nobody ended up with solution for now, so it probably doesn’t have simple/good enough solution

All the more reason to discuss it then

As soon as you start working with event based frameworks, there’s no such thing as “this 0..N will complete because of its type so I can safely put code *afterwards*” What you can do it keep going with the flow (no puns intended). Are you using any reactive library? Good, use it. Do you need something to be executed when the flow completes? Go for `onCompletion` method Are you in an Rx library? Then you have `doOnComplete` method As I read in a presentation, unless you can model everything synchronously, a single piece of asynchronous code can break your model. So model everything event based

Yeah, I think that’s good advice. But still, forget about putting code ‘afterwards’ because of its type. The same problem still exists with

onCompletion()

- without looking at the stream, you don’t know whether this will ever be called.

But. as mentioned before, you can’t be sure that neither • the flow will emit a limited number of items so it will complete

That’s true. But we are left to look at the stream and draw these conclusions ourselves

Even if you had a type for that “CompletableFlow”, nothing prevents it from completing in several years

Yes, I agree - in fact, I made all the same points. Again, I’m just lamenting that you can’t codify this somehow. The fact that a consumer can expect a flow to complete - if you can indicate that somehow in a variable name, then I’m just entertaining the notion that you could indicate that with a type.

If I see a

filesInFolder

method I know for sure that it will finish in a reasonable amount of time

databaseChanges

will never finish (unless the app gets closed and some cleanup needs to be done)

time

(not sure why, but just as an example) will be there forever

Yeah, that’s exactly right

In the end… I think that it’s not a matter of having an specific type or name for it. It’s a matter of the mental model that async libraries “impose” (I can’t think of a better word 😅) The same way any developer will not put anything in a finally after a

while(true)

in imperative programming, any developer with the asynchronous model in the head should not rely on putting anything after a

collect

…should not rely on anything they put after collect being executed *

If you wanted to limit the “scope” of your

CompletableFlow

you could go for an inline class

inline class CompletableFlow<T>(private val flow: Flow<T>)

So you can have functions that only accept a

CompletableFlow

val flow: Flow<Int> = ...
val completable: CompletableFlow<Int> = ...
fun demo(c: CompletableFlow<Int>) = ...
demo(completable) // works
demo(flow) // doesn't work

I’m not sure about inline classes and generics, though

Timeout! Guaranteed to complete.

Now, I know you want to ask: How would one indicate this behaviour?

I think this would be a great use case for dependent types, which kotlin doesn't have and probably never will. https://en.m.wikipedia.org/wiki/Dependent_type

I think it’s overkill to solve this with dependent types

Is it? This thread is a discussion of how there isn't really a good way to express the "completability" of a flow in the type system. And expressing stuff like list size in the type of the list is a basic use case for dependent types, isn't it?

Even if something is completable in theory, it doesn’t make it completable on practice

I don't know what the solution would look like, my (very limited) exposure to DTs is with pure functional languages that don't have imperative calls at all, let alone suspending ones. But as a pattern match, the problem of "how to express some runtime property in the type system" sounds like a dependent types problem to me.

Thanks Zach. I won’t pretend to understand anything in this article, but it’s nice to know that a language feature exists out there that might solve at least similar problems to the one I’m describing