I read somewhere that supporting `try-catch` insid...

# compose

[Youssef Shoaib [MOD]]

I read somewhere that supporting try-catch inside composables was planned. Is that still being worked on? I'm trying to use Arrow's Raise with Compose and Molecule, which I guess makes it a doubly-niche use case.

[Stylianos Gakis]

Where would you be doing such a thing in composition and not inside an effect?
Even in molecule and especially with arrow + raise I am having a hard time to see where you’d need to do a try/catch in composition

[Youssef Shoaib [MOD]]

E.g. if I want to do a complicated filtering of values. Yes I can just use Option, but I'd like to instead have the ability to just do raise(None).
Sorry if it wasn't clear, but I mentioned try-catch because that's what Raise uses to function. I'm not directly using that, I'm instead `raise`ing inside an option block

[Stylianos Gakis]

I think I’d like to see some code for this because I still don’t see why you’d raise an error completely out of your molecule instead of handling it inside there and have the result of that be the output of your molecule.

[Youssef Shoaib [MOD]]

This very basic example works for some reason:

@Test
  fun filtering2() = runTest {
    lateinit var itemChannel : SendChannel<Int>
    val items = callbackFlow {
      itemChannel = this
      awaitClose()
    }
    val flow = moleculeFlow(RecompositionMode.Immediate) {
      option {
        val item by items.collectAsState(0)
        if (item == 2) raise(None) else item
      }
    }.filterIsInstance<Some<Int>>().map { it.value }
    flow.test(10.seconds) {
      awaitItem() shouldBe 0
      itemChannel.send(1)
      awaitItem() shouldBe 1
      itemChannel.send(2)
      itemChannel.send(3)
      awaitItem() shouldBe 3
      cancel()
    }
  }

However, when trying to extract the plumbing out of it, I get a `ComposeRuntimeError`:

@Test
fun filtering2() = runTest {
  lateinit var itemChannel : SendChannel<Int>
  val items = callbackFlow {
    itemChannel = this
    awaitClose()
  }
  val flow = myMoleculeFlow {
    val item by items.collectAsState(0)
    if (item == 2) raise(None) else item
  }
  flow.test(10.seconds) {
    awaitItem() shouldBe 0
    itemChannel.send(1)
    awaitItem() shouldBe 1
    itemChannel.send(2)
    itemChannel.send(3)
    awaitItem() shouldBe 3
    cancel()
  }
}
inline fun <T> myMoleculeFlow(
  crossinline body: @Composable OptionRaise.() -> T
) = moleculeFlow(RecompositionMode.Immediate) {
  option {
    body()
  }
}.filterIsInstance<Some<T>>().map { it.value }

[Youssef Shoaib [MOD]]

Seemingly, I can't ever catch an exception coming out of a composable, even if that composable is being inlined, but I can have exceptions inside a composable function? I'm not entirely sure how to achieve what I want here. I hope that at least the example makes sense: I want to achieve filtering

[Stylianos Gakis]

What you see as an annotation internally changes the shape of that lambda more than you may think (I've only read about it too, don't know details), but yeah I don't think they expect exceptions to be thrown out.

But as you can see if you never throw an exception through a composable "scope" (if that makes any sense) maybe that should be fine

[Stylianos Gakis]

I still feel like you can surely do this without the exceptions through the composables right? 😅

[Youssef Shoaib [MOD]]

I know compose does crazy things, I'm just surprised that inline doesn't play that nicely with it (my expectation would be that manually inlining an inline method shouldn't change whether the code works or not). I wonder if there's a "compose way" to do this.

[Stylianos Gakis]

I'm still not 100% sure what this is here when you say a compose way to do this.
If you show like the real use case here you have with the existing code it might be much easier to discuss

[Youssef Shoaib [MOD]]

The issue is that I'm creating something kind of abstract: I'm trying to create something akin to monad comprehensions. I'm thus trying to have an easy way of filter ing unwanted values.

[Stylianos Gakis]

If it's just to return null inside a molecule flow, you can just do that operation inline as normal Kotlin code, without a raise or whatever 🤷‍♂️

[Stylianos Gakis]

Yeah, and can that not happen inside the presenter itself? Or with a normal non-compose function which takes input and returns the filtered output back?

[Youssef Shoaib [MOD]]

Yeah that's true, it just runs into the same issue though that Raise is intending to solve: I would have to explicitly pass on the null value from any other possibly-filtering function I callo
> normal non-compose function which takes input and returns the filtered output back
The issue is that I want to filter based on flows that were `collectAsState`ed.
I want to be able to basically "cancel" a current composition if you will. Maybe that's the terminology I should be using. I want to cancel the current composition because it can't produce a useful value, and instead I'd like to wait until another state change causes a recomposition

[Stylianos Gakis]

"cancel a current recomposition", so do you want it to return nothing? Because that may be going against what compose wants to do in the first place. Instead of "cancelling" it shouldn't you just short circuit your calculation and return the "none" case, whatever that may be

[Youssef Shoaib [MOD]]

Well exactly, I want to return a none case, but I either have to do that explicitly by wrapping in Option, which is inelegant, or with exceptions and Raise, which compose doesn't support easily

[Stylianos Gakis]

but why does the function that is an extension on Raise need to be a composable itself? And not a normal function?

[Youssef Shoaib [MOD]]

That's... very true actually. I guess that function can either run its own molecule and expose that as a Flow, or it can be a Composable and return an Option. The annoying bit, I guess, is that both aren't possible at once, but I guess that's not that significant of a drawback

[Pablichjenkov]

Personally I prefer not using exceptions in the UI code. Makes the code not so appealing.

[Stylianos Gakis]

This was for molecule, so it wasn't UI code.

[Pablichjenkov]

Ah sorry didn't go through the thread tbh, just so the headline and went straight to comment.
I had seen code try/catching in Composable or view related code and wanted to express that is not good looking.
I am pretty sure there are other opinions but in my case I would avoid any exception handling in Composable UI/View land

[Stylianos Gakis]

Yes, but again, this isn't about compose UI 😅

[Pablichjenkov]

Sorry 😁