Is there a trick to using composable function refe...

# compose

[Mark Murphy]

Is there a trick to using composable function references?

@Preview
@Composable
fun ListifyLambda() {
  AdapterList(data = listOf("foo", "bar", "goo")) {
    ListItem(it)
  }
}

@Preview
@Composable
fun ListifyFnRef() {
  AdapterList(data = listOf("foo", "bar", "goo"), itemCallback = ::ListItem)
}

@Composable
fun ListItem(item: String) {
  Text(item)
}

Here, ListifyLambda() and ListifyFnRef() seem like they should be equivalent. ListifyLambda() compiles and runs without issue. ListifyFnRef() fails to compile, with:

Type mismatch: inferred type is KFunction1<String, Unit> but (String) -> Unit was expected

(dev13 and AS 4.2 C1)

It doesn't seem like the Kotlin itself is flawed. Function references that match the function type should be usable, and ListItem() seems to match the itemCallback . If you try swapping compatible lambda expressions and function references outside of composables, it works as expected.

I have run into this a few times and just wrote it off as being glitchy prerelease stuff, but https://stackoverflow.com/q/62362663/115145 spurred me to try to get to the bottom of this. Are we doing something wrong, or is this a bug/limitation of something (e.g., Kotlin compiler plugin)?

[Adam Powell]

known bug, afaik

[Adam Powell]

agreed that both should work

[Mark Murphy]

I didn't see it in the public issue tracker, though it's possible I'm searching for the wrong thing

[Adam Powell]

hmm, I know I've had discussions with the team about this one but it might not be tracked. Feel free to add one and we'll dupe it if we find an existing one, or if the existing one is internal we'll dupe that against the external one

[Mark Murphy]

filed as https://issuetracker.google.com/issues/158892712 -- thanks for the help!

[Leland Richardson [G]]

missed this over the weekend. i duped the bug with the existing one. definitely want this to work but it has proven challenging. i upped the bug’s priority though as i’m not sure we were tracking this as a priority item for the alpha timeframe

[Mark Murphy]

It's not the end of the world if we're stuck with lambda expressions. IIRC, you switched to changing the code at the call site, and I can see where function references could make that interesting.

Push come to shove, if you find that it's just not realistic to offer function reference support... if we could get a dedicated Lint error, or perhaps a clearer compile error, that would help.

[Leland Richardson [G]]

yeah i think we will try and have a compiler error if we can’t provide support so we can add support eventually in a “non-breaking” way