Please take a look at this function at jetpack com...

# compose

[Kazemihabib1996]

Please take a look at this function at jetpack compose sample:
https://github.com/android/compose-samples/blob/master/JetNews/app/src/main/java/com/example/jetnews/ui/home/HomeScreen.kt#L97

@Composable
private fun HomeScreenTopSection(post: Post) {
    ProvideEmphasis(emphasis = EmphasisLevels().high) {
        Text(
            modifier = LayoutPadding(start = 16.dp, top = 16.dp, end = 16.dp, bottom = 0.dp),
            text = "Top stories for you",
            style = MaterialTheme.typography().subtitle1
        )
    }
    Ripple(bounded = true) {
        Clickable(onClick = {
            navigateTo(Screen.Article(post.id))
        }) {
            PostCardTop(post = post)
        }
    }
    HomeScreenDivider()
}

As you see it's an incomplete composable function because we are calling 3 other functions without a Row or Column, and HomeScreenTopSection can draw correctly just because the parent has called it inside a column:
https://github.com/android/compose-samples/blob/master/JetNews/app/src/main/java/com/example/jetnews/ui/home/HomeScreen.kt#L88

Column(modifier = modifier) {
            HomeScreenTopSection(post = postTop)
            HomeScreenSimpleSection(posts = postsSimple)
            HomeScreenPopularSection(posts = postsPopular)
            HomeScreenHistorySection(posts = postsHistory)
        }

So if I call HomeScreenTopSection outside a column or row, there are multiple possible solutions:
1) the compose framework can decide to put it inside a Row for example
2) Keep the first one and throw away others and seems that compose follows this approach at the moment.
3) Threw error

I think first and second solutions are not appropriate solutions and seems that throwing compile time error is not possible and just remains run time error.

Our functions return Unit, if they return something the HomeScreenTopSection should return a list and the problem is solved we can throw compile time errors.
but with returning functions we loose lots of its benefits so that's not an option here,
but what about adding annotations like ThisShouldBeCalledInsideRowOrColumn (I know it's a worst naming you ever seen 😬) ?

[Leland Richardson [G]]

this is not a runtime exception because all contexts in which you are able to call a composable are contexts that have a layout policy implied by the parent

[Leland Richardson [G]]

as a result, HomeScreenTopSection may have a different behavior if you put it inside of a Row vs putting it inside of a Column

[Leland Richardson [G]]

but it is always valid

[Kazemihabib1996]

Yes, but is it valid if I call it outside of Row or Column? and nothing prevents me from doing that.

[Leland Richardson [G]]

if you call it inside of a composable function, a layout policy exists

[Leland Richardson [G]]

and that is guaranteed

[Leland Richardson [G]]

compose is building a tree

[Leland Richardson [G]]

and all nodes in the tree allow an arbitrary number of children

[Leland Richardson [G]]

and as such, any composable function can always create any number of nodes in the tree at that location

[Leland Richardson [G]]

basically, in terms of the tree that is built, there is no difference between

Column {
  Node()
  Node()
  Node()
}

and

Column {
  Nodes()
}

Where

@Composable fun Nodes() {
  Node()
  Node()
  Node()
}

[Leland Richardson [G]]

and both are equally valid

[Leland Richardson [G]]

this would be a problem if some nodes in the tree only allowed or expected a single child node, but we have explicitly decided that allowing an arbitrary number of children is a required contract for nodes in the tree

[Kazemihabib1996]

Got it, Thanks