How to write Composable function using Kotlin Functional (SAM) Interface

Question

We can write functional interfaces in Kotlin like this - function-interfaces

fun interface Sum {
    fun add(a: Int, b: Int): Int
}

val sumImpl = Sum { a, b ->
    return@Sum a + b
}

val testSum = sumImpl.add(4, 5)

How can we write Jetpack Composable function in same way? Below code is not working. `

fun interface SampleText {
    @Composable
    fun text(data : String)
}

val textImpl = SampleText { data ->
    return@SampleText @Composable { Text(data) }
}

@Composable
fun testText() = textImpl.text("Data")

I have tried this as well, but this also didn't work.

fun interface SampleText {
    fun text(data : String) : @Composable () -> Unit
}

val textImpl = SampleText { data ->
    @Composable { Text(data) }
}

@Composable
fun testText() = textImpl.text("Data")

Answer 1

The first version is not compiling in its lambda form because your interface function returns a Unit and your'e actually having a Type mismatch error, its just weird the compiler reports Internal Error when you try to return a @Composable annotated function, but the issue becomes clear if you simply return something like a String.

vs

To solve your first version, either you fully declare an object of the class like this (though its useless since you want a lambda version of your SAM interface not an actual object in the first place)

val textImpl = object: SampleText {
    
    @Composable
    override fun text(data: String) {
        Text(data)
    }
}

, but it will work just by simply calling the testText() function like this.

testText()

Or change it to your second version.

---

Now for your second version, since your interface returns a @Composable lambda, you have to invoke it as well in the call-site, making two function invocations to make it work,

testText()() // two invocations

first call invokes your testText() function, second pair of parenthesis invokes the @Composable lambda from your interface.

Or simply call .invoke()

testText().invoke() // just call the .invoke() of the returned composable lambda

Either of the implementations and calls display the text "Data"