[Kotlin 1.4] Suspend conversion on callable references is supported

From Kotlin 1.4, suspend conversion on callable references is supported 🎉 Release note doesn't say much about this feature but it's really helpful when you use Flow#combine().

Let's pretend you want to combine two flows and use them in collect(). Before Kotlin 1.4, you may write code like this:

suspend fun before4_1() {
    combine(
        flowA, flowB
    ) { a, b ->
        a to b
    }.collect {
        val a = it.first
        val b = it.second
        println("$a and $b")
    }
}

val flowA = flowOf(1, 2)
val flowB = flowOf(3, 4)

This code is kind of redundant and doesn't look great (too much as and bs! 😩)

From Kotlin 1.4, you can simplify the above code.

suspend fun from1_4() {
    combine(
        flowA, flowB, ::Pair
    ).collect { (a: Int, b: Int) ->
        println("$a and $b")
    }
}

val flowA = flowOf(1, 2)
val flowB = flowOf(3, 4)

Before Kotlin 1.4, this code causes compile error because the type of the 3rd parameter of combine() is suspend function. Pair class constructor is not a suspend function so compile error will occur. From Kotlin 1.4, this 3rd parameter lambda will be converted to suspend function automatically and work fine.

Bounus: I use destructuring declarations for the parameter of collect().

    ).collect { (a: Int, b: Int) ->

We can name values of pair.first and pair.second here. Type definition can be omitted but () is necessary.