Trailing Lambdas and Function References

If the last parameter is a function, the lambda can go outside the parentheses. If it is the only argument, the parentheses can be dropped. This is why Kotlin code (and Jetpack Compose) looks like Column { ... }.

fun measure(label: String, block: () -> Unit) {
    println("Start $label")
    block()
    println("End $label")
}

fun main() {
    measure("sync", { println("  working (inside parentheses)") })
    measure("sync") { println("  working (trailing lambda)") }
    listOf(1, 2, 3).forEach { print("$it ") }
    println()
}

Start sync
  working (inside parentheses)
End sync
Start sync
  working (trailing lambda)
End sync
1 2 3

::functionName turns an existing function into a value, so you can pass it where a lambda is expected. It also works for constructors (::User) and members (String::length).

data class User(val name: String)

fun isAdult(age: Int) = age >= 18

fun main() {
    val ages = listOf(12, 25, 17, 40)
    println(ages.filter(::isAdult))
    println(listOf("asha", "ravi").map(String::uppercase))
    println(listOf("Asha", "Ravi").map(::User))
    listOf("done").forEach(::println)
}

[25, 40]
[ASHA, RAVI]
[User(name=Asha), User(name=Ravi)]
done