Gist as reply for https://devstyle.pl/2018/04/30/jak-i-po-co-pisac-funkcyjnie-w-c-sharp

FirstClassFunctions.kt

//Type inference, global variable and function as object
var x = fun(x:Int, y:Int): Int {
    return x + y
}

//Top level functions
fun main(args: Array<String>) {
    println("Hello world")
}

//Function as parameter and expression-bodied methods
fun adding(a: Int, b: Int, addingFunction: (Int, Int) -> Int) = addingFunction.invoke(a, b)

fun example1() {
    //Nested functions
    fun increment(i: Int, incrementFunction: (i: Int) -> Int) = incrementFunction.invoke(i)
    //Beautiful notation
    val incrementedValue = increment(1) { it + 1 }
}

//Returning functions
fun returningFunction() = fun(s: String) = s.length