Functions are first-class citizens in Kotlin

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fun add(a: Int, b: Int) = a + b

fun subtract(a: Int, b: Int) = a - b

fun calculator(a: Int, b: Int, sum: (Int, Int) -> Int): Int {
    return a + b + sum(a, b)
}

fun main(args: Array<String>) {
    val functions = mutableListOf(::add, ::subtract)

    println(functions[0])
    // fun add(kotlin.Int, kotlin.Int): kotlin.Int

    println(functions[0](12,30))
    // 12+30 = 42

    println(functions[1](57,15))
    // 57-15 = 42

    println(calculator(11, 10, { a, b -> sum(a, b) }))
    // 11+10+(11+10) = 42
}