reduceOrNull
Accumulates value starting with the first character and applying operation from left to right to current accumulator value and each character.
Returns null if the char sequence is empty.
import kotlin.test.*
fun main(args: Array<String>) {
//sampleStart
val strings = listOf("a", "b", "c", "d")
println(strings.reduceOrNull { acc, string -> acc + string }) // abcd
println(strings.reduceIndexedOrNull { index, acc, string -> acc + string + index }) // ab1c2d3
println(emptyList<String>().reduceOrNull { _, _ -> "" }) // null
//sampleEnd
}Parameters
operation - function that takes current accumulator value and a character,
and calculates the next accumulator value.