inline fun <S, T : S> Sequence<T>.reduceOrNull(
    operation: (acc: S, T) -> S
): S?

Accumulates value starting with the first element and applying operation from left to right to current accumulator value and each element.

Returns null if the 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 an element, and calculates the next accumulator value.

The operation is terminal.