reduceIndexedOrNull

Accumulates value starting with the first element and applying operation from left to right to current accumulator value and each element with its index in the original sequence.

inline fun <S, T : S> Sequence<T>.reduceIndexedOrNull(operation: (index: Int, acc: S, T) -> S): S?(source)

fun main() {
    // Non‑empty sequence: weighted sum = 0*10 + 1*20 + 2*30 = 80
    val weightedSum = listOf(10, 20, 30).asSequence()
        .reduceIndexedOrNull { index, acc, value -> acc + value * index }

    println(weightedSum)   // prints: 80

    // Empty sequence: returns null
    val emptySum = emptySequence<Int>()
        .reduceIndexedOrNull { index, acc, value -> acc + value * index }

    println(emptySum)      // prints: null
}

Source