reduceOrNull

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

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

val numbers = arrayOf(1, 2, 3, 4, 5)
val sum = numbers.reduceOrNull { acc, n -> acc + n }
println(sum)   // prints 15

Source