Three flavours of Kotlin fold...

folding.kt

fun <A, B> foldLeft(l: List<A>,
                    seed: B,
                    func: (b: B, a: A) -> B) : B {

    return when {
        l.isEmpty() -> seed
        else        -> foldLeft(l.tail, func(seed, l.head), func)
    }
}

fun <A, B> foldRightOverflows(l: List<A>,
                              seed: B,
                              func: (a: A, b: B) -> B) : B {

    return when {
        l.isEmpty() -> seed
        else -> func(l.head, foldRightOverflows(l.tail, seed, func))
    }
}

fun <T> foldRight(l: List<T>,
                  seed: T,
                  func: (a: T, b: T) -> T) : T {

    return foldLeft(l.asReversed(), seed, func)
}