Basically, I want the same thing as Clojure’s flatten: user=> (flatten [1 2 3 4]) (1 2 3 4) user=> (flatten [1 [2 [3] 4] 5]) (1 2 3 4 5)

It is quite a common thing to do for, say, Collections, Lists: private fun <E> flatten(list: List<E>) = mutableListOf<E>().apply { val queue = LinkedList(list) while (!queue.isEmpty()) { val e = queue.remove() when (e) { is List<*> -> queue.addAll(e as List<E>) else -> add(e) } } } This works just fine (apart from 1 unchecked cast) for any collection of any elements. I’d like to implement the same thing, but for Sequences

My initial thought was: oh, that is an easy thing to do, just replace all List in the method above with Sequence:

private fun flatten(seq: Sequence<Any?>) = mutableListOf<Any?>().apply {
        val queue = LinkedList<Any?>().apply { addAll(seq); }
        while (!queue.isEmpty()) {
            val e = queue.remove()
            when (e) {
                is Sequence<*> -> queue.addAll(e)
                else -> add(e)
            }
        }
    }

But this doesn’t work as expected