Hi! kotlin.sequences package has a flatten method...

# announcements

[kovrik]

Hi!
kotlin.sequences package has a flatten method (https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.sequences/flatten.html).
But it works only on a sequence of sequences (Sequence<Sequence<T>>).
Are there any alternatives that will work on any sequence (Sequence<T>)?
Something that behaves more like a flatten function in Clojure.

[orangy]

I’m not sure what you think it should do? flatten gets inner sequences and joins (flattens) them together. Like ( (1,2), (3,4) ) to (1,2,3,4). For a generic T, what should it do?

[kovrik]

For a sequence of (1,2,3,4) it should return (1,2,3,4), which is already flat.

[kovrik]

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)

[orangy]

That’s get us into a land of dynamic typing, no?

[kovrik]

Well, kind of. It just becomes Sequence<Any?> as we don’t know (anymore) if given seq element is another sequence or some other object.
But that is fine by me, in that particular case I’d be happy with Sequence<Any?>

[kovrik]

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

[kovrik]

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

[kovrik]

println(flatten(listOf(1, listOf(2, 3).asSequence(), 4).asSequence()))   // prints [1, 4, 2, 3], not [1, 2, 3, 4]

[orangy]

Well, again, there is no such function in standard library. You can easily write your own 🙂

[kovrik]

yeah, just wanted to confirm. Anyway, thanks for help! 🙂