Kotlin Programming Cookbook

How it works…

Let's take a look at the implementation of the flatten() function:

public fun <T> Iterable<Iterable<T>>.flatten(): List<T> {    val result = ArrayList<T>()    for (element in this) {        result.addAll(element)    }    return result}

As you can see, it's just adding the items from iterables (array or lists) in a new list and returning that list.