Flat family

Say you have a list of other lists. You probably got it from different database queries, or maybe from different configuration files:

val listOfLists = listOf(listOf(1, 2),
listOf(3, 4, 5), listOf(6, 7, 8))

// [[1, 2], [3, 4, 5], [6, 7, 8]]

And you want to turn them into a single list such as the following:

[1, 2, 3, 4, 5, 6, 7, 8]

One way to merge those lists is to write some imperative code:

val results = mutableListOf<Int>()

for (l in listOfLists) {
    results.addAll(l)
}

But calling flatten() will do the same for you:

listOfLists.flatten()

You can also control what happens with those results using flatMap():

println(listOfLists.flatMap {
    it.asReversed()
})

Note that in this case, it refers to one of the sublists. 

You can also decide to use flatMap() for type conversions:

println(listOfLists.flatMap {
    it.map { it.toDouble() }
//  ^        ^
// (1)      (2)
})

The preceding code prints the following output:

[1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0]

We converted all integers...