How to destruct a Kotlin list into sublists?

Question

How can I destruct a list in Kotlin into two sublists? Currently I do something like this:

val (first, rest) = listOf("one", "two", "three")

But doing so, first is "one" and rest is "two". I want them to be first =["first"] and rest = ["two", "three"].

Is this even possible using this "destructor" syntax?

Answer 1

Destructuring translates to calling component1, component2, etc. operator functions on an object. In the case of a List, these are defined as extensions in the standard library, and return the Nth element respectively.

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You could define your own extension extension that splits the list as desired and returns a Pair, which can then be destructured:

fun <T> List<T>.split() = Pair(take(1), drop(1))

This could be used like so:

val (first, rest) = listOf("one", "two", "three").split()

println(first) // [one]
println(rest)  // [two, three]

Perhaps naming it something better than split would be smart though.

Answer 2

You could also define your own component functions:

operator fun <T> List<T>.component2(): List<T> = this.drop(1)

And then this works as expected:

val (head, rest) = listOf("one", "two", "three")
println(head) // "one"
println(rest) // ["two", "three"]