Implementing a functional list

With everything that we've learned in the first two chapters, we can implement a pure functional list:

sealed class FunList<out T> {
   object Nil : FunList<Nothing>()

   data class Cons<out T>(val head: T, val tail: FunList<T>) : FunList<T>()
}

The FunList class is a sealed class; just two possible subclasses exist—Nil, an empty list (in other books you can see this defined as Null or Empty) and Cons (a construct, name inherited from Lisp, that holds two values).

The T type is marked out; this is for variance, which we'll cover variance in future chapters.

Nil is an object (we don't need different instances of Nil) extending FunList<Nothing> (remember that Nothing is the bottom of Kotlin's type hierarchy).

The Cons value contains two values—head, a single T, and...