Recursive ADT

List.kt

package dev.kioba.ilist

sealed class IList<A>
class INil<A> : IList<A>()
class ICons<A>(val head: A, val tail: IList<A>) : IList<A>()

fun <A> iListOf(vararg arg: A): IList<A> = 
  arg.foldRight<A, IList<A>>(INil()) { element, acc -> ICons(element, acc) }

fun <A> IList<A>.forEach(f: (A) -> Unit): Unit = 
  fold(Unit) { element, _ -> f(element) }

fun <A, B> IList<A>.map(f: (A) -> B): IList<B> =
    foldR<A, IList<B>>(INil()) { element, acc -> ICons(f(element), acc) }

fun <A, B> IList<A>.foldR(initial: B, f: (A, B) -> B): B = when (val it = this) {
    is INil -> initial
    is ICons -> f(it.head, it.tail.foldR(initial, f))
}

fun <A, B> IList<A>.fold(initial: B, f: (A, B) -> B): B = when (val it = this) {
    is INil -> initial
    is ICons -> it.tail.fold(f(it.head, initial), f)
}