package fp.kotlin.example.chapter09.solution

import fp.kotlin.example.chapter09.Foldable
import fp.kotlin.example.chapter09.SumMonoid

/**
 *
 * 연습문제 9-10
 *
 * 리스트 모노이드를 ``Foldable`` 타입 클래스의 인스턴스로 만들어서 ``foldLeft`` 함수를 작성하고, ``foldMap`` 함수를 테스트해 보자.
 */
fun main() {

    val list1 = Cons(1, Cons(2, Cons(3, Cons(4, Nil))))

    require(list1.foldLeft(0) { acc, value -> acc + value } == 10)

    require(list1.foldMap({ x -> x + 1 }, SumMonoid()) == 14)
    require(list1.foldMap({ x -> x * 2 }, SumMonoid()) == 20)
}

sealed class FunList<out T> : Foldable<T>

object Nil : FunList<Nothing>() {
    override fun <B> foldLeft(acc: B, f: (B, Nothing) -> B): B = acc
}

data class Cons<out T>(val head: T, val tail: FunList<T>) : FunList<T>() {
    override fun <B> foldLeft(acc: B, f: (B, T) -> B): B = tail.foldLeft(f(acc, head), f)
}
