Scala 関数型デザイン&プログラミング:Exercize3.1

続き

サンプルコード

package fpinscala.datastructures

sealed trait List[+A]

case object Nil extends List[scala.Nothing]

case class Cons[+A](head: A, tail: List[A]) extends List[A]

object List {

  def sum(ints: List[Int]): Int = ints match {
    case Nil         => 0
    case Cons(x, xs) => x + sum(xs)
  }

  def product(sd: List[Double]): Double = sd match {
    case Nil          => 1.0
    case Cons(0.0, _) => 0.0
    case Cons(x, xs)  => x * product(xs)
  }

  def apply[A](as: A*): List[A] =
    if (as.isEmpty) Nil else Cons(as.head, apply(as.tail:_*))
}

があるときに、下記の結果はなんだ！？

  val x = List(1,2,3,4,5) match {
    case Cons(x, Cons(2, Cons(3, Cons(4, _)))) => x
    case Nil => 42
    case Cons(x, Cons(y, Cons(3, Cons(4, _)))) => x + y
    case Cons(h, t) => h + List.sum(t)
    case _ => 101
  }

見たまんま