fpis practise chapter 3

gistfile1.txt

object chapter3 {

  println("chapter 3")                            //> chapter 3

  import Basket._

  val emptyBag = Empty                            //> emptyBag  : Basket.Empty.type = Empty

  val intBag = Cons[Int](23, Cons[Int](12, Empty))//> intBag  : Basket.Cons[Int] = Cons(23,Cons(12,Empty))

  // sum(intBag)
  
  import Basket.Bag._
  
  val bagInts = Bag(1,2,3)                        //> bagInts  : Basket.Bag[Int] = Cons(1,Cons(2,Cons(3,Empty)))
 
 	sum(bagInts)                              //> res0: Int = 6
}

object Basket {

  trait Bag[+A]

  case object Empty extends Bag[Nothing]

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

  object Bag {
    def apply[A](a: A*): Bag[A] = {
      if (a.isEmpty) Empty
      else Cons(a.head, apply(a.tail: _*))
    }
    def sum(bag: Bag[Int]): Int = {
      bag match {
        case Empty      => 0
        case Cons(h, t) => h + sum(t)
      }
    }
  }

}