scalaAgdt.scala

// AGDT - Algebraic Data type 
// List implementation
//
sealed trait  AList[+A]
case object ANil extends  AList[Nothing]
case class  Cons[A](head: A, tail: AList[A]) extends AList[A]

scala> Cons(1, Cons(10, Cons(30, ANil)))
res74: Cons[Int] = Cons(1,Cons(10,Cons(30,ANil)))

// Singleton object - Works like a ML-module.
object AList {

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


scala> val xs =  Cons(1, Cons(2, Cons(3, ANil)))
xs: Cons[Int] = Cons(1,Cons(2,Cons(3,ANil)))

scala> AList.sum(xs)
res81: Int = 6