package Main

abstract class SemiGroup[A] {

  def add(x: A, y: A): A

}

abstract class Monoid[A] extends SemiGroup[A] {

  def unit: A

}

object ImplicitTest extends App {

  implicit object StringMonoid extends Monoid[String] {

    def add(x: String, y: String): String = x concat y

    def unit: String = ""

  }

  implicit object IntMonoid extends Monoid[Int] {

    def add(x: Int, y: Int): Int = (x + y)

    def unit: Int = 0

  }

  def sum[A](xs: List[A])(implicit m: Monoid[A]): A =

    if (xs.isEmpty) m.unit

    else {

      println("head: " + xs.head);

      println("tail: " + xs.tail);

      m.add(xs.head, sum(xs.tail))

    }

  println(sum(List(1, 2, 3)))

  println(sum(List("a", "b", "c")))

}