1 package Main

     2 

     3 abstract class SemiGroup[A] {

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

     5 }

     6 

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

     8   def unit: A

     9 }

    10 

    11 object ImplicitTest extends App {

    12   implicit object StringMonoid extends Monoid[String] {

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

    14     def unit: String = ""

    15   }

    16 

    17   implicit object IntMonoid extends Monoid[Int] {

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

    19     def unit: Int = 0

    20   }

    21 

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

    23     if (xs.isEmpty) m.unit

    24     else {

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

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

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

    28     }

    29 

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

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

    32 }