typeclasses.scala

// Our interface
trait Monoid[A] {
  def zero: A
  def plus(a: A, b: A): A
}

// Implementation for integers
implicit object IntegerMonoid extends Monoid[Int] {
  override def zero: Int = 0
  override def plus(a: Int, b: Int): Int = a + b
}

// Implementation for strings
implicit object StringMonoid extends Monoid[String] {
  override def zero: String = ""
  override def plus(a: String, b: String): String = a.concat(b)
}

// Could be implementation for custom classes, etc..

// Our generic function that knows which implementation to use based on type parameter 'A'
def sum[A](values: Seq[A])(implicit ev: Monoid[A]): A = values.foldLeft(ev.zero)(ev.plus)