Example Semigroup and Monoid Typeclasses

typeclasses.scala

// Paste this into the scala shell after typeing `:paste`

trait Semigroup[A] {
  def combine(x: A, y: A): A
}

object Semigroup {
  def apply[A](implicit ev: Semigroup[A]): Semigroup[A] = ev

  private implicit class SemigroupOps[A](val lhs: A) extends AnyVal {
    def combine(rhs: A)(implicit semi: Semigroup[A]): A =
      semi.combine(lhs, rhs)

    /** Alias for `combine` */
    def |+|(rhs: A)(implicit semi: Semigroup[A]): A =
      semi.combine(lhs, rhs)
  }

  trait SemigroupSyntax {
    implicit def toSemigroupOps[A: Semigroup](lhs: A): SemigroupOps[A] =
      new SemigroupOps(lhs)
  }

  object syntax extends SemigroupSyntax

  implicit val intSemigroup: Semigroup[Int] = (a, b) => a + b

}

trait Monoid[A] extends Semigroup[A] {
  def empty: A
}

object Monoid {
  def apply[A](implicit ev: Monoid[A]): Monoid[A] = ev

  object syntax extends Semigroup.SemigroupSyntax

  implicit val stringMonoid: Monoid[String] = new Monoid[String] {
    def combine(x: String, y: String): String = x + y

    def empty: String = ""
  }
}