Monoidppoi.scala

trait Monoid[A] {
  def zero: A
  def append(x: A, y: A): A
}

trait MonoidFunctions {
  implicit class MonoidFunction[A: Monoid](x: A) {
    val m = implicitly[Monoid[A]]

    def |+|(a: A) = m.append(x, a)
  }
}

trait MonoidInstances {
  implicit val intMonoid = new Monoid[Int] {
    def zero = 0
    def append(a: Int, b: Int) = a + b
  }

  implicit val stringMonoid = new Monoid[String] {
    def zero = ""
    def append(a: String, b: String) = a + b
  }
}

object Monoid extends MonoidInstances with MonoidFunctions


/*
scala> import Monoid._
import Monoid._

scala> 1 |+| 1
res1: Int = 2

scala> "a" |+| "b"
res2: String = ab

scala> 1.0 |+| 1.0
<console>:15: error: could not find implicit value for evidence parameter of type Monoid[Double]
              1.0 |+| 1.0
              ^
*/