Main.scala

import cats._

object Moneys:
  opaque type Money = Double

  object Money:
    def apply(d: Double): Money = d

  given Monoid[Money] = {
    import cats.implicits._
    summon[Monoid[Double]]
  }

@main def main: Unit =
  import cats.implicits._
  import Moneys._
  println(Money(1.0) |+| Money(2.0))