Demonstrate contravariant Functor with Show typeclass without using Cats

ShowMeTheMoneyApp.scala

package org.teckhooi

import scala.language.implicitConversions

trait Show[A] {
  def show(a: A): String
}

trait ShowOps {
  def show: String
}

implicit def showOps[A: Show](a: A): ShowOps = new ShowOps {
  override def show: String = implicitly[Show[A]].show(a)
}

trait ContraMap[F[_]] {
  def contraMap[A, B](fa: F[A])(f: B => A): F[B]
}

case class Money(amount: Int)
case class Salary(size: Money)

object ShowMeTheMoneyApp extends App {

  implicit val showContraMap: ContraMap[Show] = new ContraMap[Show] {
    override def contraMap[A, B](fa: Show[A])(f: B => A): Show[B] = (b: B) => fa.show(f(b))
  }

  implicit val showMoney: Show[Money] = (m: Money) => s"$$${m.amount}"

  implicit val showSalary: Show[Salary] =
    showContraMap.contraMap[Money, Salary](showMoney)(salary => salary.size)

  println(Salary(Money(1000)).show)
}