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polyglotpiglet/ContravariantFunctorExample.scala

Created April 6, 2020 20:08
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ContravariantFunctorExample.scala
// has a contramap method
trait MyContravariantFunctor[F[_]] {
def contramap[A, B](fa: F[A], f: B => A): F[B]
}
// We can use contramap to build a show for Salary from the show for Money
import cats._
import cats.implicits._
case class Money(amount: Int)
case class Salary(size: Money)
implicit val showMoney: Show[Money] = Show.show(m => s"$$${m.amount}")
implicit val showSalary: Show[Salary] = showMoney.contramap(_.size)
println(Money(5).show)
println(Salary(Money(50)).show)
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