Example of Contravariant Predicate

Predicate.sc

 import $ivy.`org.typelevel::cats-core:1.5.0`
 import cats.Contravariant
 import cats.implicits._

{
  trait Predicate[A] {
    def getPredicate(a:A): Boolean
  }
  object Predicate {
    def apply[A](f: A => Boolean): Predicate[A] = new Predicate[A]{ def getPredicate(a: A) = f(a) }
    def and[A](p1: Predicate[A], p2: Predicate[A]): Predicate[A] = Predicate( a => p1.getPredicate(a) && p2.getPredicate(a))
    implicit val instance: Contravariant[Predicate] = new Contravariant[Predicate] {
      def contramap[A,B](fa: Predicate[A])(f: B => A): Predicate[B] = Predicate {(b:B) => fa.getPredicate(f(b)) }
  }
  }
  }
import Predicate._
case class Person(name: String, age: Int, iq: Int)
val isAdult: Int => Boolean = _ > 18
val isSmart: Int => Boolean = _ > 90
val isAdultP: Predicate[Person] = instance.contramap(Predicate(isAdult))(_.age)
val isSmartP: Predicate[Person] = instance.contramap(Predicate(isSmart))(_.iq)
val isSmartAdultP: Predicate[Person] = Predicate.and(isAdultP,isSmartP)

isSmartAdultP.getPredicate(Person("benji", 5, 50)) //false
isSmartAdultP.getPredicate(Person("benji", 5, 100)) //false
isSmartAdultP.getPredicate(Person("benji", 19, 100))//true
isSmartAdultP.getPredicate(Person("benji", 19, 10)) //false