MeepSpec.scala

import org.scalacheck._, Prop.forAll, Gen._, Arbitrary._

object MeepSpec extends Properties("Meep") {
  case class Meep(id: Int, name: String)
  implicit lazy val arbMeep: Arbitrary[Meep] = Arbitrary(for (i <- arbitrary[Int]; n <- alphaStr) yield Meep(i, n))

// using ScalaCheck 1.12.5, this outlandish property would have passed:
  property("meep1") = forAll { (f: Meep => Meep, m1: Meep, m2: Meep) => f(m1) == f(m2) }

// using ScalaCheck 1.13.0, we have to define how function input influences function output via an instance of Cogen:
  implicit lazy val coMeep: Cogen[Meep] = Cogen(_.id)

// `coMeep` is used to create `f`, so that in this example, only the `id` of the Meep input is "mixed" into the seed
// of the deterministic random generator used to generate the Meep output (of `f`), effectively emulating functions
// that do nothing with the `name` field of a Meep:

  property("meep2") = forAll { (f: Meep => Meep, m: Meep, s: String) => f(m) == f(Meep(m.id, s)) }
}