use scalaz's === for adt's

Question

I have just started to learn scalaz from eugene's blog post. I am trying to use === in my code but it returns a compile error value === is not a member of object Red

Here's is my code

import scalaz._

sealed trait TrafficLight
case object Red extends TrafficLight
case object Yellow extends TrafficLight
case object Green extends TrafficLight

object Equality {

import Scalaz._

def eqInstance[A](f: (A,A) => Boolean): Equal[A] = new Equal[A]{
    def equal(a1: A, a2: A): Boolean = f(a1,a2)    
}

def trafficLightEqualFunction(a1: TrafficLight, a2: TrafficLight): Boolean = (a1, a2) match {
    case (Red, Red) => true
    case (Yellow, Yellow) => true
    case (Green, Green) => true
    case _ => false
}

implicit val trafficLightEqual: Equal[TrafficLight] = eqInstance(trafficLightEqualFunction)

def main(args: Array[String]){
    println(Red === Red)
}

}

Answer 1

That's exactly what his blog complains. The type of Red is Red.type and there is no instance of scalaz.Equal for this type because Equal is invariant, so despite:

Red <: TrafficLight

it doesn't follow

Equal[Red] <: Equal[TrafficLight].

You can amend that by making Scala see Red as TrafficLight:

println((Red:TrafficLight) === (Red:TrafficLight))