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)
}

}

Works for me on scala 2.11.7 and scalaz 7.1.3, produces: `true`. What version are you using? — Alvaro Carrasco, Jul 05 '16 at 15:59

score 1 · Accepted Answer · answered Jul 05 '16 at 16:04

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))

answered Jul 05 '16 at 16:04

pedrofurla

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Yes...I got that...made it work by doing `Red.asInstanceOf[TrafficLight] === Red.asInstanceOf[TrafficLight]` But, what is the solution? Both `Red:TrafficLight === Red.TrafficLight` and `Red.asInstanceOf[TrafficLight] === Red.asInstanceOf[TrafficLight]` looks bad – xeonzion Jul 05 '16 at 16:36
I like the solution he shows in the post: have you own constructors like `val red:TrafficLight = Red`. – pedrofurla Jul 05 '16 at 17:43

use scalaz's === for adt's

1 Answers1