SpeakingCreatureATM.scala

// Inspiration : Existensial Types (Abstract type Member) Make OOP great again! Julien R. Foy 
// https://www.youtube.com/watch?v=6j5kZj17aUw
// Compare this with the type class version : https://gist.github.com/wibisono/28d8808159e0c7103042d7c5c668edf7
sealed trait Animal
case class Dog(name : String) extends Animal
case class Cat(name : String) extends Animal

trait Speaking {
  type A
  def speak(creature : A) : String
}

type SpeakingType[C] = Speaking {type A = C}


// Providing this instance only dog can speak 
implicit val speakingDog : SpeakingType[Dog] = new Speaking {
  type A = Dog

  def speak(dog: Dog): String = s"I am speaking dog ${dog.name}"
}

val lassie = Dog("Lassie")
val garfield = Cat("Garfield")

// Direct usage
speakingDog.speak(lassie)

object Speaking {
  def speak[C](creature : C)(implicit speakingC : SpeakingType[C]) = speakingC.speak(creature)
}

// Ok also with interface object
Speaking.speak(lassie)

//Speaking.speak(garfield)
//Error: ... could not find implicit value for parameter speakingC: SpeakingType[Cat]


// Now the interface syntax style for pimping up existing type with instances
object SpeakingSyntax {
  implicit class SpeakingCreature[C](creature : C) {
      def speak(implicit speakingC : SpeakingType[C]) = speakingC.speak(creature)
  }
}

import SpeakingSyntax._
lassie.speak

//garfield.speak
//Error: ... could not find implicit value for parameter speakingC: SpeakingType[Cat]

implicit val speakingCat : SpeakingType[Cat] = new Speaking {
  type A = Cat

  def speak(cat: Cat): String = s"I am speaking cat ${cat.name}"
}

// Only now garfield can speak
garfield.speak