Example usage of Cats typeclasses that should be easy enough to follow

output.txt

Using default instances
false
false
Using custom Eq instance
true
Comparisons
0
1
-1
Show
Item name:Cat price:10.0
list is List(Item(Ant,1.0), Item(Mouse,5.0), Item(Cat,10.0), Item(Dog,20.0))
list is List(Item name:Ant price:1.0, Item name:Mouse price:5.0, Item name:Cat price:10.0, Item name:Dog price:20.0)
Enum example
false
true
a man is Gender Male

Typeclasses.scala

import cats._
import cats.data._
import cats.implicits._

object Typeclasses extends App {

  case class Item(name: String, price: Double)

  object Item {
    // note we are using SAM support to define these instances
    // https://gist.github.com/fancellu/713602e68746336b1dc41c9dab85d4a5
    implicit val eq: Eq[Item] = (x: Item, y: Item) => x == y
    implicit val order: Order[Item] = (x: Item, y: Item) => x.price.compare(y.price)
    implicit val show: Show[Item] = (x: Item) => s"Item name:${x.name} price:${x.price}"
  }

  val cat = Item("Cat", 10)
  val mouse = Item("Mouse", 5)
  val dog = Item("Dog", 20)
  val ant = Item("Ant", 1)

  println("Using default instances")
  println(cat === Item("Cat", 1000))
  println(cat === mouse)

  println("Using custom Eq instance")

  { //in scope Eq only cares about name
    implicit val ItemEqName: Eq[Item] = (x: Item, y: Item) => x.name == y.name
    println(cat === Item("Cat", 1000))
  }

  println("Comparisons")
  println(cat compare cat)
  println(cat compare mouse)
  println(cat compare dog)

  println("Show")
  println(cat.show)

  val items = List(cat, mouse, dog, ant).sorted // sorted by Item.order, i.e. price

  println(s"list is $items")
  //vs
  println(show"list is $items")

  println("Enum example")

  sealed trait Gender

  object Gender {

    private case object Male extends Gender

    private case object Female extends Gender

    private case object Unknown extends Gender

    // the horror of enums in Scala, sorry, need to upcast due to Eq nonvariant subtyping
    def male: Gender = Male

    def female: Gender = Female

    def unknown: Gender = Unknown

    implicit val eq: Eq[Gender] = (x: Gender, y: Gender) => x == y
    implicit val show: Show[Gender] = (x: Gender) => s"Gender $x"
  }

  println(Gender.male === Gender.female)
  println(Gender.male === Gender.male)
  println(show"a man is ${Gender.male}")

}