area.scala

class Elipse(val width: Double, val height: Double)

// circle is a special case of elispse where width and height are the same
class Circle(r: Double) extends Elipse(r, r)


// typeclass to calculate the Area of a T
trait Area[-T] {
  def area(t: T): Double
}

// given a way to calculate the area of a circle (a), determine if the left circle is bigger
def leftIsBiggerCircle(a: Area[Circle])(left: Circle, right: Circle): Boolean = {
  val leftArea = a.area(left)
  val rightArea = a.area(right)

  return leftArea > rightArea
}

// here is a way to calculate the area of any elipse (and remember a circle is an elipse)
val elipseArea = new Area[Elipse] {
  def area(e: Elipse): Double = e.height * e.width * 3.14
}

// I can call my leftIsBiggerCircle function using elsipseArea, because Area[Elipse] is a subtype of Area[Circle]
// if you were to redifine Area[-T], as just Area[T], this won't compile
println(leftIsBiggerCircle(elipseArea)(new Circle(2.0), new Circle(1.5)))