Structural typing in Scala

gistfile1.scala

case class Circle(radius: Double) {
  def area = radius * radius * Math.PI
}

case class Rectangle(side: Double) {
  def area = side * side
}

case class Triangle(base: Double, height: Double) {
  def area = 0.5d * base * height
}

These classes do not share a common parent type (aside from AnyRef, which is Scala's equivalent of java.lang.Object). They don't implement a common interface. But we can still get strong typing via structural typing -- we declare a dependency on the method they have in common. They each have a method named area which takes no arguments and returns a Double.

def areaPrinter(thing: { def area: Double }) =
  println(s"thing has area ${thing.area}")

val circle = Circle(1.0)
val triangle = Triangle(1.0, 2.0)
val rectangle = Rectangle(4)

areaPrinter(circle)
areaPrinter(triangle)
areaPrinter(rectangle)

The parameter thing: { def area: Double } says that the areaPrinter method depends on something with a method named area which takes no arguments and returns a Double.

So the above examples would compile, but this would not:

areaPrinter("foobar")

since String doesn't have an area() method.