Examples of idiomatic Scala polymorphism

Polymorphism.scala

// Runtime polymorphism

trait Closeable {
  def name: String

  def close: String
}

case class Connection(override val name: String, database: String) extends Closeable {
  override def close: String = s"Closing connection $name"
}

case class File(override val name: String, opener: String) extends Closeable {
  override def close: String = s"Closing file $name"
}

def showClosing(c: Closeable): String = c.close

val w = Connection("MyConnection", "PostgreSQL")
val f = File("file.txt", "TextEdit")
showClosing(w)
showClosing(f)


// Parameteric polymorphism (generics) with List[T]

val numbers = List(1, 2, 3)
val firstNumber: Int = numbers.head

val strings = List("1", "2", "3")
val firstString: String = strings.head

def use[T <: File](t: T) = s"Using ${t.name}"

use(f)


// Typeclass polymorphism

case class Complex(real: Double, imaginary: Double)

object Complex {
  implicit object ComplexOrdering extends Ordering[Complex] {
    def compare(a: Complex, b: Complex): Int = a.real.compare(b.real)
  }
}

List(Complex(3, 5), Complex(10, 2), Complex(1, 6)).sorted