Polymorphism

Polymorphism is the provision of a single interface to entities of different types.

Polymorphism types:

• Ad-hoc polymorphism: when a function has different implementations depending on a limited range of individually specified types and combinations. Ad-hoc polymorphism is supported in many languages using function overloading. All of the following works due to add operation (__add__ method) is implemented for list, float, int etc.

Ad-hoc polymorphism (Python)

    >>> [1, 2, 3] + [4, 5, 6]
    [1, 2, 3, 4, 5, 6]
    >>> 1 + 2
    3
    >>> 4.5 + 1
    5.5

• Parametric polymorphism (generics): when code is written without mention of any specific type and thus can be used transparently with any number of new types.

Parametric polymorphism (Scala)

    class Stack[T] {
      private var elements: List[T] = Nil

      def push(x: T): Unit = elements = x :: elements

      def isEmpty: Boolean = elements.isEmpty

      def pop: T = {
        val (x, xs) = (elements.head, elements.tail)
        elements = xs
        x
      }
    }

    val stackOfInts = new Stack[Int]
    // stackOfInts: Stack[Int] = Stack@4cd70290
    stackOfInts.push(1)
    stackOfInts.push(2)
    stackOfInts.push(3)
    stackOfInts.pop
    // res1: Int = 3
    stackOfInts.pop
    // res2: Int = 2

    val stackOfStrings = new Stack[String]
    // stackOfStrings: Stack[String] = Stack@2da3e198
    stackOfStrings.push("a")
    stackOfStrings.push("b")
    stackOfStrings.push("c")
    stackOfStrings.pop
    // res9: String = c
    stackOfStrings.pop
    // res10: String = b

• Subtyping: refers to compatibility of interfaces. A type Duck is a subtype of Bird if every function that can be invoked on an object of type Bird can also be invoked on an object of type Duck.

Subtyping (Scala)

  trait Bird {
    val sound: String
  }

  class Duck extends Bird {
    override val sound: String = "Quack!"
  }

  class Owl extends Bird {
    override val sound: String = "Hoot!"
  }

  def getSound(bird: Bird): String = bird.sound

  val owl = new Owl
  // owl: Owl = Owl@199697be

  val duck = new Duck
  // duck: Duck = Duck@c03ff84

  getSound(owl)
  // res1: String = Hoot!

  getSound(duck)
  // res2: String = Quack!

TODO: http://www.cmi.ac.in/~madhavan/courses/pl2009/lecturenotes/lecture-notes/node28.html