polymorphism

Ways to get polymorphism

• subtyping:
  • Key example: You have 2 sub-types of an Animal: Cat, Dog. Both of them implement a make_noise() method. Say we have Animal a = new Cat(). Then, at runtime, a.make_noise() will cause a table lookup into the a object, which happens to be a cat. This table lookup will contain the location of the make_noice() function to run for a Cat. See (wikipedia article on late binding). This is conceptually similar to dynamic linking (see Linker)
  • You can do this in Java
• parametric polymorphism
  • in haskell, this is done with type variables. So, id:: a -> a must be able to ttake any type a as input
  • In Java and Rust this is done with generics
• ad hoc polymorphism
  • basically, overloading methods
  • choose the method at compile according to the type of the arguments
    • contrasted with subtyping, which uses late binding, described above
  • In haskell, this is achieved via typeclasses, e.g., you can define what == means for any type that is an instance of the Eq type class
  • In java, this is done with method overloading
  • see also multiple dispatch