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Polymorphism (computer science)

Using one interface or symbol with regards to multiple different types

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In object-oriented programming, polymorphism is the provision of one interface to entities of different data types. The concept is borrowed from a principle in biology in which an organism or species can have many different forms or stages.

The most commonly recognized major forms of polymorphism are:

  • Ad hoc polymorphism: defines a common interface for an arbitrary set of individually specified types.
  • Parametric polymorphism: does not specify concrete types and instead uses abstract symbols that can substitute for any type.
  • Subtyping (also called subtype polymorphism or inclusion polymorphism): when a name denotes instances of many different classes related by a common superclass.

History

Interest in polymorphic type systems developed significantly in the 1990s, with practical implementations beginning to appear by the end of the decade. Ad hoc polymorphism and parametric polymorphism were originally described in Christopher Strachey's Fundamental Concepts in Programming Languages, where they are listed as "the two main classes" of polymorphism. Ad hoc polymorphism was a feature of ALGOL 68, while parametric polymorphism was the core feature of ML's type system.

In a 1985 paper, Peter Wegner and Luca Cardelli introduced the term inclusion polymorphism to model subtypes and inheritance, citing Simula as the first programming language to implement it.

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