Type polymorphism

In programming languages and type theory, polymorphism (from Greek , polys, "many, much" and , morphē, "form, shape") is the provision of a single interface to entities of different types. A polymorphic type is one whose operations can also be applied to values of some other type, or types. There are several fundamentally different kinds of polymorphism:

The interaction between parametric polymorphism and subtyping leads to the concepts of variance and bounded quantification.

Ad hoc polymorphism and parametric polymorphism were originally described in Fundamental Concepts in Programming Languages, a set of lecture notes written in 1967 by British computer scientist Christopher Strachey. In a 1985 paper, Peter Wegner and Luca Cardelli introduced the term inclusion polymorphism to model subtypes and inheritance. However, implementations of subtyping and inheritance predate the term "inclusion polymorphism", having appeared with Simula in 1967.

Chris Strachey chose the term ad hoc polymorphism to refer to polymorphic functions that can be applied to arguments of different types, but that behave differently depending on the type of the argument to which they are applied (also known as function overloading or operator overloading). The term "ad hoc" in this context is not intended to be pejorative; it refers simply to the fact that this type of polymorphism is not a fundamental feature of the type system. In the Pascal / Delphi example below, the Add functions seem to work generically over various types when looking at the invocations, but are considered to be two entirely distinct functions by the compiler for all intents and purposes:

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