polymorphism

n 1: (chemistry) the existence of different kinds of crystal of

the same chemical compound [syn: {pleomorphism}]

2: the existence of two or more forms of individuals within the

same species (independent of sex differences)

polymorphism

A concept first identified by Christopher Strachey (1967) and

developed by Hindley and Milner, allowing types such as list

of anything. E.g. in {Haskell}:

length :: [a] -> Int

is a function which operates on a list of objects of any type,

a (a is a type variable). This is known as parametric

polymorphism. Polymorphic typing allows strong type checking

as well as generic functions. {ML} in 1976 was the first

language with polymorphic typing.

Ad-hoc polymorphism (better described as {overloading}) is the

ability to use the same syntax for objects of different types,

e.g. ":" for addition of reals and integers or "-" for unary

negation or diadic subtraction. Parametric polymorphism

allows the same object code for a function to handle arguments

of many types but overloading only reuses syntax and requires

different code to handle different types.

See also {generic type variable}.

In {object-oriented programming}, the term is used to describe

variables which may refer at run-time to objects of different

{class}es.