polymorphism

theory, programming 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

a variable that may refer to objects whose class is not

known at compile time and which respond at run time

according to the actual class of the object to which they

refer.

(2002-08-08)