Polymorphism

A value is polymorphic if, depending on the context where it's used, it can take on more than one type.

There are different kinds of polymorphism.

1. Parametric polymorphism; mostly found in functional languages
2. Ad-hoc polymorphism or overloading
3. Inclusion polymorphism; mostly found in object oriented languages

Examples

foldr :: forall a b. (a -> b -> b) -> b -> [a] -> b

foldr is a parametric polymorphic function. When actually used, it may take on any of a variety of types, for example:

::(Char ->Int->Int)->Int->String->Int
::(String->String->String)->String->[String]->String

An "integer literal" is a parametric polymorphic data type:

1 :: forall t. (Num t) => t

References

• On Understanding Types, Data Abstraction, and Polymorphism (1985), by Luca Cardelli, Peter Wegner in ACM Computing Surveys.

• Type polymorphism at Wikipedia