Difference between revisions of "Polymorphism"
From HaskellWiki
(Improve wording, and add Wikipedia link.) |
Benmachine (talk | contribs) (→Examples) |
||
| Line 10: | Line 10: | ||
== Examples == | == Examples == | ||
<haskell> | <haskell> | ||
| − | foldr :: | + | foldr :: (a -> b -> b) -> b -> [a] -> b |
</haskell> | </haskell> | ||
| − | <hask>foldr</hask> is a | + | <hask>foldr</hask> is a parametrically polymorphic [[function]]. When actually used, it may take on any of a variety of types, for example: |
<haskell> | <haskell> | ||
| − | ::(Char ->Int->Int)->Int->String->Int | + | :: (Char -> Int -> Int) -> Int -> String -> Int -- a = Char, b = Int |
| − | ::(String->String->String)->String->[String]->String | + | :: (String -> String -> String) -> String -> [String] -> String -- a = b = String |
</haskell> | </haskell> | ||
| − | + | Numeric literals are overloaded (i.e. subject to ad-hoc polymorphism): | |
<haskell> | <haskell> | ||
| − | 1 :: | + | 1 :: (Num t) => t |
</haskell> | </haskell> | ||
Revision as of 18:00, 30 April 2012
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.
- Parametric polymorphism; mostly found in functional languages
- Ad-hoc polymorphism or overloading
- Inclusion polymorphism; mostly found in object oriented languages
Examples
foldr :: (a -> b -> b) -> b -> [a] -> b
foldr is a parametrically polymorphic function. When actually used, it may take on any of a variety of types, for example:
:: (Char -> Int -> Int) -> Int -> String -> Int -- a = Char, b = Int
:: (String -> String -> String) -> String -> [String] -> String -- a = b = String
Numeric literals are overloaded (i.e. subject to ad-hoc polymorphism):
1 :: (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