Difference between revisions of "Polymorphism"
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foldr :: (a -> b -> b) -> b -> [a] -> b |
foldr :: (a -> b -> b) -> b -> [a] -> b |
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</haskell> |
</haskell> |
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| − | <hask>foldr</hask> is a parametrically polymorphic [[function]]. When actually used, it may take on any of a variety of types, for example: |
+ | The type of <hask>foldr</hask> involves unrestricted type variables, so it is a parametrically polymorphic [[function]]. When actually used, it may take on any of a variety of types, for example: |
<haskell> |
<haskell> |
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:: (Char -> Int -> Int) -> Int -> String -> Int -- a = Char, b = Int (note String = [Char]) |
:: (Char -> Int -> Int) -> Int -> String -> Int -- a = Char, b = Int (note String = [Char]) |
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Numeric literals are overloaded (i.e. subject to ad-hoc polymorphism): |
Numeric literals are overloaded (i.e. subject to ad-hoc polymorphism): |
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<haskell> |
<haskell> |
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| − | 1 :: (Num t) => t |
+ | 1 :: (Num t) => t |
</haskell> |
</haskell> |
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| + | The difference is that the type variable here is constrained – it must be an instance of <hask>Num</hask>. |
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== References == |
== References == |
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Revision as of 16:20, 7 May 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
The type of foldr involves unrestricted type variables, so it 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 (note String = [Char])
:: (String -> String -> String) -> String -> [String] -> String -- a = b = String
Numeric literals are overloaded (i.e. subject to ad-hoc polymorphism):
1 :: (Num t) => t
The difference is that the type variable here is constrained – it must be an instance of Num.
References
- On Understanding Types, Data Abstraction, and Polymorphism (1985), by Luca Cardelli, Peter Wegner in ACM Computing Surveys.
- Type polymorphism at Wikipedia