Difference between revisions of "Ad-hoc polymorphism"
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Benmachine (talk | contribs) (Moved content to Polymorphism) |
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| + | #REDIRECT [[Polymorphism#Ad-hoc polymorphism]] |
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| − | [[Category:Glossary]] |
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| − | A value is [[polymorphism|polymorphic]] if, depending on its context, it can assume more than one type. If the value is specified separately for each type it can have, this is called '''ad-hoc polymorphism''', also known as '''overloading'''. Ad-hoc polymorphic values may be used at a limited number of pre-specified types. |
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| − | For instance, in C++, the addition operator may be used on any numeric type, like <tt>int</tt> or <tt>float</tt>, and may be applied to new types provided you write a definition for it using those types as arguments. The range of possible types of <tt>+</tt> in C++ is limited to those built-in and those for which the programmer has provided a definition. Similarly, in C, the operator <tt>+</tt> may operate on <tt>int</tt> or <tt>float</tt> or pointers. However, the programmer cannot supply their own definitions, and only the built-in types can be used. |
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| − | In Haskell, type classes are used as a mechanism for "principled overloading", i.e. ad-hoc polymorphism. For instance, the following function: |
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| − | <haskell> |
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| − | show :: (Show a) => a -> String |
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| − | </haskell> |
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| − | may adopt any of the following types: |
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| − | <haskell> |
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| − | Int -> String |
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| − | Float -> String |
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| − | (Maybe Int, String, Either Char Float) -> String |
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| − | </haskell> |
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| − | The types which <hask>show</hask> can adopt are precisely those which have an instance of the standard <hask>Show</hask> class defined. Each instance gives a definition of the <hask>show</hask> function to be used when the types match. Unlike in C++ function overloading, Haskell also allows overloaded ''values'', like <hask>maxBound</hask>, that adopt a different value based on their type (e.g. <hask>maxBound :: Bool</hask> is equal to <hask>True</hask>, while <hask>maxBound :: Int</hask> may be <hask>2147483647</hask>). |
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| − | Contrast ad-hoc polymorphism with [[parametric polymorphism]]: e.g. the function <hask>length :: [a] -> Int</hask> may be applied to lists of ''any'' type, but has only one definition which it uses regardless of which type it adopts. |
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Revision as of 21:59, 4 September 2012
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