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| [[Category:Glossary]] | | #REDIRECT [[Polymorphism#Ad-hoc polymorphism]] |
| A value is [[polymorphism|polymorphic]] if, depending on its context, it can assume more than one type. If the possible types are limited and must be individually specified before use, this is called ad-hoc polymorphism (or overloading).
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| In object-oriented languages, ad-hoc polymorphism is often called ''overloading.'' For instance, in C++, the operator '''<hask>+</hask>''' may have type (in Haskell notation) <hask>Int -> Int -> Int</hask> or <hask>String -> String -> String</hask>, 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 '''<hask>+</hask>''' in C++ is limited to those built-in and those explicitly defined.
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| Many non-object-oriented languages use ad-hoc polymorphism too. In C, the operator '''<hask>+</hask>''' may have the types <hask>Int -> Int -> Int</hask> or <hask>Float -> Float -> Float</hask> or <hask>Double -> Int -> Double</hask> or <hask>Ptr -> Int -> Ptr</hask>. The range of possible types of <hask>+</hask> in C is limited to those built-in.
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| In Haskell, ad-hoc polymorphism is achieved by type classes. For instance, the function <hask>show</hask> may have type of the form <hask>Int -> String</hask> or <hask>Float -> String</hask> or <hask>(Maybe Int, String, Either Char Float) -> String</hask> depending on its context, but the possible types of <hask>show</hask> are limited. The user must ''specify'' the type <hask>show</hask> will take by defining the type of its first argument as an instance of <hask>Show</hask>. (This is reflected in its signature <hask>show :: Show a => a -> String</hask>.)
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| Contrast ad-hoc polymorphism with [[parametric polymorphism]] of the function <hask>length :: [a] -> Int</hask>. <hask>length</hask> may be applied to lists of ''any'' type--that is, may take on an unlimited number of types (of the form <hask>[a] -> Int</hask>)--and the user may apply it to any list without needing to specify the type beforehand.
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