They have more in common with interfaces, in that they specify a series of methods or values by their type signature, to be implemented by an instance declaration. In Haskell, this is achieved via the system of type classes and class instances.ĭespite the similarity of the name, Haskell's type classes are quite different from the classes of most object-oriented languages. Other languages like C++ allow programmers to provide their own overloading, supplying multiple definitions of a single function, to be disambiguated by the types of the arguments. Most languages support at least some ad-hoc polymorphism, but in languages like C it is restricted to only built-in functions and types. For example, the + operator essentially does something entirely different when applied to floating-point values as compared to when applied to integers – in Python it can even be applied to strings as well. This is a somewhat limiting but extremely useful property known as parametricity.Īd-hoc polymorphism refers to when a value is able to adopt any one of several types because it, or a value it uses, has been given a separate definition for each of those types. Since a parametrically polymorphic value does not "know" anything about the unconstrained type variables, it must behave the same regardless of its type. the result type of id must be the same as the argument type, and the input and output types of the function given to map must match up with the list types. Note, however, that if a single type variable appears multiple times, it must take the same type everywhere it appears, so e.g. Likewise, the empty list :: belongs to every list type, and the polymorphic function map :: ( a -> b ) -> -> may operate on any function type. In Java and some similar languages, generics (roughly speaking) fill this role.įor example, the function id :: a -> a contains an unconstrained type variable a in its type, and so can be used in a context requiring Char -> Char or Integer -> Integer or ( Bool -> Maybe Bool ) -> ( Bool -> Maybe Bool ) or any of a literally infinite list of other possibilities. In Haskell, this means any type in which a type variable, denoted by a name in a type beginning with a lowercase letter, appears without constraints (i.e. Parametric polymorphism refers to when the type of a value contains one or more (unconstrained) type variables, so that the value may adopt any type that results from substituting those variables with concrete types.
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