How do functional programming languages handle polymorphism?

Functional programming languages handle polymorphism through parametric and ad-hoc polymorphism, using type classes and higher-order functions.

In more detail, functional programming languages like Haskell and Scala use two main types of polymorphism: parametric and ad-hoc. Parametric polymorphism allows a function or a data type to be written generically, so it can handle values uniformly without depending on their type. This is similar to generics in Java or templates in C++. For example, a function that reverses a list can work with a list of any type: integers, strings, etc. This is because the operation of reversing a list does not depend on what types of elements are in the list.

Ad-hoc polymorphism, on the other hand, allows a function to be applied to arguments of different types in a way that may behave differently for each type. This is achieved using type classes in Haskell or traits in Scala. For example, the equality function (==) in Haskell is an instance of the Eq type class. This means that any type that has an instance of the Eq type class can use the equality function. This allows the function to behave differently depending on the type of its arguments, hence the term 'ad-hoc'.

Higher-order functions also play a role in polymorphism in functional programming. A higher-order function is a function that takes other functions as arguments or returns a function as a result. This allows for a high degree of reusability and modularity, as functions can be passed around and used in different contexts. This can also be seen as a form of polymorphism, as the same function can be used with different functions as arguments, potentially leading to different behaviours.

In conclusion, functional programming languages handle polymorphism through a combination of parametric and ad-hoc polymorphism, using type classes and higher-order functions. This allows for a high degree of flexibility and reusability in code, as functions and data types can be written in a generic way that can handle different types uniformly.