In Haskell, inheritance is implemented through the concept of typeclasses. Typeclasses define a set of functions that must be implemented for a type to be considered a member of that class.
To demonstrate inheritance in Haskell using typeclasses, let's consider an example. Suppose we want to define a hierarchy of shapes, starting with a basic Shape class and then deriving specific sub-classes such as Square and Circle.
We can define the Shape class as follows:
class Shape a where
area :: a -> Float
data Square = Square Float
data Circle = Circle Float
instance Shape Square where
area (Square side) = side * side
instance Shape Circle where
area (Circle radius) = pi * radius * radius
In this example, we define a Shape class which has only one function area. We then define Square and Circle as type constructors. To make Square and Circle members of Shape class, we define instances of the Shape class for them.
The area function is defined differently for each type. When area is called on a Square object, it returns the area of the square (which is the square of its side length). When area is called on a Circle object, it returns the area of the circle (which is pi times the square of its radius).
This example shows how inheritance is achieved through the use of typeclasses in Haskell. By defining instances of a class for different types, we can implement hierarchical relationships and reuse common functionality across multiple types.