Haskell data constructor

In computer science, a type class is a type system construct that supports ad hoc polymorphism . This is achieved by adding constraints to type variables in parametrically polymorphic types.

The Eq class defines equality ( == ) and inequality ( /= ) . All the basic datatypes exported by the Prelude are instances of Eq , and Eq may be derived for any datatype whose constituents are also instances of Eq . The Haskell Report defines no laws for Eq .

6 Predefined Types and Classes

Haskell isn’t an object-oriented language . All of the functionality built here from scratch already exists in a much more powerful form, using Haskell’s type system. Many of the ideas used in this section will come up again, but rather than hacking together objects, you’ll be creating types.

Everything before the => symbol is called a class constraint. We can read the previous type declaration like this: the equality function takes any two values that are of the same type and returns a Bool. The type of those two values must be a member of the Eq class (this was the class constraint).

An instance of a class is an individual object which belongs to that class . In Haskell, the class system is (roughly speaking) a way to group similar types. (This is the reason we call them “type classes”). An instance of a class is an individual type which belongs to that class.