# Eq – Equality Types

The

`Eq`

class supports the **comparison methods**(equality and inequality), so any two values of a type that is an instance of the`Eq`

class can be compared using the functions:(==) :: a -> a -> Bool

(/=) :: a -> a -> Bool

We have already used these functions many times, and we were able to because all the basic types (

`Char`

, `Bool`

, `Int`

....) are instances of the `Eq`

class. Classes are defined using the `class`

keyword followed by the **class name**, the**type specification**and the`where`

keyword, followed by the **default definitions**of the class methods:class Eq a where

(==), (/=) :: a -> a-> Bool

-- Minimal complete definition:

-- (==) or (/=)

x /= y = not (x == y)

x == y = not (x /= y)

We see the two default methods above defined in terms of each other, which means we need to define one of them in a clear way to have a complete definition for an instance. For example, the

`Bool`

type can be made an instance of the `Eq`

class with:instance Eq Bool where

False == True = False

True == False = False

_ == _ = True

-- or using the (/=) method

instance Eq Bool where

False /= True = True

True /= False = True

_ /= _ = False

Keep in mind that the definitions of the required functions can be as simple or as complicated as you like, and default definitions can be overwritten when declaring instances.

Last modified 3mo ago