Derived Instances

The deriving keyword can be used to make a new type into an instance of other built-in classes Eq, Ord, Show, Read, and Enum without the need for defining any of the methods. For example, the Bool type can be declared as:

data Bool = False | True
  deriving (Eq, Ord, Show, Read)

It is as if we are stating that the new data type Bool can have two values (two nullary constructors True and False) and it should also be made an instance of the classes Eq, Ord, Show and Read, but we let the compiler write the actual code for us using the default definitions. Note that for the Ord class, the default ordering will be the order in which the constructors are defined – in this case, True comes after False and is, therefore "greater than" False. With this definition, we can use methods of all the class instances included with the type Bool:

ghci> True == True
True

ghci> False < True
True

ghci> show True
"True"

ghci> read "False"
False