Types with Class

Type classes are a way to overload functions or operators by putting constraints on polymorphism.

For example, we have seen that:

(+) :: a -> a -> a 

is not OK because we want to restrict addition to numbers.

Likewise,

 (<) :: a -> a -> Bool

is not OK because it is not clear a priori how to compare to arbitrary types.

To address this issue Haskell provides type classes. These restrict the polymorphism. For example:

 (+) :: Num a => a -> a -> a 

says that the type a must be a numeric type, and

 (<) :: Ord a => a -> a -> Bool

says that a must be orderable.