Introduction to Type Classes

In the previous lesson, we defined our own data type for geometric shapes. In order to make them printable, we had to add deriving (Show) to our type declaration.

Show is an example of a Haskell type class. We also encountered the type class Eq, already, in type annotations of polymorphic functions (e.g. elem), where it acted as a constraint. In this lesson, we will learn in more detail what type classes are and how we can use them with our data types.

The Show type class

A type class is a collection of types that share a common property. For example, the type class Show is the class of all types that can be turned into a string using the show function (note the difference in capitalization). Its definition is:

class Show a where
  show :: a -> String

A type class declaration starts with the class keyword. What ...