To read that definition, you need to understand that Haskell has a notion of "type variables", and 'data' is used to introduce new algebraic data types, which y

To read that definition, you need to understand that Haskell has a notion of "type variables", and 'data' is used to introduce new algebraic data types, which you can think of as a sort of tagged union. The definition states "For all types a, the data type Maybe a consists of 'Nothing' or 'Just a'", where the a in 'Just a' is a value of type a. Nothing and Just are polymorphic constructors that create the values of any "Maybe a" type. So if you take a concrete type, say String, you get a new concrete type called "Maybe String" whose values are Nothing, Just "Hello", Just "World", etc. For Maybe Int the values are similarly Nothing (polymorphism!) and Just 1, Just 2, etc. Extra: 'a' can be any type so Maybe (Maybe Int) is a perfectly valid type, with values like Nothing, Just Nothing, and Just (Just 1). Something like that would be useful when retrieving a "possibly empty" value from a database: Nothing means it's not in the database, Just Nothing means it's there, but it's empty, and Just (Just x) means it's there and it has a value.