Haskell Cookbook

How it works...

List is defined as follows:

    data [] a = []       -- Empty list or       | a : [a]  -- An item prepended to a list, is also a list

There are two data constructors. The first data [] constructor shows an empty list, and a list with no elements is a valid list. The second data constructor tells us that an item prepended to a list is also a list.

Also, notice that the type constructor is parameterized by a type parameter a. It means that the list can be constructed with any type a.