data Date = -- name of type
  DMY       -- name of constructor
    Int     -- day
    Int     -- month
    Integer -- year
  deriving Show

data Person = -- name of type
  Person      -- constructor name can be same as type name 
    String    -- first name
    String    -- last name
    Bool      -- married
    Date      -- birthday
  deriving Show

today = DMY 11 10 2021
myself = Person "Rene" "Thiemann" True today 

data Brand = Audi | BMW | Fiat | Opel deriving Show 
data Vehicle = 
    Car Brand Double -- horsepower
  | Bicycle
  | Truck Int -- number of wheels
  deriving Show

data Expr = 
    Number Integer
  | Plus Expr Expr
  | Negate Expr
  deriving Show

data List = 
    Empty
  | Cons Integer List
  deriving Show