-- read http://learnyouahaskell.com/making-our-own-types-and-typeclasses

data Shape = Circle Float Float Float | Rectangle Float Float Float Float
--data Shape = Circle Float Float Float | Rectangle Float Float Float Float   deriving (Show) 

surface (Circle _ _ r) = pi * r ^ 2  
surface (Rectangle x1 y1 x2 y2) = (abs $ x2 - x1) * (abs $ y2 - y1)  

-- :s +t
-- :t Circle
-- surface (Circle 1 2 3)
-- surface ( Rectangle 1 2 3 4)
-- Circle 1 2 3
-- map (Circle 10 20) [4,5,6,6]