haskell polymorphism and lists

Question

Suppose I have the following:

class Shape a where
    draw a :: a -> IO ()

data Rectangle = Rectangle Int Int

instance Shape Rectangle where
    draw (Recta         


        

Answer 1

A variant of Ganesh's solution using the existential quantification syntax instead.

{-# LANGUAGE ExistentialQuantification #-}
class IsShape a where
    draw :: a -> String

data Rectangle = Rectangle Int Int

instance IsShape Rectangle where
    draw (Rectangle length width) = ""

data Circle = Circle Int Int

instance IsShape Circle where
    draw (Circle center radius) = ""

data Shape = forall a. (IsShape a) => Shape a

shapes = [Shape (Circle 5 10), Shape (Circle 20 30), Shape (Rectangle 10 15)]