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

How to deal with a heterogeneous list of shapes in Haskell — Abstract polymorphism with type classes: http://pastebin.com/hL9ME7qP via @pastebin

CODE:

{-# LANGUAGE GADTs #-}

class Shape s where
 area :: s -> Double
 perimeter :: s -> Double

data Rectangle = Rectangle {
 width :: Double,
 height :: Double
} deriving Show

instance Shape Rectangle where
 area rectangle = (width rectangle) * (height rectangle)
 perimeter rectangle = 2 * ((width rectangle) + (height rectangle))

data Circle = Circle {
 radius :: Double
} deriving Show

instance Shape Circle where
 area circle = pi * (radius circle) * (radius circle)
 perimeter circle = 2.0 * pi * (radius circle)

r=Rectangle 10.0 3.0 
c=Circle 10.0
list=[WrapShape r,WrapShape c]

data ShapeWrapper where
 WrapShape :: Shape s => s -> ShapeWrapper

getArea :: ShapeWrapper -> Double
getArea (WrapShape s) = area s

getPerimeter :: ShapeWrapper -> Double
getPerimeter (WrapShape s) = perimeter s

areas = map getArea list
perimeters = map getPerimeter list