Challenge 1: A Shape Typeclass With a Default describe — Possible Solution
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Main.hs:

class Shape a where
  area :: a -> Double
  perimeter :: a -> Double
  describe :: a -> String
  describe s = "Area: " ++ show (area s) ++ ", Perimeter: " ++ show (perimeter s)

data Circle = Circle Double
data Square = Square Double

instance Shape Circle where
  area (Circle r) = pi * r * r
  perimeter (Circle r) = 2 * pi * r

instance Shape Square where
  area (Square s) = s * s
  perimeter (Square s) = 4 * s

main :: IO ()
main = do
  putStrLn (describe (Circle 3))
  putStrLn (describe (Square 4))

Output:

    Area: 28.274333882308138, Perimeter: 18.84955592153876
    Area: 16.0, Perimeter: 16.0

Explanation:
Neither Circle's nor Square's instance overrides describe -- both
only supply area and perimeter, the two methods with no default body.
describe is inherited automatically from Shape's own default
implementation, which itself calls area and perimeter through
whichever instance is actually in play for each type -- exactly the
default-method inheritance the chapter introduces.

WHY THIS WORKS AS AN ANSWER
------------------------------
This defines a real multi-method typeclass with a default method and
two genuine instances, demonstrating the default method being
inherited and used correctly without either instance overriding it.