Challenge 2: A Shape3D Typeclass Requiring Shape as a Superclass — Possible Solution
====================================================================

Main.hs:

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

class Shape a => Shape3D a where
  volume :: a -> Double

data Cube = Cube Double

instance Shape Cube where
  area (Cube s) = 6 * s * s        -- surface area
  perimeter (Cube s) = 12 * s      -- total edge length

instance Shape3D Cube where
  volume (Cube s) = s * s * s

main :: IO ()
main = do
  let c = Cube 3
  print (area c)
  print (volume c)

Output:

    54.0
    27.0

Explanation:
`class Shape a => Shape3D a` means any type with a Shape3D instance
must ALSO have a Shape instance -- the constraint is checked at
compile time, the same way haskell2-2's own Applicative required
Functor. Cube genuinely provides both instances, so both area
(inherited from the Shape requirement) and volume (Shape3D's own
method) can be called on the same Cube value.

WHY THIS WORKS AS AN ANSWER
------------------------------
This builds a real superclass-constrained typeclass hierarchy exactly
as the chapter introduces, with a single type (Cube) satisfying both
the base and the derived typeclass, and both methods called
successfully on it.