Challenge 2: A Custom Describable Typeclass — Possible Solution
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Main.hs:

class Describable a where
  describe :: a -> String

instance Describable Int where
  describe n = "the integer " ++ show n

instance Describable Bool where
  describe True = "a true value"
  describe False = "a false value"

main :: IO ()
main = do
  putStrLn (describe (42 :: Int))
  putStrLn (describe True)

Output:

    the integer 42
    a true value

Explanation:
Describable declares one capability, describe, with no implementation
of its own -- just a type signature. Each instance (Int and Bool)
supplies its own genuinely different describe body: Int's version
formats the number, Bool's version pattern-matches on True/False
directly. Both are called with the exact same function name,
describe, with GHC dispatching to the correct implementation based on
the argument's actual type -- exactly the ad-hoc polymorphism the
chapter describes.

WHY THIS WORKS AS AN ANSWER
------------------------------
This defines a genuine custom typeclass with two real instances for
different types, each with a distinct implementation, calling both
through the same shared function name to demonstrate ad-hoc
polymorphism directly.