Challenge 1: A TrafficLight Sum Type With a Cycling Function — Possible Solution
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Main.hs:

data TrafficLight = Red | Yellow | Green
  deriving (Show)

next :: TrafficLight -> TrafficLight
next Red = Green
next Green = Yellow
next Yellow = Red

main :: IO ()
main = do
  print (next Red)
  print (next Green)
  print (next Yellow)

Output:

    Green
    Yellow
    Red

Explanation:
TrafficLight is a sum type with three no-argument constructors --
Red, Yellow, and Green are each complete, distinct values of the type
on their own, no additional data attached. next pattern-matches on
each possible constructor directly, one equation per case, defining
the exact Red -> Green -> Yellow -> Red cycle requested. deriving
(Show) lets each value print as its own constructor name automatically,
which is why print (next Red) shows "Green" rather than an error.

WHY THIS WORKS AS AN ANSWER
------------------------------
This defines a genuine sum type with three no-argument alternatives,
matching the chapter's own Shape-style data declaration shape, and
implements the requested cycle using one pattern-matched equation per
constructor.