traffic.hs

data TrafficLight = Red | Yellow | Green

instance Eq TrafficLight where
    Red == Red = True
    Green == Green = True
    Yellow == Yellow = True
    _ == _ = False

instance Show TrafficLight where
    show Red = "Red light"
    show Yellow = "Yellow light"
    show Green = "Green light"

main = do
    print $ Red == Red
    print $ Red == Yellow
    print $ Red `elem` [Red, Yellow, Green]
    print $ [Red, Yellow, Green]

-- True
-- False
-- True
-- [Red light,Yellow light,Green light]