从Haskell中的用户定义数据类型生成随机值

import System.Random

data Color = Red | Green | Blue deriving (Show, Eq, Enum, Bounded)

instance Random Color where
  randomR (lo, hi) g = let (a, g') = randomR (fromEnum lo, fromEnum hi) g
                       in (toEnum a, g')
  random = randomR (minBound, maxBound)

main :: IO ()
main = do
  gen <- newStdGen
  let randomColor = randomR (Red, Blue) gen
  putStrLn $ "Random color: " ++ show (fst randomColor)