在Haskell (Trifecta)中解析三位数的最简洁方法

import Text.Trifecta

parseThreeDigitNumber :: Parser Int
parseThreeDigitNumber = do
  num <- decimal
  if num >= 100 && num <= 999
    then return num
    else fail "Not a three-digit number"

main :: IO ()
main = do
  let input = "123"
  case parseString parseThreeDigitNumber mempty input of
    Success num -> putStrLn $ "Parsed number: " ++ show num
    Failure err -> putStrLn $ "Parsing failed: " ++ show err