gistfile1.txt

module Main where

import System.Environment (getArgs)

data A = A deriving Show
data B = B deriving Show

parseA :: String -> Maybe A
parseA "A" = Just A
parseA _ = Nothing

parseB :: String -> Maybe B
parseB "B" = Just B
parseB _ = Nothing

data AorB = ItsA A | ItsB B deriving Show

parseAorB :: String -> Maybe AorB
parseAorB s = case parseA s of
  Just a -> Just $ ItsA a
  Nothing -> case parseB s of
    Just b -> Just $ ItsB b
    Nothing -> Nothing

concatA :: A -> String -> String
concatA A = ('A':)

concatB :: B -> String -> String
concatB B = ('A':)

main :: IO ()
main = do
  [s] <- getArgs
  case parseAorB s of
    Nothing -> print "Sorry"
    Just x -> print x