gistfile1.hs

import Control.Monad.State

data Message = MsgA | MsgB
  deriving (Show, Eq)

class Monad m => MonadHmud m where
  getMessage :: m Message

instance MonadHmud IO where
  getMessage = do
    l <- getLine
    case l of
      [] -> return MsgA
      otherwise -> return MsgB

-- instance MonadHmud (State [Message]) where
    -- getMessage [] = (MsgA, [])
    -- getMessage (m:ms) = (m, ms)

main = do
  m <- getMessage
  print m