Reader Monad Example

gistfile1.hs

import Control.Monad.Reader (runReader, ask)

data Game = Game { playerOne    :: String
                 , playerTwo    :: String
                 , currentValue :: Int
                 }

data Move = Move { player :: String
                 , value  :: Int
                 }

-- Argument passing style
makeMove game move bannerText = (game { currentValue = currentValue game - value move}, message)
  where message = gameMessage (value move) bannerText

gameMessage value bannerText = bannerText ++ " -> " ++ (show value)

-- Monadic style
makeMoveR game move = do
  message <- gameMessageR $ value move
  return (game { currentValue = currentValue game - value move}, message)

gameMessageR value = do
  bannerText <- ask
  return $ bannerText ++ " -> " ++ (show value)

main = do
  let game = Game "Michael" "Wai" 10
      move = Move "Michael" 2

  let (_, firstMoveText) = makeMove game move "Michael making a move"
  putStrLn firstMoveText

  let (_, firstMoveTextR) = runReader (makeMoveR game move) "Michael making a move"
  putStrLn firstMoveTextR