StateSpendings.hs

module Main where

  import Control.Monad.State (StateT(..), runStateT, modify, get)
  import Control.Monad.Reader (ReaderT(..), runReaderT, ask)
  import Control.Monad.Trans.Class (lift)
  import Control.Monad.IO.Class (liftIO)

  type MySuperMonad = ReaderT String (StateT Int IO)

  runMySuperMonad :: String -> Int -> MySuperMonad a -> IO (a, Int)
  runMySuperMonad username initialBalance m = runStateT stateThing initialBalance
      where stateThing = runReaderT m username

  spend :: Int ->  MySuperMonad Int
  spend money = do
      appName <- ask
      currentBalance <- get
      if currentBalance >= money then 
          (logSpending appName) >> modify (\x -> x - money) >> return money
        else (logError appName) >> return 0

    where logSpending appName = liftIO . putStrLn $ "Spent " ++ appName ++ ": " ++ (show money)
          logError appName = liftIO . putStrLn $ "Error trying to spend " ++ appName ++ ": " ++ (show money)

  performSpendings = do
      spend 100
      spend 100
      spend 1000

  performLotsOfSTuff = do
    performSpendings
    performSpendings
    performSpendings
    performSpendings

  main :: IO ()
  main = do
    print =<< runMySuperMonad "Vasiliy" 100 performLotsOfSTuff

no no no david blaine)