import System.IO
import Control.Concurrent
import Control.Concurrent.STM

type Account = TVar Int

transfer :: Account -> Account -> Int -> IO ()
transfer from to amount
    = atomically (do deposit to amount
                     withdraw from amount)

withdraw :: Account -> Int -> STM ()
withdraw acc amount
    = do bal <- readTVar acc
         writeTVar acc (bal - amount)

deposit :: Account -> Int -> STM ()
deposit acc amount = withdraw acc (- amount)