-- A concurrent bank account

-- Messages for the bank account
data Message = Deposit  Int  -- argument: amount of payment
             | Withdraw Int  -- argument: amount of waithdraw
             | Balance  Int  -- argument: current balance

-- The bank account "server":
account :: Int -> [Message] -> Bool
account balance messages = case messages of
  []                -> True  -- end of process execution
  (Deposit  a : ms) -> account (balance + a) ms
  (Withdraw a : ms) -> account (balance - a) ms
  (Balance  b : ms) -> b === balance & account balance ms