-- A concurrent bank account

-- Messages for the bank account
data Message = Deposit Int | Withdraw Int | Balance Int

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