-- 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" with current state and incoming messages
-- as arguments.
account :: Int -> [Message] -> Bool
account balance messages = case messages of -- suspends if free variable
  []                -> 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













-- Create account and send some messages:
-- > account 0 ms  &  ms =:= [Deposit 100, Withdraw 20, Balance b] where b,ms free