Consider a use case for a vending machine that accepts credit card payments

1. A customer, Tom, swipes his credit card in the vending machine slot.
2. The vending machine relays the card number over the network to a bank, requesting authorization for a small charge.
3. The bank responds with an authorization number, indicating that the transaction is pre-approved.
4. Tom selects a diet soda.
5. The vending machine sends the charge (authorization number and the cost of the soda) to the bank.
6. The vending machine dispenses the soda to Tom.

Let us start with the following diagram:

#
customer:Customer[a]
card:CreditCard[a]
machine:VendingMachine[a]
bank:Bank[a]
#
customer:soda=machine.swipe(card)
machine:auth_number=bank.authorize(card)
machine:selection=customer.displaySelectionMessage()
machine:bank.sendCharge(auth_number,cost)

First we replace the anonymous customer object with a named customer object:

#
tom:Customer
card:CreditCard[a]
machine:VendingMachine[a]
bank:Bank[a]
#
tom:soda=machine.swipe(card)
machine:auth_number=bank.authorize(card)
machine:selection=tom.displaySelectionMessage()
machine:bank.sendCharge(auth_number,cost)

We then remove displaySelectionMessage() and split the call to VendingMachine:

tom:Customer
card:CreditCard[a]
machine:VendingMachine[a]
bank:Bank[a]
#
tom:message=machine.swipe(card)
machine:auth_number=bank.authorize(card)
tom:soda=machine.select("diet soda")
machine:bank.sendCharge(auth_number,cost)

How would we modify this for the alternate case where Tom’s card is declined?

tom:Customer
card:CreditCard[a]
machine:VendingMachine[a]
bank:Bank[a]
#
tom:message=machine.swipe(card)
machine:auth_number=bank.authorize(card)
#
[c:alt message="make selection"]
tom:soda=machine.select("diet soda")
machine:bank.sendCharge(auth_number,cost)
[/c]