Domain Modeling Made Functional

Change 3: Adding Support for Promotion Codes

Let’s move on to a different scenario: the sales team wants to do some promotions, and they want to offer a promotion code that can be provided when placing an order to get discounted prices.

After discussion with the sales team, we have these new requirements:

• When placing an order, the customer can provide an optional promotion code.
• If the code is present, certain products will be given different (lower) prices.
• The order should show that a promotional discount was applied.

Some of these changes are easy, but the last requirement, which looks so simple, will create surprisingly powerful ripples throughout the domain.

Adding a Promotion Code to the Domain Model

Let’s start with the new promotion code ...