designing-for-correctness

WIP

Heavily derivative of these excellent posts on Scott Wlaschin's blog:

• https://fsharpforfunandprofit.com/posts/designing-for-correctness/
• https://fsharpforfunandprofit.com/csharp/union-types-in-csharp.html

For all implementations:

Use Cases:

• create order
• add order item to order
• remove order item from order
• pay for order
• refund order
• complete order

Business Rules:

• cannot modify paid order
• cannot pay for order with no items, or already paid order
• can only refund paid orders, which are not already refunded or completed
• can only complete paid orders, which are not refunded

Properties / Invariants:

• Always true:
  1. paid order must have items
  2. refunded order must be paid
  3. therefore: refunded order must have items
  4. completed order must be paid
  5. therefore: completed order must have items
• Never true:
  1. refunded order must not be completed
  2. therefore: completed order must not be refunded