Domain Modeling Made Functional

Chapter 7Modeling Workflows as Pipelines

In the previous two chapters, we saw how we could use types to do domain modeling in a general way. In this chapter, we’ll apply what we’ve learned there to our order-placing workflow. Along the way, we’ll look at a number of techniques that are useful for modeling any workflow. The goal, as always, is to have something that is readable by a domain expert.

So let’s revisit the steps in the Place Order workflow. Here’s the summary of what we need to model:

workflow "Place Order" =
    input: UnvalidatedOrder
    output (on success):
        OrderAcknowledgmentSent
        AND OrderPlaced (to send to shipping)
        AND BillableOrderPlaced (to send to billing)
    output (on error):
        ValidationError

    // step 1
 do ValidateOrder ...