Domain Modeling Made Functional

Wrapping Up

In this chapter, we concentrated entirely on implementing the steps in the pipeline and working with dependencies. The implementation for each step was narrowly focused on doing just one incremental transformation and was easy to reason about and test in isolation.

When it came time to compose the steps, the types didn’t always match up, so we introduced three important functional programming techniques:

• Using an “adapter function” to transform a function from one “shape” to a different shape—in this case, to change the output of checkProductCodeExists from a bool to a ProductCode

• “Lifting” disparate types into a common type, as we did with the events, converting them all to the common PlaceOrderEvent type

• Using partial application ...