Domain Modeling Made Functional

Chapter 9Implementation: Composing a Pipeline

So far, we’ve spent a lot of time modeling the domain using only types. We haven’t yet implemented anything! Let’s remedy that now.

In the next two chapters, we’ll work with the design that we did earlier (Chapter 7, ​Modeling Workflows as Pipelines​) and implement it using functional principles.

To recap the design in that chapter, the workflow can be thought of as a series of document transformations—a pipeline—with each step in the pipeline designed as a section of “pipe.”

From a technical point of view, we have the following stages in our pipeline:

• Start with an UnvalidatedOrder and convert it into a ValidatedOrder, returning an error if the validation fails.

• Take the output of the validation ...