Domain Modeling Made Functional

Composing the Workflow from the Steps

We’ve now got definitions for all the steps; so when we have implementations for each of them, we should be able to just wire the output of one step to the input of the next one, building up the overall workflow.

But it won’t be quite that simple! Let’s look at the definitions of all the steps in one place, with the dependencies removed so that only the inputs and outputs are listed.

type ValidateOrder =
  UnvalidatedOrder                                       // input
    -> AsyncResult<ValidatedOrder,ValidationError list>  // output

type PriceOrder =
  ValidatedOrder                            // input
    -> Result<PricedOrder,PricingError>     // output

type AcknowledgeOrder =
  PricedOrder                                // input
    -> Async<OrderAcknowledgmentSent option> // output ...