Domain Modeling Made Functional

Change 4: Adding a Business Hours Constraint

So far, we’ve looked at adding new data and behavior. Now let’s look at adding new constraints on how the workflow is used. Here’s our new business rule:

• Orders can only be taken during business hours.

For whatever reason, the business has decided that the system should only be available during business hours (perhaps under the assumption that people accessing the site at four o’clock in the morning are probably not real customers). So how can we implement this?

We can use a trick that we’ve seen before, which is to create an “adapter” function. In this case we’ll create a “business-hours-only” function that accepts any function as input and outputs a “wrapper” or “proxy” function that has exactly ...