Type-Safe Domain Modeling in Kotlin

“If it compiles, it works” with Valiktor and Konad

Photo by Georgy Rudakov on Unsplash

In Domain Driven Design there is a concept of ubiquitous language. Being trivial, this is usually related to the names you give to the entities in the domain model.

However, it’s possible to take it a step further. We can turn the code into an unequivocal expression of the domain.

For example, here it follows how an American domain expert may describe a “contact” and some business rules.

A contact has a name, a surname, and an email. The name can have a middle initial. The email must be verified. You can send password recovery only to verified emails.

A typical, persistence oriented, modeling could look like this:

Does this code express any of the requirements? Yes, it enumerates all the data. Anyway, is there any constraint on strings? Is all the data mandatory, or may some be missing?

We can do better by declaring this information in the model.

Declarative Domain model

First, recall the basics.

Cohesion

If some of the data is related, then it’s a good idea to group them.

This model expresses which information is related. Maintaining high cohesion also favors reusability.

Declare the constraints

The string attributes of PersonalName should not be empty. This is not clear in the code, so let’s clarify it.