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What Makes a Good Domain Model?

We all strive to build high-quality software and that begins with a clean and well-defined domain model. But what do we mean when we say "clean" or "well-defined"? It is difficult to decide whether a change in the domain model is an improvement without specific measures. In this post, we'll dive into how we can measure and improve the quality of our domain models.

Making illegal state unrepresentable is a well-known principle in the functional-programming community which refers to avoiding ambiguity that leaves the model open to more than one interpretation. A simple example is when a model uses strings instead of enums. Strings are ambiguous because they leave the interpretation completely to the consumer while enums limit the possible values significantly.

Is there a way to measure the ambiguity? One approximation we can use is the cardinality of types. Taking the above example a String has an infinite cardinality while an enum is always finite, usually less than 100 values. So a String is infinitely more ambiguous than an enum.

Stepping up a level, we can derive a few more useful metrics by looking at type cardinalities in the context of functions:

Domain-to-Range Cardinality Ratio

Take a simple example of a String -> Enum function. This takes an ambiguous value and turns it into a less ambiguous one which is great since it decreases ambiguity. Taking the opposite Enum -> String though turns a relatively unambiguous value to a completely ambiguous one, which decreases the quality of the output model. So clearly, you want to avoid the latter.

Domain Test Coverage

Adding test cases to the picture, you can measure what percentage of possible inputs are covered by test cases. When the cardinality of the input type is finite, it is possible to achieve full coverage which means you can be confident that the function works as expected.

More to come ...

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