I don’t get it. Even in a statically typed language like C# you can change the domain model and your application will still compile as if nothing happened. In this post we will address this and see how F# can help with such pitfalls of C# enumerations.
What’s the problem?
The problem with enumerations is simply that when we add new cases especially in a large code base, it can be really hard to find and update all dependent source code which might lead to errors at runtime.
The compiler will not help to find all the places that have to be updated. A text-based search isn’t reliable.
Here is how things can go wrong.
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There are two great articles by Scott Wlaschin on how functional programming can be used for the domain design of real-world applications by implementing a Tic-Tac-Toe game. He demonstrates how business rules can be modeled with types, how data hiding, encapsulation, logging and capability-based security can be achieved with functional programming and more.
What I found remarkable is that in the second article he completely re-engineered the first design. Even though to me the original implementation appeared to be very appealing.
I liked that the data-centric domain model was concise, totally clear and very close to the natural notion of the game. Eventually it couldn’t meet high security standards since there were ways for malicious users of the API to manipulate the data.
The second function-centric implementation introduced the concept of capability-based security. The design smells of the previous version could be resolved. But I argue that the API of the second version is not as intuitive as the first one anymore. Technically it is also quite simple. However, the recursive structure doesn’t come totally natural to me. Also an indicator that the second version is more complex is that logging becomes trickier.
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