What if the specification changes ? Being static is has both advantages and disadvantages. You can do the checks without running the code, but it doesn't guarantee run-time correctness. I think a better strategy is to type check the actual code via inference, maybe adding some doctype comments to help the static analyzer, then add run-time checks where things are likely to break. And you can make your API easier to use by checking the parameters, throwing helpful and descriptive errors, allowing several different types as input, so the user doesn't have to convert his/her model to your model.
The spec changing is the best part! (Or conversely, the worst part when you are 100% dynamic). You change your types to match the new spec (eg adding a case to a discriminated union) and you immediately get feedback on places in your code that are definitely wrong/incomplete now. It won’t find every change you need to make but it’s a big load off and frees up some cognitive capacity for what’s left.
