I don't find knowing the shape of the data to be problematic in the applications I work on. I find the REPL tends to play a big part for me here. Any time I dev

I don't find knowing the shape of the data to be problematic in the applications I work on. I find the REPL tends to play a big part for me here. Any time I develop a feature, I run the code to see what it's doing. So, say I'm adding a new feature that needs some data from the database. I'll run the query to get the data and see it, then I'll write a function to transform it in the desired way, run it, see the output, take the next step, and so on. When I'm updating or changing the feature, I do the same thing. I'll get the application in the desired state, find the spot where I want to make the change, run the current code, and work from there. Meanwhile, without immutability the types do little to help with tracking side effects which create implicit coupling. To know whether it's safe to change data, you have to know how it's used in every location it's referenced. This completely precludes the ability to do local reasoning about your code. This is a completely separate problem from concurrency.