> The second part is where functional programming shines -- you have a simple way to just recompute all the derived things. And since it's presumably the same way the derived things were computed in the first place, you don't have to worry about its logic getting out-of-sync.
Thinking this further, this is also performance related. I think there is an interesting relationship between FP and DOD:
A technique of data oriented design is to keep state minimal and lazily derive data when you actually need it, which may involve recomputing things. The rationale is that compressed, normalized data requires less fetching from memory and computation on it is faster.
In contrast caching and buffering, both of which are heavily stateful and require a lot of additional memory, are often necessary, because they minimize inherently slow operations that are out of your control. Those kinds of things are often best implemented as (computational) objects with encapsulated, internal state and small, general interfaces, like OO has taught us.
But once the data in your control, this mindset has to be flipped on its head. You want to model your in-memory data not that differently from how you'd model for databases: Neatly aligned, normalized data, with computed columns, views and queries to get richer answers.
Interestingly if you follow this approach, then code starts to look more similar to functional code, because you potentially need the whole context to derive values from it and a lot less like independent objects that send messages to each other.