Wójtowicz's subtle but important point: mathematical purity isn't what matters for understanding your system. A function that caches to Redis with a swallowed error can be treated as a calculation if callers get the same result regardless of timing, even though it does I/O. A pure CPU-heavy function can be treated as an action because you care when it runs. What matters is whether a piece of code's output depends on when or how many times it's called — because those are the things you must coordinate, and because anything touching an action becomes an action.