I've heard this before but don't really understand it. Most of my reducers are storing and maybe updating data loaded from the server. What's wrong with this pattern?
[0] https://redux.js.org/style-guide/style-guide/#model-actions-...
[1] https://github.com/dmmulroy/talks/blob/master/event-driven-r...
[2] https://youtu.be/K6OlKeQRCzo?t=2626 / https://rangle.slides.com/yazanalaboudi/deck#/
I don't think actions are what they're talking about, but reducers as setters vs reducers with more complex logic. I stumbled against the same thing last week, and rather than duplicating the logic in two reducers I settled on putting the logic in the action and turning both reducers into simple setters.
When you mentally model an action as a "setter", like `SET_PIZZAS_ORDERED`, the reducer usually has almost no logic and just blindly accepts whatever value was in the action. The work of calculating the value was done before the action was dispatched.
If you model actions conceptually as "events", the corollary is that the work of calculating the new state typically ends up in the reducer.