Update style-guide.md

[davidkpiano]

Add detailed docs for #3692

PR Type

Does this PR add a new page, or update an existing page?

Checklist

• Is there an existing issue for this PR?
  • [Docs Rewrite] Create "Treat Reducers as State Machines" page #3692
• Have the files been linted and formatted?

What docs page is being added or updated?

https://redux.js.org/style-guide/style-guide/#treat-reducers-as-state-machines

For Updating Existing Content

What updates should be made to the page?

Adding detailed explanation for "Treat Reducers as State Machines"

Do these updates change any of the assumptions or target audience? If so, how do they change?

No

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[markerikson]

Sweet, thanks!

My one stylistic nitpick is that I'm generally trying to move our examples away from hardcoded constants and switch statements, but other than that, nice explanation with a good example.

I briefly debated asking for a side-by-side comparison of implementing the same logic via both a "state-first" and "action-first" approach, but we can save that for a longer docs page. I want to keep the "Style Guide" page shorter and focused overall.

[davidkpiano]

I'm generally trying to move our examples away from hardcoded constants and switch statements

I'd like to do that; how would you suggest I refactor my example with that in mind?

[markerikson]

Erm... that's a good question, really.

Problem is, that's primarily a createSlice thing, which is of course action-type-oriented, and going for any kind of a state machine abstraction is probably more distracting than helpful for the code example in this specific page.

Totally fine with showing, say, "baby's first FSM abstraction" in the larger docs page (as well as the obvious mentions of the small and full XState libs as full-blown solutions in that page as well).