Who decides what an event does?

Time for another smart move, Ben. Instead of calling setStatus from anywhere, we send named events — and the current state decides what to do with them. We need a type for them. Let's call it Event. It will have: SAVE, SUCCESS, FAIL, RETRY, and DONE — one type per thing that can happen.

When a SAVE event arrives while we sit in idle, idle should have a rule for it — one that moves us to saving.

What if the event arrives twice?

Ben

But... wait. What if the same SAVE arrives while we're already saving?

Good question Ben. A rule for that shouldn't exist — and the machine should just stay put.

Now, if FAIL arrives while saving, another rule should exist — one that moves the machine to failed.

Ben

Hmm... and say we sit back in idle — what about FAIL arriving there?

No rule, Ben. Ignored. And that's how impossible transitions disappear — impossible rules are simply never written.

Can we see every rule at once?

Now let's put it all in one place — states, events, and the rules between them. State plus event decides what comes next.

idle gets one rule: SAVE moves us to saving. saving gets two — SUCCESS lands in saved, FAIL lands in failed. Then RETRY from failed, and DONE from saved. Every missing rule is a blocked path — not by accident, but because no one wrote it.

Congratulations to us — this little table is our first state machine. Read it and you know every valid path. No handlers to trace all around our codebase, no scattered conditions.

George

I could hear the celebration through the wall, gentlemen! Don't get too comfortable. When a save fails, I need to know how many times it failed — and why. Can your little table tell me that?

Easy, George — nothing is wrong. Failing is a path we planned for — it sits right here on the map. But how many times, and why... that needs memory. And memory is something our little table doesn't have — yet.

events.ts

1 type Event =

2 | { type: 'SAVE' }

3 | { type: 'SUCCESS' }

4 | { type: 'FAIL' }

5 | { type: 'RETRY' }

6 | { type: 'DONE' }