write atom on async atom?

[prrrcl]

hi!

If i write this:

export const DATA = atom(
  async () => {
    // ...fetch data and return json
  },
  (get, set, update) => {
    set(DATA, (previous) => ({ ...previous, ...update }));
  }
);

I got: Error: no atom init

What is the correct way for update DATA atom?

[dai-shi]

It's prohibited to write a value directly to a derived atom. (because it may cause inconsistencies.)

This pattern is implemented as atomWithDefault in jotai/utils. You could also do the similar way on your end.

[prrrcl]

Oh, I didn’t know about the inconsistencies on that.

Thanks a lot @dai-shi

[dai-shi]

Well, the inconsistencies are the issues when we use get on read.
So, in your use case it won't be a big issue. (but the library doesn't distinguish the difference.)
Anyway, writing directly to derived atoms is prohibited, because it's derived.

[tukaz]

this worked. i think this example has to be more visible in the documentation since is very common we get something async and we have to modify it later.