An atom that sets in the get no longer triggers the sub in newer versions of jotai

[ayroblu]

Bug Description

This is something that we use quite heavily to orchestrate systems.
See reproduction for example

Reproduction Link

https://stackblitz.com/edit/vitest-dev-vitest-n3jqc4qm?file=test%2Fatom-shallow-equal.test.ts

[dai-shi]

Could you specify which exactly the version introduced the change?

[dmaskasky]

Here is one way to adjust your utility atom, which will work across all versions of Jotai.

function atomShallowEqual<T>(fn: (get: Getter) => T): Atom<T> {
  const previousValueRefAtom = atom<{ value?: T | void }>(() => ({}));
  return atom((get) => {
    const previousValueRef = get(previousValueRefAtom);
    const oldValue = previousValueRef.value;
    const newValue = fn(get);
    previousValueRef.value = newValue;
    if (!oldValue) {
      return newValue;
    }
    return shallowEquals(newValue, oldValue) ? oldValue : newValue;
  });
}

[dmaskasky]

jotai-history can also help here.

import { withHistory } from 'jotai-history'

function atomShallowEqual<T>(fn: (get: Getter) => T): Atom<T> {
  const historyAtom = withHistory(atom<T>(fn), 2);
  return atom((get) => {
    const history = get(historyAtom);
    const [newValue, oldValue] = history;
    if (history.length < 2) {
      return newValue;
    }
    return shallowEquals(newValue, oldValue) ? oldValue : newValue;
  });
}

https://stackblitz.com/edit/vitest-dev-vitest-yvbsk43y

[ayroblu]

Though I appreciate that using a ref style may work, it's not very intuitive.
I've used a lot of sets in gets, it's a very convenient pattern and very powerful imo

[dmaskasky]

Another option is to use jotai-effect

import { withAtomEffect } from 'jotai-effect';

function atomShallowEqual<T>(fn: (get: Getter) => T) {
  const previousValueAtom = atom<T>();
  const shallowAtom = withAtomEffect(atom<T>(), (get, set) => {
    const oldValue = get.peek(previousValueAtom);
    const newValue = fn(get);
    set(previousValueAtom, newValue);
    if (!oldValue) {
      set(shallowAtom, newValue);
      return;
    }
    set(shallowAtom, shallowEquals(newValue, oldValue) ? oldValue : newValue);
  });
  return shallowAtom;
}

https://stackblitz.com/edit/vitest-dev-vitest-yvbsk43y

[ayroblu]

Maybe you can hard crash and put a suggestion about what to do instead. Current behaviour is not backwards compatible or "correct"