Support reducers for primitive atoms

Allow primitive atoms to define reducers (setters), which will allow to transform the update. I don’t think you should need to create a pair of a primitive and a derived atoms (where you don’t export the primitive atom at all) just to accomplish this.

It seems it’s somewhat supported even now, but it’s undocumented and not typed correctly:

type List = { id: string; value: number }[];
const sortedListAtom = atom<List, List>([], (get, set, update) => {
  return set(
    sortedListAtom,
    update.sort((a, b) => a.value - b.value)
  );
});

Sandbox

Use case: transform updates so that callers don’t have to do it themselves in each call site. In the above example, we want to update the list and ensure it’s sorted. The updater may be much more complex and you don’t want to force (and ensure) consumers do that themselves.

[dwelle]

Although I wouldn’t add this in core

I meant just adding the type (if we manage to fix it — I haven’t looked at it yet). If not, it can indeed live in utils as a helper method.

[dai-shi]

Although I wouldn’t add this in core

I might change my mind in the future, if there were a better way.