Handling mutation errors on per-store level

[dkzlv]

First mentioned in #27.

The proposed solutions are:

• adding onError to createMutationStore interface. Maybe, same should be applied to createFetcherStore?
• (not sure I like this direction) add a conditional to createMutationStore, if mutate should throw or not.

[martinmckenna]

Just wanted to pop in here to make a case for potentially adding an onError argument to mutate().

We have a use-case, where we're letting the user bulk-delete things like so:

const { mutate: deleteThing } = useStore($deleteThing);

const handleBulkDelete = (values) => {
  Promise.all(
    (selectedOptions).map(async (eachOptionToDelete) => {
      await deleteThing({
        thingID: eachOptionToDelete.id,
      });
    }),
  );
};

And since some can error, some can succeed, it would be good to handle errors on a per-request level, which means we would either need muatate to take it's own onError or have the option to have it throw

[dkzlv]

@martinmckenna Your particular code sample seems to be a strange usage of mutations 🤔 I'll set aside the fact that you perform N API operations instead of one (maybe, that's just a limitation of your backend, even though it's quite horrible, sometimes that's life). I particularly dislike the usage of the same mutation store in N parallel operations.
Even though it'll work, that's a bit misleading, because you know what those $deleteThing.get().loading/error will reflect? Only last mutation, and you'll never know how many of them are running, for example.
See the thing is that it could have been one mutation, right?

const $bulkDeleteThing = createMutationStore<{ id: string }[]>(
  async ({ data: selectedOptions }) => {
    return Promise.all(
      selectedOptions.map((opt) =>
        // That should be a direct API call, not mutation call
        deleteThing({ thingID: opt.id }).catch(() => {
          // There you have it! That's your per-operation `onError` right there.
        })
      )
    );
  }
);

So I think your particular use case is more or less covered by current API. You just need to use promises creatively.

[dkzlv]

@martinmckenna Btw, after giving it a good thought, I think that your specific code will break in some of future releases 😬 Read issue #34 I just created for more details.