yield* call sometimes infers type as Generator

[rssfrncsrenishaw]

const { d1, d2  } = yield* all({
    d1: call(fetchD1, options),
    d2: call(fetchD2, options),
  });

I've had sitatuions where the above example fetchD1 and fetchD2 are both other sagas that I am calling with call but fetchD1 infers the correct type ({ from: string; to: string; featureName: string | undefined; name: string; data: MeasurementCharacteristicPresentation[]; }) and fetchD2 infers,

Generator<SelectEffect | AllEffect<SagaGenerator<{
    from: string;
    to: string;
    featureName: string | undefined;
    name: string;
    data: MeasurementCharacteristicPresentation[];
}, CallEffect<...>>>

The difference between the two sagas return,

fetchD1

return yield* call(
    (from, to, take, machineId) =>
      client.fetchD1Data.unique({
        query: {
          from,
          to,
          take,
          machineId
        }
      }).promise,
    from,
    to,
    constants.maxTake,
    machineId
  );

fetchD2

return yield* all(
    items.map(({  name }) =>
      call(fetchD2Data, { from, to,  name })
    )
  );

[rssfrncsrenishaw]

Doing yield* yield* call() works... so it does seem to think i'm returning a generator though i'm not sure how I am as I am doing return yield* all

[rssfrncsrenishaw]

works in the sense it stops typescript complaining but breaks runtime behaviour

[danielnixon]

I may have caused this in #13

[danielnixon]

Could you formulate this as a dtslist $ExpectType failure in index.test.ts?

[kristian-puccio]

Hi @danielnixon you can see an example in #92 .
Thanks!

PS more than happy to tidy up the PR if you find it useful

[rssfrncsrenishaw]

hi is there any update on this?

[danielnixon]

PRs welcome

[ivan-aksamentov]

I was struggling with this as well, and in the end it was my fault, because I was calling a saga which yielded a mixture of generators and non-generators in places where there is conditional program flow - because I forgot to check all the branches and to turn some of the yield to yield*.

TLDR: If you have this issue, first go and check all the callees recursively - you might have forgot to change yield to yield* somewhere and it waits for you to fix it.

Real example:

export function* parseSaga({ threadParse, input }: ParseParams) {
  yield put(parseAsync.started())
  try {
    const result = yield* call(threadParse, input)
    yield put(parseAsync.done({ result }))
    return result
  } catch (error) {
    yield put(parseAsync.failed({ error: sanitizeError(error) }))
  }
  return undefined
}

const parseResult = yield* call(parseSaga, { threadParse, input })
// type of parseResult is inferred as `Generator<bla bla bla>`

Notice how in parseSaga() I use yield* call(threadParse, input), like typed-redux-saga readme suggests, but I forgot to change all 3 yield put(...). The put() from typed-redux-saga returns a generator and you must use yield* with it. And if you now, then the type of the entire saga becomes a generator. So, if I change those to yield* put(...) (with an asterisk), then the type is inferred correctly.

It is a super silly mistake to make, but took me some time to figure. Hope it helps someone to resolve their type errors.

[danielnixon]

Thanks @ivan-aksamentov that's a good insight. I wonder if there's something we can do with eslint to catch that 🤔

[danielnixon]

That might also explain #27

[nzankich]

I ran into this when using eventChannel which isn't typed

If anyone needs a hack to get around this issue you can do something like this

const foo = *function() {
      // @ts-ignore
      const something = yield* call(bar) as ReturnType<typeof barTyped>;

     // something now has correct type
}

// Do not call.  Used for typing
const barTyped = function *() {
  return yield* yield* call(bar);
};

const bar = function *() {
   return yield call(something)
}

Avoiding yield is still the best choice if you have that option though