ActionCreatorWithNonInferrablePayload when using createSlice() creator

[amcsi]

Here is a TypeScript playground link.

After creating a (dynamic) slice, the payloads for the reducer/actions I create are not inferrable. It seems like there may be some disconnect between GenericFetchState<T> and State (even though State needs to extend GenericFetchState<T>), because this works if I don't allow the freedom of expanding the state shape and only allowed it to be exactly GenericFetchState<T>.

[phryneas]

That's perfectly valid and a TypeScript thing. T could be any supertype of number in this context, it cannot be explicitly narrowed down by inference just based on the state type.

For example, as you wrote it, it would also allow for const slice = createFetchSlice<string | number, GenericFetchState<number>>('hey', getInitialState<number>(3));

See this twitter thread by Ryan Cavanaugh: https://twitter.com/SeaRyanC/status/1337176976444932099

[amcsi]

@phryneas

1. But if T is string|number, and State extends GenericFetchState<T>, then wouldn't State have to extend GenericFetchState<string|number>?
2. If not, then how am I expected to be able to restrict State that the {data: T} bit would always be a given? I most certainly want State to be compatible with GenericFetchState<T>.
3. This is a slimmed down example. In my real code there's a bunch of complex stuff happening, but all the typing (regarding State) works exactly as I expect, with the only exception being reaching into these action creators from the created slice.

[phryneas]

You could try it without adding T.

see playground

[phryneas]

As for the questions:

1. TS is TS. Put it in, be appalled :/
2. see last comment
3. this has nothing to do with RTK, but is normal TS behaviour, as you can see in this codesandbox

[amcsi]

I do need T, because I create slices of many data shapes. But it seems like I was able to get it to work with your example but keeping T like this.
It's still a bit weird that I couldn't have a generic parameter of T itself like in the example in the redux-toolkit docs. But I'm still a bit new to typescript, so there's not much I can do for now than to accept that this is normal for some reason.
Thanks for your help!

[amcsi]

Also I don't get it... back to what I was originally trying to do, how come it's recognized here (as I'm expecting) that initialState.data is of type T? It does look like TypeScript recognizes my type properly; it's just that something doesn't make that typing get all the way to the returned slice's action creators :/

[phryneas]

Well, as you saw from point 3 in my last answer: that type just does not make it out of that function. I have no idea why. But again, this has nothing to do with action creators or anything RTK. It's just how that generic flows there.

[amcsi]

Oh yeah, apparently if I return that initialState.data, suddenly it appears as unknown. OK, I get it, this really does look to be a TypeScript issue.

[lukemcgregor]

It seems to happen if you have a type like Partial eg where {} is a possible value. You can get around it by nesting this {thing: Partial}