Fix PayloadAction inference

[tydira]

By wrapping the P match in SliceActionCreator<P> with a single-element tuple, it prevents the conditional from being checked distributively, which otherwise has the effect of converting a union into an intersection when used with contra-variant types.

Without this change, the modified test emits the following error:

Argument of type '2' is not assignable to parameter of type 'number & number[]'. Type '2' is not assignable to type 'number[]'.

Argument of type 'number[]' is not assignable to parameter of type 'number & number[]'. Type 'number[]' is not assignable to type 'number'.

My original two comments on this issue are here: #133 (comment) and here: #133 (comment). More information about distributive conditionals can be found here: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html. They're very useful and typically match our intent when combining unions with other types, but in cases like these they end up transforming the resulting type into something entirely useless.

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[markerikson]

I'll similarly ask for help on this one. You think it's worth merging / ready to merge?

@hedgerh @Dudeonyx @denisw @Jessidhia

[hedgerh]

Looks like @denisw may have more context here. I'll take some more time to look over this tonight if he's not available, though.

[denisw]

I didn't know about distributive conditional types - thanks for the pointer! The PR looks good to me. The only thing I'd add is a comment at the definition of SliceActionCreator explaining why the single-element tuple type is used, as it's far from obvious. :)

[tydira]

Cheers, @denisw. I didn't know about distributive conditionals either until I ran into this and wanted to figure out what was happening.

Does anyone have an explanatory comment in mind? I'm thinking something appended to the bottom of SliceActionCreator<P>'s original comment that says:

The P generic is wrapped with a single-element tuple to prevent the
conditional from being checked distributively, thus preserving unions
of contra-variant types.

I'll commit and push this if there are no objections.

[markerikson]

Sure, seems good. (I still don't quite grok the meaning, but sounds plausible :) )

[renatoruk]

Thank you for this library, it really improves the workflow with redux. I am currently using it on a react typescript project for a short time, but can already see the benefit. 👏

I ran into the same issue as @kroogs with the first reducer I wrote using createSlice.

Slice instance looks like this:

export const module = createSlice({
    initialState: {
        visible: true,
    },
    reducers: {
        setVisible(state: ModuleState, action: PayloadAction<boolean>) {
            state.visible = action.payload;
        },
    },
});

Trying to dispatch that action from outside resulted with error:
TS: 2345 Argument of type 'boolean' is not assignable to parameter of type 'false & true'.

This is code in the container:

const visible = true;
dispatch(module.actions.setVisible(visible));

I just modified the dist node_modules for the time being as this PR fixes the problem (thank you @kroogs !), but it's only a temporary solution.

Is this PR ready to be merged? If not, what can we do to improve it?

[Retsam]

As perhaps an alternate way of reaching the same end, perhaps:

// Since TS only distributes conditionals when the checked type is a naked type parameter
// intersecting with an empty interface will prevent it from distributing, without changing the contract of T
type NoDistribute<T> = T & {};

export type SliceActionCreator<P> = NoDistribute<P> extends void ? //...

If you wanted to link the docs on distributive conditional types for context in the comment, they're here: https://www.typescriptlang.org/docs/handbook/advanced-types.html#distributive-conditional-types.

[markerikson]

In the absence of further comments, I'm going to assume this is okay and go with it.