Infer action types from action creators instead of using separate Flow types

[kumar303]

This article has a recipe for inferring action types from action creator functions instead of defining, exporting, and maintaining separate action types. This will make everything simpler.

For example, instead of this:

export const SET_NAME = 'SET_NAME';
export const SET_AGE = 'SET_AGE';

export type SetName = {type: 'SET_NAME', payload: string}
export type SetAge = {type: 'SET_AGE', payload: number}

const setName = (name: string): SetName => {
    return {type: SET_NAME, payload: name}
}

const setAge = (age: number): SetAge => {
    return {type: SET_NAME, payload: age}
}

export type Actions = SetName | SetAge;

We can do this:

export const SET_NAME = 'SET_NAME';
export const SET_AGE = 'SET_AGE';

const setName = (name: string) => {
    return {type: SET_NAME, payload: name}
}

const setAge = (age: number) => {
    return {type: SET_AGE, payload: age}
}

export type Actions =
    ExtractReturn<typeof setName> |
    ExtractReturn<typeof setAge>

[kumar303]

There is a better version here: facebook/flow#4002 (comment)

[kumar303]

Well, huh. I can get things to work in Flow Try but not in our code. It seems to always set action and all of its properties to any which is not helpful. That is, it's not actually extracting the type of the returned object.

[bobsilverberg]

I just tried doing this as well, for a new action I was adding to the collections reducer, and noticed that flow reports the type as any | {|payload: {|username: string|}, type: "UNLOAD_USER_COLLECTIONS"|}, so the any is breaking Flow's ability to type check. I'm also not sure why it works on Flow Try but not in our code.

[bobsilverberg]

Actually, I take that back. I think it does work. I'll put up a PR and someone else can check to see if it looks like it's working.

[bobsilverberg]

Ugh, now I cannot get it to work again. :-( Nevermind me.

[stale]

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