update combineReducers typings for ts 2.1

[Rendalf]

Since TypeScript corresponds keyof and Lookup Types we can check shape of passed reducers map.

Next code is valid in current version, but invalid with updated typings.

type A = number
type B = string
type MyState = {
  a: A
  b: B
}

const reducerA: Reducer<A> = (state = 0, action) => {
  ...
}

const reducerB: Reducer<B> = (state = 'hello', action) => {
  ...
}

export default combineReducers<MyState>({
  x: reducerA,
})

Updated typings will check shape of combineReducers properly.

...
export default combineReducers<MyState>({
  a: reducerA,
  b: reducerB,
})

[timdorr]

How prevalent is TS 2.1 usage? I don't want to break others' usage of these typings if they haven't upgraded TS.

(This is exactly what I don't like about having typings in the source repo, btw)

[aikoven]

Already merged into the next branch: #2182

@timdorr Switching typings for 2.1 would always be a breaking change. Unfortunately there are little alternatives to bundling typings with Redux.

To publish typings to @types NPM org one has to add them to DefinitelyTyped mono-repo, but this makes them quite inconvenient to maintain.

Another option is to publish them as a separate NPM package, but then TS won't be able to automatically discover it and users would have to explicitly link their compilation to that package like this: #2182 (comment). This also introduces a risk of fragmentation if no single package is considered official.

That said, the current situation already makes us use explicit linking (as in #2182 (comment)) until the next major release, which I don't expect to happen soon.

I'd propose to put typings in a separate GitHub repo under https://github.com/reactjs/, publish as a separate NPM package, and make a link to it in Redux readme.

There's an ongoing discussion that would hopefully let us link that package with @types someday: microsoft/types-publisher#4

[Rendalf]

Thanks to @aikoven for explanation

Won't be merged because it already exists into the next branch

[timdorr]

Thanks for the insight, @aikoven!