[Type] cannot subscribe observable from array of promise's concatAll()

[kwonoj]

const sources = Rx.Observable.fromArray([
  new Promise((res: any) => { res(0); }),
  new Promise((res: any) => { res(1); }),
  new Promise((res: any) => { res(2); }),
  new Promise((res: any) => { res(3); }),
]);

sources.concatAll().subscribe();

won't be allowed as

31 sources.concatAll().subscribe(
~~~~~~~~~
error TS2339: Property 'subscribe' does not exist on type 'Promise<{}>'.

This issue does not affect operator behavior.

[kwonoj]

I've thought this bit more and start to think this is somewhat expected, since via concatAll flattening and trying to access <T> directly here. It's same for other type of object as well such as number, string which in non-subscribable - I'd consider this as case for requiring explicit casting if nessary.

@david-driscoll - would you mind share opinion here? I looked into RxJS 4's type definition and seems it behaves similar.

[david-driscoll]

The signature for concatAll() looks like:

export interface ConcatAllSignature<T> {
  (): T;
}

Ideally we could do something like:

interface Observable<U, T extends ObservableInput<U>> {
    concatAll(): Observable<U>;
}

The problem is, with TypeScript anyway, there is no way to make a specific prototype method have a different generic constraint than the class it's attached to. So in this scenario the only thing we can do is to return T and assume T is an Observable<U>.

To help with this problem, we can certainly make the interface a little better, so that the manual type can be given.

export interface ConcatAllSignature<T> {
  (): T;
  <R>(): Observable<R>;
}

[lock]

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