toObservable() is not compatible with Rx (observer is not partial)

[felixfbecker]

IxJS version:

Code to reproduce:

import {from} from 'rxjs'
import {AsyncIterableX, toObservable} from 'ix/asynciterable'

from(toObservable(AsyncIterableX.from([1,2,3])))

Expected behavior:
Should compile

Actual behavior:
Fails with

[ts]
Type 'Observable<T>' is not assignable to type 'Subscribable<T>'.
  Types of property 'subscribe' are incompatible.
    Type '(observer: Observer<T>) => Subscription' is not assignable to type '{ (observer?: NextObserver<T> | ErrorObserver<T> | CompletionObserver<T> | undefined): Unsubscribable; (next?: ((value: T) => void) | undefined, error?: ((error: any) => void) | undefined, complete?: (() => void) | undefined): Unsubscribable; }'.
      Types of parameters 'observer' and 'next' are incompatible.
        Type '((value: T) => void) | undefined' is not assignable to type 'Observer<T>'.

This is because Observable as defined in Ix takes an Observer where all of next, error and complete have to be provided, instead of being optional.

Additional information:

[trxcllnt]

Ah yes this has been bothering me too. Ix also doesn't add the Symbol.observable property to the returned Observable, which I've been working around in JS this way:

import symbolObservable from 'symbol-observable';
function asyncIteratorToObservable(x) {
    const obs = AsyncIterable.as(x).toObservable();
    obs[symbolObservable] = () => obs;
    return Observable.from(obs);
}

[arseneyr]

I think this fix broke compatibility in the other direction (creating an AsyncIterable from a rxjs Observable):

import { AsyncIterable } from "ix";
import { from } from "rxjs";

AsyncIterable.from(from([1, 2, 3]));

works fine with ix@2.3.5 but fails on ix@2.4.3 with

[ts]
Argument of type 'Observable<number>' is not assignable to parameter of type 'AsyncIterableInput<{}>'.
  Property '[Symbol.observable]' is missing in type 'Observable<number>' but required in type 'Observable<{}>'. [2345]

This might be related to ReactiveX/rxjs#3890 where [Symbol.observable] doesn't make it into the typings for rxjs observables.

Note that if you work around this problem with

declare module "rxjs/internal/Observable" {
  interface Observable<T> {
    [Symbol.observable]: () => Observable<T>;
  }
}

it still fails with

[ts]
Argument of type 'Observable<number>' is not assignable to parameter of type 'AsyncIterableInput<number>'.
  Type 'import("./node_modules/rxjs/internal/Observable").Observable<number>' is not assignable to type 'import(".node_modules/ix/observer").Observable<number>'.
    Types of property 'subscribe' are incompatible.
      Type '{ (observer?: NextObserver<number> | ErrorObserver<number> | CompletionObserver<number> | undefined): Subscription; (next?: ((value: number) => void) | undefined, error?: ((error: any) => void) | undefined, complete?: (() => void) | undefined): Subscription; }' is not assignable to type '(observerOrNext?: NextObserver<number> | ErrorObserver<number> | CompletionObserver<number> | ((value: number) => void) | undefined, error?: ((err: any) => void) | undefined, complete?: (() => void) | undefined) => Subscription'.
        Types of parameters 'observer' and 'observerOrNext' are incompatible.
          Type 'NextObserver<number> | ErrorObserver<number> | CompletionObserver<number> | ((value: number) => void) | undefined' is not assignable to type 'NextObserver<number> | ErrorObserver<number> | CompletionObserver<number> | undefined'.
            Type '(value: number) => void' is not assignable to type 'NextObserver<number> | ErrorObserver<number> | CompletionObserver<number> | undefined'.
              Property 'complete' is missing in type '(value: number) => void' but required in type 'CompletionObserver<number>'. [2345]

[trxcllnt]

Dang, lemme see what I can do about that. Thanks for flagging this @arseneyr!