fix(catch): fix catch typings

[ghetolay]

Description:

The catch operator is returning an observable with the type of the innerObservable (observable returned when error occur).
It should be a union of the outerObservable's type and the innerObservable's type :

function _catch<T, R>(this: Observable<T>, selector: (err: any, caught: Observable<T>) => ObservableInput<R>): Observable<T | R>

[coveralls]

Coverage remained the same at 97.689% when pulling c6c80d9 on ghetolay:catch_typings into 01e1343 on ReactiveX:master.

[kwonoj]

Would you mind share examples of It should be a union of the outerObservable's type?

[david-driscoll]

Would be great to see the use case for this... catch allows you to handle an error and decide what to do. There is nothing stopping you from doing something like...

o.catch((e, c) => c);

Which would make T -> R. With this change then the output would always be a special union of two potentially different types.

[ghetolay]

Any example where you would return an observable with a different type on the catch would reveal the need of union. Take the first example given on docs :

Observable.of(1, 2, 3, 4, 5)
   .map(n => {
 	   if (n == 4) {
 	     throw 'four!';
     }
	   return n;
   })
   .catch(err => Observable.of('I', 'II', 'III', 'IV', 'V'))
   .subscribe(x => console.log(x));

x should be of type number | string and not just number.

But I think this apply for all cases because when you use the same type like :

obsv$ = Observable.of(1)
  .catch( err => Observable.of(0) );

the type of obsv$ is Observable<number> but not because the catch returns that type, it's because the union number | number is equal to type number.

[ghetolay]

With this change then the output would always be a special union of two potentially different types.

@david-driscoll isn't it what exactly happens after a catch ? it's either the source observable or the one defined on catch, you can't know which one and thus can't know for sure the type of emitted item.

[david-driscoll]

That is actually a fair point, I had forgotten about how that behavior would flow. I have been corrected 👍

[david-driscoll]

After above comments these changes make sense.

[lock]

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