partition makes no sense as a lettable/pipeable operator

RxJS version: 5.5.1

partition is one of the new lettable/pipeable operators in the operators directory. However, it’s not really lettable, as it does not return an Observable:

export function partition<T>(
  predicate: (value: T, index: number) => boolean,
  thisArg?: any
): UnaryFunction<Observable<T>, [Observable<T>, Observable<T>]> {
  return (source: Observable<T>) => [
    filter(predicate, thisArg)(source),
    filter(not(predicate, thisArg) as any)(source)
  ] as [Observable<T>, Observable<T>];
}

And with it defined as it is, the usage is somewhat clumsy:

const source = of(1, 2, 3, 4);
const [odds, evens] = partition(value => value % 2)(source);

Are all of the operators in the operators directory supposed to be lettable? Should it be defined elsewhere? And in a way that makes its calls more straightforward? That is, is there any point in its returning a function if that function cannot be passed to pipe (or to let)?

The solution for toPromise was to move it to the prototype, but is there are alternative solution for these types of ‘operators’ (if that term is even appropriate)?

[bratter]

@maxime1992, @crimx, not sure if you were suggesting you’d solved or not, but if not… you need to do as follows:

const source = of(1, 2, 3, 4);
const [odds, evens] = partition(val => val % 2)(source);

Note that the partition function returns a function that has to then be called with the source observable as per felix’s comment above, and therefore cannot be used with pipe().

[maxime1992]

Indeed, I thought I could do something like that:

of(1, 2, 3, 4)
  .pipe(
    partition((x) => x % 2 === 0)
  );

But it ends up with the following error:

Argument of type ‘UnaryFunction<Observable<any>, [Observable<any>, Observable<any>]>’ is not assignable to parameter of type ‘UnaryFunction<Observable<number>, Observable<Observable<any>>>’. Type ‘[Observable<any>, Observable<any>]’ is not assignable to type ‘Observable<Observable<any>>’. Property ‘_isScalar’ is missing in type ‘[Observable<any>, Observable<any>]’.

Repro: https://stackblitz.com/edit/angular-fwfubg?file=app%2Fapp.component.ts