An alternative chainFirst'ish operator

🚀 Feature request

Current Behavior

pipe(of('a'), chainFirst(() => EMPTY)) is equivalent to EMPTY. pipe(of('a'), chainFirst(() => of('b'))) is equivalent to of('a'). pipe(of('a'), chainFirst(() => from(['b', 'c']))) is equivalent to from(['a', 'a']). pipe(of('a'), chainFirst(() => pipe(from(['b', 'c']), chainIOK(a => () => console.log(a))))) is equivalent to from(['a', 'a']) and prints b c.

Desired Behavior

pipe(of('a'), chainFirst(() => EMPTY)) is equivalent to of(a). pipe(of('a'), chainFirst(() => of('b'))) is equivalent to of('a'). pipe(of('a'), chainFirst(() => from(['b', 'c']))) is equivalent to of(a). pipe(of('a'), chainFirst(() => pipe(from(['b', 'c']), chainIOK(a => () => console.log(a))))) is equivalent to of('a') and prints b c.

Suggested Solution

Did anybody else need this ever?

The need for this becomes more apparent for ObservableEither, because then errors from the inner observable would be merged into the outer observable.

// I don't know what name is appropriate
const kindaLikeChainFirst = <A>(
  f: (a: A) => Observable<B>,
): ((ma: Observable<A>) => Observable<A>) =>
  pipe(ma, chain(a => pipe(f(a), $.filter(() => false), $.startWith(of(a)))));

Implementation for ObservableEither would be something like this:

// I don't know what name is appropriate
const kindaLikeChainFirst = <E, A>(
  f: (a: A) => ObservableEither<E, B>,
): ((ma: Observable<E, A>) => Observable<E, A>) =>
  pipe(ma, chain(a => pipe(f(a), $.filter(Either.isLeft), $.startWith(Either.right(a)))));

gcanti / fp-ts-rxjs