toTaskX functions do not consider empty observables

[mlegenhausen]

🐛 Bug report

Current Behavior

import * as _ from 'fp-ts-rxjs/Observable'

const t = await _.toTask(_.zero<number>())() // Returned type number, Expected type number | undefined
assert.deepStrictEqual(t, undefined)

Expected behavior

Instead of Task<A> we need to return Task<Option<A>>

export const toTaskOption = <A>(o: Observable<A>): Task<O.Option<A>> => () => o.toPromise().then(O.fromNullable)

Reproducible example

See current behavior

Suggested solution(s)

We need to change all toTaskX function to toTaskOptionX which get really ugly and before providing a PR I would like to discuss how the function for ObservableThese and ObservableEither should look like. I would actually consider to deprecate or even to remove them cause they are buggy and add no additional value over the toTaskOption function from Observable.

Additional context

Currently it is only a runtime bug. With rxjs@7 we will get also a type error cause the signature of toPromise has changed to cover the undefined case.

Your environment

Which versions of fp-ts-rxjs are affected by this issue? Did this work in previous versions of fp-ts-rxjs?

All versions since 0.6.5

Software	Version(s)
fp-ts	2.10.4
fp-ts-rxjs	0.6.14
rxjs	6.6.7
TypeScript	4.2.4

[gcanti]

@mlegenhausen thanks for reporting, toPromise's signature is inaccurate:

toPromise<T>(this: Observable<T>): Promise<T>

Is it possible to detect if an observable completes without emitting a value?

[mlegenhausen]

Yes, this is demonstrated in a new rxjs@7 function lastValueFrom.

[gcanti]

Ok, so I guess we can do the same and define an internal lastValueFrom helper (except we can't reject) and use that instead of toPromise

[mlegenhausen]

So we would not resolve when a complete occurred and no value was emitted?

[gcanti]

Yes, like Task's never.

[mlegenhausen]

PR updated.