Defer factory should return ObservableInput, not just Subscribable | PromiseLike

[felixfbecker]

Observable.defer() takes a factory that takes Subscribable | PromiseLike. That is very annoying when the value is available synchronously and often results in a lot of boiler plate when usingconcat(). Simplified example:

const contents = new Map<string, string>();

getFilesInWorkspace()
  .mergeMap(uri =>
    // ensureContent emits no items, just completes once content is ensured
    ensureContent(uri)
      // After that, we can get the content synchronously
      // Need Observable.of() here because just returning an array is not allowed
      .concat(Observable.defer(() => Observable.of(contents.get(uri))))
  )
  .mergeMap(content => ...)

[kwonoj]

what's type of contents[uri]? is it Array<object> or Array<Observable<object>>? given snippet above does not provide information to actually reproduce issues.

[felixfbecker]

Updated example.

[kwonoj]

Ok, so now I'm more confused. By subject, I guess Observable.defer(() => Observable.of(contents.get(uri))) is part you'd like to discuss, with // Need Observable.of() here because just returning an array is not allowed - is that correct?

but,

1. comment says defer factory function returns array but code returns factory function returns string
2. either case it doesn't work anyway cause defer expects function to return Observable object, so wrapping it via Observable.of is expected.

Would you clarify what's exact issue you're experiencing?

[felixfbecker]

I would like defer() to accept ObservableInput, just like mergeMap, catch etc. do:

.concat(Observable.defer(() => [contents.get(uri)]))

[kwonoj]

OK, I see now.

I personally don't agree with those as it makes defer have polymorphic behavior by allowing ArrayLike<T> other than Observable itself. As you've already said those can be achievable trivially via current operator / static creation method set which doesn't require huge code boilerplate or technical difficulties to do so, I do not see huge value to it.

I guess I was confused by your subject stating Defer factory should return... makes me think some of incompatible type signatures are there around operators, but this seems more about feature suggestion. I'll update label and /cc @benlesh for second opinion.

[felixfbecker]

But why does it accept Subscribable and PromiseLike then? It is already polymorphic, what is the benefit of not allowing ArrayLike? The concat+defer pattern is needed a lot when you want to have a promise.then-like chain of Observables where an Observable doesn't emit any items but only has a completion event.

[kwonoj]

Subscribable and PromiseLike is Observable-like, both can be subscribe into which isn't polymorphic in terms of subscribable behavior. All of operator which accepts observale can accept either subscribable or promiselike directly.

ArrayLike is not direct subscribable one need additional execution branching in code, reason I state it as make operator more polymorphic.

[felixfbecker]

one need additional execution branching in code

Isn't the the extra branching required for ArrayLike the exact same extra branching required for PromiseLike? ArrayLike needs to check for typeof obj.length === 'number', PromiseLike needs to check for the presence of typeof obj.then === 'function'

[felixfbecker]

Fixed in #2586