Filter an observable by an observable

What you've got is fine and, tbh, I can't think of a more concise way of doing it. I'd probably use flatMap rather than concatMap if out-of-order apples aren't an issue.

If readibility is an issue for you, just move the implementation into it's one function (eg. filterObservable that accepts a function that takes a value and returns an IObservable<bool>)