title: How can I chain HTTP calls in Angular 2?
body: <p>I'm new to Angular 2 and HTTP Observables. I have a component which calls an HTTP service and returns an Observable. Then I subscribe to that Observable and it works fine.</p>
  <p>Now, I want, in that component, after calling the first HTTP service, if the call was successful, to call another HTTP service and return that Observable. So, if the first call is not successful the component returns that Observable, opposite it returns Observable of the second call.</p>
  <p>What is the best way to chain HTTP calls? Is there an elegant way, for example like <a href="https://en.wikipedia.org/wiki/Monad_(functional_programming)" rel="noreferrer">monads</a>?</p>