title: When to use Angular 2 factory functions?
body: <p>I can't imagine a situation where I need to use a factory provider. </p>
  
  <p>According to the offical docs <a href="https://angular.io/docs/ts/latest/guide/dependency-injection.html" rel="noreferrer">https://angular.io/docs/ts/latest/guide/dependency-injection.html</a> the situation is that one may not be able to access a service (<em>service-b</em>) from within another service (<em>service-a</em>), but, the factory function does (have access to <em>service-b</em>). So, when would something like this really happen?</p>