title: RxSwift How to use combineLatest?
body: <p>I have defined:</p>
  
  <pre><code>let currentHours:Variable&lt;Float&gt; = Variable(0.0)
  let currentRate:Variable&lt;Float&gt;  = Variable(0.0)
  </code></pre>
  
  <p>and I would like to make an Observable with combineLatest to sum these two value:</p>
  
  <pre><code>Observable.combineLatest(currentHours, currentRate, { (h, r) -&gt; Float in
      return Float(h+r)
  })
  </code></pre>
  
  <p>and I also try:</p>
  
  <pre><code>let c =  Observable.combineLatest(currentHours, currentRate) { $0 + $1 }
  </code></pre>
  
  <p>I always get compiler error.
  thanks</p>