title: Swift error: (int) not convertible to int
body: <p>I am going through the "Swift Programming Language" guide from Apple and came across the following issue. The book defines a sumation function as:</p>
  
  <pre><code>func sumOf(numbers: Int...) -&gt; Int {
      var s = 0
      for number in numbers {
          s += number
      }
      return s
  }
  </code></pre>
  
  <p>and includes an experiment to "write a function that calculates the average of it's arguments", which I am trying to re-use sumOf to define:</p>
  
  <pre><code>func averageOf(numbers: Int...) -&gt; Float {
      var sum = sumOf(numbers)
      return Float(sum)/Float(numbers.count)
  }
  </code></pre>
  
  <p>Xcode is returning an error calling sumOf(numbers): "'[Int]' not convertible to 'Int'" 
  Ignoring the obvious divide by zero issue, why do variadic parameters lose their type inside the method and is there a way to cast this in a way to pass the variadic parameter to the sumOf function?</p>