title: What does the arrow mean in a function's argument in Swift 3.x?
body: <pre><code>some_function(y_array: [Double], f_array: [(([Double], Double) -&gt; Double)]) -&gt; [Double] {
  
  //some code
  
  return some_array
  
  }
  </code></pre>
  
  <p>I'm new to Swift and don't understand this notation. What does the <strong><em>first</em></strong> arrow mean in this context? This is an argument to a function.</p>