title: Don't understand closures example in Swift
body: <p>I'm trying to learn about swift and closures.
  I'm stuck on this example.</p>
  
  <pre><code> numbers.map({
      (number: Int) -&gt; Int in
      let result = 3 * number
      return result
   })
  </code></pre>
  
  <p>What is (number: Int) -> Int? Is it a function? Where is it defined?
  <a href="https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/GuidedTour.html#//apple_ref/doc/uid/TP40014097-CH2-ID1" rel="noreferrer">https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/GuidedTour.html#//apple_ref/doc/uid/TP40014097-CH2-ID1</a></p>
  
  <p>What does the keyword "in" do? The docs say to use "to separate the arguments and return type from the body". I'm not sure I understand this. Why isn't "in" used to separate "let result = 3 * number" from "return result".</p>