title: How to extend functions onto imported types in Golang
body: <p>I have an imported type</p>
  
  <pre><code>type ExternalType struct {
     quantity int
  }
  
  
  type ExternalArray []*ExternalType
  </code></pre>
  
  <p>I want to be able to implement the sort interface for ExternalArray for so that I sort it by quantity.</p>
  
  <p>However, I am not sure how I could do that?</p>
  
  <p>A concrete example is this:</p>
  
  <p><a href="https://play.golang.org/p/bEPtJ8NHQK" rel="nofollow">https://play.golang.org/p/bEPtJ8NHQK</a></p>