title: Unable to append string to mutable array in Swift
body: <p><strong>Context</strong> - I'm currently learning Swift <code>Struct</code>. So I decided to create my own Phone structure in Playground. (see below)</p>
  
  <p><strong>Problem</strong> - 
  The <code>downloadApp</code> method on the <code>phone</code> struct is throwing the following error.. <code>Cannot use mutating member on immutable value: 'self' is immutable</code>. </p>
  
  <p><strong>Expect outcome</strong> - To peacefully append new strings to my <code>apps</code> property, which is an array of strings. </p>
  
  <p><strong>Swift code</strong></p>
  
  <pre><code>struct Phone {
      let capacity : Int
      let Brand : Sting
      var name: String
      let model: String
      var apps: [String]
      var contacts: [String]
  
      var formatCapacity: String {
          return "\(capacity)GB"
      }
  
      func downloadApp(name: String){
          apps.append(name) // ERROR 
      }
  }
  </code></pre>