title: Cannot convert return expression of type '()' to return type 'Double'
body: <p>what's a a expression of type '()' ?</p>
  <p>and this version is not correct</p>
  <pre><code>var compteEnBanque: Double = 999.5
  
  
  func shopping(_ new: Double) -&gt; Double {
      guard compteEnBanque &gt;= new else {
          return compteEnBanque
      }
      return compteEnBanque -= new
  }
  </code></pre>
  <p><strong>error : Cannot convert return expression of type '()' to return type 'Double'</strong></p>
  <p>but this version is correct</p>
  <pre><code>    var compteEnBanque: Double = 999.5
  
  
  func shopping(_ new: Double) -&gt; Double {
      guard compteEnBanque &gt;= new else {
          return compteEnBanque
      }
      return compteEnBanque - new
  }
  </code></pre>
  <p>Sorry, I'm not english native and thanks for your help.</p>