title: spark dataframe explode function error
body: <p>I'd like to use the explode function on a DF, I just write the code like the document:</p>
  
  <pre><code>    case class Url(url:String)
      val temp3 = temp2.explode($"urls"){
          case Row(urls:Array[String]) =&gt; urls.map(Url(_))
      }
  </code></pre>
  
  <p>However , it came out:</p>
  
  <pre><code>error: not found: value Row
  </code></pre>
  
  <p>The DF temp2 is like:</p>
  
  <pre><code>temp2.printSchema()
  root
   |-- userid: string (nullable = true)
   |-- urls: array (nullable = true)
   |    |-- element: string (containsNull = true)
  </code></pre>