title: Use of lit() in expr()
body: <p>The line:</p>
  
  <pre><code>df.withColumn("test", expr("concat(lon, lat)")) 
  </code></pre>
  
  <p>works as expected but</p>
  
  <pre><code>df.withColumn("test", expr("concat(lon, lit(','), lat)"))
  </code></pre>
  
  <p>produces the following exception:</p>
  
  <blockquote>
    <p>org.apache.spark.sql.AnalysisException: Undefined function: 'lit'. This function is neither a registered temporary function nor a permanent function registered in the database 'default'.; line 1 pos 12
            at org.apache.spark.sql.catalyst.analysis.Analyzer$LookupFunctions$$anonfun$apply$15$$anonfun$applyOrElse$49.apply(Analyzer.scala:1198)</p>
  </blockquote>
  
  <p>Why? And what would be the workaround?</p>