title: Spark Implicit $ for DataFrame
body: <p>Where in the <code>sqlContext.implicits._</code> does it define the $"string" to represent a dataframe call to the parent dataframe's column? Specifically I was confused on seeing something like the following:</p>
  
  <pre><code>import sqlContext.implicits._
  df.where($"type".isin("type1","type2") and $"status".isin("completed","inprogress"))
  </code></pre>