title: Pyspark to scala. String join function on elements after grouping
body: <p>I have to translate pyspark to scala.
  I have a function like this: <code>lambda x: ';'.join(map(str, x)</code>.
  Then we use</p>
  <pre><code>our_dataframe.groupBy('column').agg(our_lambda_function(F.collect_set('other_col')))
  </code></pre>
  <p>So we group by some column first and then get values as an array of strings. Our lambda function makes a string out of this array.</p>
  <p>Example. Instead of</p>
  <blockquote>
  <h2>|column|</h2>
  <p>|[a,b,c]|
  |[d,e,f]|</p>
  </blockquote>
  <p>we want to have</p>
  <blockquote>
  <h2>|column|</h2>
  <p>|a;b;c|
  |d;e;f|</p>
  </blockquote>
  <p>How to make that in scala?</p>