Report.scala

val stringify = udf((vs: Seq[String]) => vs.mkString(";"))

groupedKvKRecords
    .map(
      r => r.kvkRecords
        .foldRight((0, 0, List.empty[String]))(
          (kvkrecord, comb) => (
            comb._1 + 1,
            kvkrecord.wptf + comb._2,
            kvkrecord.dossierNummer :: comb._3
          )
        )
    )
    .toDF("num", "wfte", "dossiernummersArray")
    .select(
      'num,
      'wfte,
      stringify('dossiernummersArray).as("dossiernummers")
    )
    .repartition(1)
    .write.mode(SaveMode.Overwrite).option("header", true).option("sep", ";").option("quote", "").option("nullValue", "").option("mode", "FAILFAST")
    .csv(Config.targetDataFile)