Scala 2.13 is now available(scala-lang.org)

  val buffer = new StringBuffer()
  buffer.add(header)
  buffer.add("\n")
  someList.forEach { item =>
    val values: List[String] = someTransformation(item)
    buffer.add(values.mkString(","))
    buffer.add("\n")
  }
  buffer.toString

  val rows: List[String] = someList.map { item =>
    val values: List[String] = someTransformation(item)
    values.mkString(",")
  }
  (header :: rows).mkString("\n") + "\n" // I forgot whether they appended the string with a line break or not...