Extra blank line when serializing mapping inside of sequence

[bishabosha]

when the serializer does "push and indent" for entering mapping/sequence, there is always a newline inserted see code, which leads to extra blank lines.

e.g. Map("foo" -> Seq(Map("bar" -> 123))).asYaml

goes to:

foo: 
  - 
    bar: 123

It would be nice to avoid the extra blank line specifically when the first entry of a sequence is a mapping. (it is fine for scalar values)

perhaps this could be configurable if there is a possibility of conflict

[bishabosha]

As a workaround, this function works to post-process the string afterwards:

def shiftLines(yaml: String): String =
  val lines = yaml.linesIterator.toVector
  def startsWith[A](a: A, it: Iterator[A]): Boolean = it.hasNext && it.next == a
  lines.tail
    .foldLeft(lines.head :: Nil) { (before, line) =>
      val prev :: rest = before: @unchecked
      val prevEnd = prev.reverseIterator.dropWhile(_.isWhitespace)
      val lineStart = line.iterator.dropWhile(_.isWhitespace)
      if startsWith('-', prevEnd) && !startsWith('#', lineStart) then
        val (padding, content) = line.span(_.isWhitespace)
        (prev.padTo(padding.size, ' ') ++ content) :: rest
      else line :: prev :: rest
    }
    .reverse
    .mkString("\n")