Deeper than flatmap, taking on whole trees in one bite!

XML Traversal

def flatTraverse[T](node: Node)(pfn: PartialFunction[Node, TraversableOnce[T]]): Seq[T] = {
  traverse(node)(pfn).flatten
}

def traverse[T](node: Node)(pfn: PartialFunction[Node, T]): Seq[T] = {
  def inner(n: Node, acc: List[T]): List[T] = n match {
    case x if (pfn isDefinedAt x) => pfn(x) :: acc
    case e: Elem => (e.child.toList map {inner(_, Nil)}).flatten ::: acc
    case _ => acc
  }
  inner(node, Nil)
}

def transform(n: Node)(pfn: PartialFunction[Node, Node]): Node = n match {
  case x if (pfn isDefinedAt x) => pfn(x)
  case e: Elem => e.copy(child = e.child.toSeq map { transform(_)(pfn) })
  case x => x
}

...

@Test
def shouldFlatTraverse() {
  val input =
    <a>
      <b>
        <d>1</d>
        <d>2</d>
        <d>3</d>
      </b>
      <c>
        <e>
          <b>
            <d>4</d>
            <d>5</d>
          </b>
        </e>
      </c>
      <c>
        <b>
          <d>6</d>
          <d>7</d>
          <d>8</d>
          <d>9</d>
        </b>
      </c>
    </a>
    val output = flatTraverse(input) {
      case x @ <b>{_*}</b> => x.child collect {case e: Elem => e.text.trim()}
    }
    output should be (Seq("1", "2", "3", "4", "5", "6", "7", "8", "9"))
  }