Combinatorial XML Transformations in Scala

XmlTest.scala

import org.scalatest.{Matchers, Spec}

import scala.xml._

class XmlTest extends Spec with Matchers {

  def `Set text inside XML element`() = {

    val doc =
      <root>
        <el1>
          <el15>
            <el2>foo</el2>
          </el15>
        </el1>
        <el3>
          <el2>bar</el2>
        </el3>
      </root>

    val expected = "new"

    import XmlTransform._

    val t = transform(
      insideElement("el1")(
        insideElement("el2")(
          setText(expected))))

    val transformed = t(doc)

    (transformed \ "el1" \\ "el2").text should equal(expected)
    (transformed \ "el3" \ "el2").text should not equal expected
  }
}

XmlTransform.scala

import scala.xml._

object XmlTransform {

  type RewriteRule = PartialFunction[Node, Seq[Node]]

  def transform(rule: RewriteRule): Node => Seq[Node] = {
    case el: Elem if rule.isDefinedAt(el) =>
      rule(el)

    case el: Elem =>
      val replaced =
        el.child.flatMap(transform(rule))
      
      if (replaced == el.child) el
      else el.copy(child = replaced)

    case other => other
  }

  def insideElement(label: String)(rule: RewriteRule): RewriteRule = {
    case el: Elem if el.label == label =>
      transform(rule)(el)
  }

  def setText(text: String): RewriteRule = {
    case el: Elem => el.copy(child = Text(text))
  }
}