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// Import the magic libraries
import scala.xml._
import scala.xml.transform._
// Source xml. In Scala, xml is literal.
val xml =
<feature id="one"/>
<feature id="two"/>
<feature id="three"/>
<bar id="qux" name="caddr"/>
<bar id="baz" name="cdddr"/>
// The problem: replace the "features" list in the above XML
// with a new features list: ["a", "b", "c", "d"]
// Define a function to take a dom and replace the existing
// list of features with a new list of features.
def replaceFeatures(dom: Node, features: List[String]): Node = {
// You can define objects (or classes) inside functions.
object replaceFeatures extends RewriteRule {
// The rule replaces any "features" element with a new
// features element we build for the occasion.
override def transform(n: Node): Seq[Node] = n match {
// match on the "features" element
case Elem(prefix, "features", attribs, scope, _*) =>
// XML literals again...
// Can embed scala inside an XML literal: In this case,
// apply an anonymous function over the list of features
// in the original parameter list of the replaceFeatures
// function. The func turns a feature name into a feature
// node.
{ features map { d => <feature id={d}/> } }
// That which we cannot speak of, we must pass over
// in silence....
case other =>
// Subclass a RuleTransformer (because it's abstract), handing
// it a vararg list of rules to use (in this case, just one).
object transform extends RuleTransformer(replaceFeatures)
// Do the transform. (A scala function's return value is the
// value of the last expression (and everything's an expression).
// Transform the xml, replacing the existing feature list
// with a new list constructed from our feature list.
val newXml = replaceFeatures(xml, List("a", "b", "c", "d"))
// Just for fun...
// Create a partial function pp so we don't have to have a nasty object
// floating around. pp(xml) is much better than printer.format(xml),
// eh? The underscore stands for the expected value.
val pp = new PrettyPrinter(123, 2).format(_:Node)
// Print before and after
println("before = \n" + pp(xml))
println("after = \n" + pp(newXml))
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