Anti-XML Group "collect" is not collecting

Question

I am cross-posting this from scala-user:

I have the following:

object XmlTest {

  import com.codecommit.antixml._

  implicit def toPicker(nodes: Group[Node]): Picker = new Picker(nodes)

  class Picker(nodes: Group[Node]) {
    def pick[A <: Node : ClassManifest]: Group[A] = nodes collect {
      case a if implicitly[ClassManifest[A]].erasure.isInstance(a) => a.asInstanceOf[A]
    }
  }

  def testCollect(elems: Group[Elem]) {
    println("size before collect = " + elems.size)
    val es = elems collect {
      case e if e.name == "c" => println("element name is " + e.name); e
    }
    println("size after collect = " + es.size)
  }

  def main(args: Array[String]) {
    val xml = XML.fromString("<a><b/><c/><d/></a>")

    // this works because <a> has only elements as children
    testCollect(xml.children.asInstanceOf[Group[Elem]])

    // pick filters collection by type
    testCollect(xml.children.pick[Elem])
  }
}

When i ran it, it printed the following:

[info] size before collect = 3           // size is 3
[info] element name is c                 // element c matches
[info] size after collect = 1              // this is correct
[info] size before collect = 3           // size is the same as the previous case
[info] element name is c                 // element "c" is matched as well
[info] size after collect = 0               // this should be 1

I am at a lost here. What is going on?

Small detail: you can write `classManifest[A]` instead of `implicitly[ClassManifest[A]]`. — Jean-Philippe Pellet, May 10 '11 at 15:02
You should have made it explicit that you are using anti-xml. — Daniel C. Sobral, May 10 '11 at 18:52

score 5 · Accepted Answer · answered May 10 '11 at 19:12

This appears to be a bug in Anti-XML. It's pretty easy to reproduce even without all of the ClassManifest magic:

val xml = <a><b/><c/><d/></a>.anti
xml.children collect { case e => e } collect { case e => e }   // => Group()

The problem is stemming from the implementation of flatMap in Zipper. Something is definitely wonky, and no surprise, given how non-trivial the implementation is. I'll add a test case and hopefully get a fix up tonight.

Anti-XML Group "collect" is not collecting

1 Answers1