Accepted answer

Given that unselect currently just returns a Zipper[Node], I don't see a way to get what you want without bossing the type system around a bit, and that's going to take something like a cast.

In this case you really do know something the type system can't, given the current state of the Anti-XML library: you know that the parent of the zipper resulting from your transformation is a Zipper[Elem], and therefore that what unselect gives you actually will be a Zipper[Elem], even though it's typed as a Zipper[Node].

So I think the best you can do is package up the unpleasantness a little more cleanly:

def toEndo(t: Elem => Zipper[Elem]): Elem => Elem =
  t andThen (_.unselect.head.asInstanceOf[Elem])

Or, to make your intent more apparent:

def toEndo(t: Elem => Zipper[Elem]) = t andThen (_.unselect.head match {
  case e: Elem => e
  case _ => throw new RuntimeException("Aaaaah! This was never meant to happen!")

Then you can write the following:

val transforms: Seq[Elem => Zipper[Elem]] = Seq(
  _ \\ "book" filter (_.attrs("year").toInt > 2000),
  _ \\ "book" filter (_.attrs("title").contains("J"))

val result = Function.chain(transforms map toEndo)(lib)

Note that I've moved the unselect into the helper to give us a bit more type safety.

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