Question

I have anti-xml Elem, something like this (auto-generated data):

<library>
  <bookshelf number="0">
    <book year="1997" title="Puzzled Coordinators" author="Lily Williams"></book>
    <book year="2005" title="Jittery Fare" author="Lucy Taylor"></book>
    <book year="2001" title="Dizzy Jurisdiction" author="Lucy Robinson"></book>
  </bookshelf>
  <bookshelf number="1">
    <book year="1997" title="Bashful Trusts" author="Lucas Wilson"></book>
    <book year="2003" title="Outrageous Consequences" author="Byron White"></book>
    <book year="1992" title="Irritated Accusations" author="Anne Roberts"></book>
  </bookshelf>
</library>

And I want to apply several transformations to it, for example:

val transforms: Seq[...] = ...
val result = transforms.foldLeft(library)(l,t) => t(l))

But I got only as far as this solution:

val transforms: Seq[Elem => Zipper[Node]] = Seq(
  x => x \\ "book" filter (_.attrs("year").toInt > 2000) unselect,
  x => x \\ "book" filter (_.attrs("title").contains("J")) unselect
)
val result = transforms.foldLeft(lib)((l,t) => t(l).head.asInstanceOf[Elem])

Is there some way to get better types for transformations (Elem => Elem) and avoid those ugly casts?

Answer 1

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.