How would I focus on only the instances of an element which have a particular attribute value? about xml-lens

[note]

Hey @howyp, thank you for your interest in xml-lens and reporting your problem. It's especially appreciated taking into account that this is early version of the lib.

I managed to make it work for your case. First let's define a general Prism[Element, Element] out of given predicate:

def being(predicate: Element => Boolean): Prism[Element, Element] =
    Prism[Element, Element]{ el =>
      if (predicate(el)) {
        Some(el)
      } else {
        None
      }
    }(identity)

Then you can define your query as:

val havingAttrValPredicate = ElementOptics.being(el => ElementOptics.attribute("example")
        .getOption(el)
        .exists(_ == "2"))
val fetch = (root \ "b" \ "c").composePrism(havingAttrValPredicate)

Now you should be able to use such defined fetch on concrete XmlDocument:

fetch.getAll(XmlParser.parse(xml).right.get)

Does it work? Yes. Is it comfortable? No, it's embarrassingly complicated and I will work to make it easier in future. In particular being method should be part of library. The general idea of improving xml-lens is first by defining a set of general lenses (namely pl.msitko.xml.optics package) and then to try to build on top of them a bunch of useful combinators (namely pl.msitko.xml.dsl package). I find the problem you reported very useful for both of objectives.

You can track the progress on this ticket: #39 if you have any other suggestion you can comment there.

Will close that ticket after above mentioned PR is merged.

[prgsmall]

I had a similar need and created a fork to work it into the app that I was creating.
Here is a link to the pull request: #44