How to change attribute on Scala XML Element

Question

I have an XML file that I would like to map some attributes of in with a script. For example:

Answer 1

So if I were in your position, I think what I'd really want to be writing is something like:

case elem: Elem => elem.copy(attributes=
  for (attr <- elem.attributes) yield attr match {
    case attr@Attribute("attr1", _, _) =>
      attr.copy(value=attr.value.text.toInt * 2)
    case attr@Attribute("attr2", _, _) =>
      attr.copy(value=attr.value.text.toInt * -1)
    case other => other
  }
)

There are two reasons this won't work out of the box:

1. Attribute doesn't have a useful copy method, and
2. Mapping over a MetaData yields an Iterable[MetaData] instead of a MetaData so even something as simple as elem.copy(attributes=elem.attributes.map(x => x)) will fail.

To fix the first problem, we'll use an implicit to add a better copy method to Attribute:

implicit def addGoodCopyToAttribute(attr: Attribute) = new {
  def goodcopy(key: String = attr.key, value: Any = attr.value): Attribute =
    Attribute(attr.pre, key, Text(value.toString), attr.next)
}

It can't be named copy since a method with that name already exists, so we'll just call it goodcopy. (Also, if you're ever creating values that are Seq[Node] instead of things that should be converted to strings, you could be a little more careful with value, but for our current purposes it's not necessary.)

To fix the second problem, we'll use an implicit to explain how to create a MetaData from an Iterable[MetaData]:

implicit def iterableToMetaData(items: Iterable[MetaData]): MetaData = {
  items match {
    case Nil => Null
    case head :: tail => head.copy(next=iterableToMetaData(tail))
  }
}

Then you can write code pretty much like what I proposed at the beginning:

scala> val elem = 
elem: scala.xml.Elem = 

scala> elem.copy(attributes=
     |   for (attr <- elem.attributes) yield attr match {
     |     case attr@Attribute("attr1", _, _) =>
     |       attr.goodcopy(value=attr.value.text.toInt * 2)
     |     case attr@Attribute("attr2", _, _) =>
     |       attr.goodcopy(value=attr.value.text.toInt * -1)
     |     case other => other
     |   }
     | )
res1: scala.xml.Elem =