Processing optional xml attributes in Scala

Question

I have code that reads an XML file. Some of the attributes of elements that I need to process are optional. I am trying to use Option[T] to manage them. I have written the following to pimp the NodeSeq type returned by the \ Node operator:

class NodeSeqWrapper(nodeSeq: NodeSeq) {
  def textOption: Option[String] = {
    val text = nodeSeq.text
    if (text == null || text.length == 0) None else Some(text)
  }
}
implicit def nodeSeqWrapper(nodeSeq: NodeSeq): NodeSeqWrapper =
  new NodeSeqWrapper(nodeSeq)

and then call it like this:

(node \ "@attr").textOption.getOrElse("Some default value")

If the node has the "attr" attribute, this code gets it value. If it does not, the value "Some default value" is returned.

How can I improve this? Is there some way to fold the class definition into the implicit method? Is there a better way of getting "optional" attribute values? Am I using Option[T] "correctly"?

You may want to call your method textOption, in the same style as a Seq's head and headOption methods. — Jean-Philippe Pellet
– Jean-Philippe Pellet, Commented Oct 12, 2011 at 12:09

Philippe · Accepted Answer · 2011-10-12 12:31:07Z

5

I would say you are doing it in a very idiomatic way, yes.

You can "fold the definitions" as follows:

implicit def enrichNodeSeq(nodeSeq: NodeSeq) = new AnyRef {
  def textOption : Option[String] = {
    val text = nodeSeq.text
    if (text == null || text.length == 0) None else Some(text)
  }
}

If you are always applying .getOrElse(...) on the result, you may also want to define a second version textOrElse(elze : String) : String:

implicit def enrichNodeSeq(nodeSeq: NodeSeq) = new AnyRef {
  def textOption : Option[String] = {
    val text = nodeSeq.text
    if (text == null || text.length == 0) None else Some(text)
  }

  def textOrElse(elze : String) : String = textOption.getOrElse(elze)
}

That will make things slightly more concise.

scala> (<b>Hello</b> : NodeSeq).textOrElse("No text found.")
resN: String = Hello
scala> (<br /> : NodeSeq).textOrElse("No text found.")
resM: String = No text found.

edited Oct 12, 2011 at 12:31

answered Oct 12, 2011 at 12:18

Philippe

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4 Comments

Ralph Over a year ago

That's what I was looking for. I tried using an anonymous class in my first attempt (new {...}) and it did not work. I also like the textOrElse method. Thanks.

Jean-Philippe Pellet Over a year ago

Beware that "folding the definitions" causes Java reflection to be used each time you call your extra methods. Declaring the extra class does not.

Philippe Over a year ago

@Jean-PhilippePellet Good point! I would suspect the JVM JIT compiler is pretty-good at handling reflective calls where the name of the method is given as a static string, though.

Ralph Over a year ago

Forgot about that. In my case, the XML file is small (configuration file), so that will have almost no performance impact.

be.outside · Accepted Answer · 2013-08-28 06:25:51Z

2

The answer can be improved on since Scala 2.10 with the introduction of implicit classes.

http://docs.scala-lang.org/overviews/core/implicit-classes.html

The example the op gave can be re-written using an implicit class like so:

object SomeExtensions {

  implicit class ExtendedNodeSeq(nodeSeq: NodeSeq) {
    def textOption: Option[String] = {
      val text = nodeSeq.text
      if (text == null || text.length == 0) None else Some(text)
    }
  }

}

Note that the example follows a couple of the restrictions for case classes:

They must be defined inside of another trait/class/object.
They may only take one non-implicit argument in their constructor.

answered Aug 28, 2013 at 6:25

be.outside

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