Consider the following XML node:
<Interval>
<P v="1"/>
<Q v="0.0"/>
</Interval>
What is the correct way to pattern match the top level element in Scala? I would expect the following to work but it does not:
def visit(node:Node):String = {
node match {
case p @ <P/> => (p \ "@v") text
case q @ <Q/> => (q \ "@v") text
case <Interval> @ children:_* </Interval> => "parent"
}
}
When you create an XML literal in Scala, the variable you assign to it is the top level element.
val node: scala.xml.Elem =
<Interval>
<P v="1"/>
<Q v="0.0"/>
</Interval>
To match the top-level element, here Interval, or any element of Interval that has child elements like your example, you can use curly braces to match the children.
node match {
case <Interval>{ children @_* }</Interval> => children.collect {
case p @ <P/> => p \@ "v"
case q @ <Q/> => q \@ "v"
}
}
The result is:
Seq("1", "0.0")
If you're not familiar with collect, it allows you to provide a partial function (read as "pattern match") and ignore cases that would otherwise fail as a match error.
This expression in scala:
<Interval>
<P v="1"/>
<Q v="0.0"/>
</Interval>
would definitely return a scala.xml.Node in scala but firstly a scala.xml.Elem. You can pattern match on it like this:
import scala.xml.Elem
def visit(elem: Elem) = {
elem match {
case e@Elem(_, "Interval",_, _, _*) => "identified!"
}
}
Or you can pattern match on the child as well, because you pattern match on object n of type Elem is like Elem(n.prefix, n.label, n.attributes, n.scope, n.child) and here child elements (it's a Seq) are matched against each remaining elements of the pattern:
def visit(elem: Elem) = {
elem match {
case Elem(_, "Interval",_, _, emptyElem, nodeIWant@(<P/>), _*) =>
(nodeIWant \ "@v").text
}
}
which return 1 here for instance.
Hope it helps.
String whom representation is 1. Anyway, don't hesitate to ask for a more specific case if that's not sufficient here.