Xml Transformation 1

Unlike most Java Xml Apis the Scala Object model consists of immutable object. This has two major consequences:

• There is no reference to the parent node because that would cause the XML to be very expensive during transformations
• Transforming the XML requires creating new nodes rather than changing the existing nodes

Both point cause non-functional programmers to feel a little uneasy but in practice only the first restriction causes any real discomfort.

Two methods for XML transformation will be demonstrated in this and the next topic.

1. scala> val xml = <library>
2.      | <videos>
3.      | <video type="dvd">Seven</video>
4.      | <video type="blue-ray">The fifth element</video>
5.      | <video type="hardcover">Gardens of the moon</video>
6.      | </videos>
7.      | <books>
8.      | <book type="softcover">Memories of Ice</book>
9.      | </books>
10.      | </library>
11. xml: scala.xml.Elem = 
12. <library>
13.        <videos>
14.        <video type="dvd">Seven</video>
15.        <video type="blue-ray">The fifth element</video>
16.        <video type="hardcover">Gardens of the moon</video>
17.        </videos>
18.        <books>
19.        <book type="softcover">Memories of Ice</book>
20.        </books>
21.        </library>
23. scala> import scala.xml._
24. import scala.xml._
26. scala> import scala.xml.transform._
27. import scala.xml.transform._
29. // Some of the books are labelled as videos
30. // not books so lets select those elements
31. scala> val mislabelledBooks = xml \\ "video" filter {e => (e \\ "@type").text == "hardcover"}
32. mislabelledBooks: scala.xml.NodeSeq = <video type="hardcover">Gardens of the moon</video>
34. // we can create a rule that will remove all the
35. // selected elements
36. scala> object RemoveMislabelledBooks extends RewriteRule {
37.      | override def transform(n: Node): Seq[Node] ={ 
38.      | if (mislabelledBooks contains n) Array[Node]()
39.      | else n
40.      | }
41.      | }
42. defined module RemoveMislabelledBooks
44. // a quick test to make sure the elements are removed
45. scala> new RuleTransformer(RemoveMislabelledBooks)(xml)
46. res1: scala.xml.Node = 
47. <library>
48.        <videos>
49.        <video type="dvd">Seven</video>
50.        <video type="blue-ray">The fifth element</video>
51.        
52.        </videos>
53.        <books>
54.        <book type="softcover">Memories of Ice</book>
55.        </books>
56.        </library>
59. // Now another rule to add them back
60. scala> object AddToBooks extends RewriteRule {                             
61.      | override def transform(n: Node): Seq[Node] = n match {                                
62.      | case e:Elem if(e.label == "books") =>                                                 
63.      |   val newBooks = mislabelledBooks map { case e:Elem => e.copy(label="book") }
64.      |   e.copy(child = e.child ++ newBooks)                                                 
65.      | case _ => n
66.      | }
67.      | }
68. defined module AddToBooks
70. // voila done
71. scala> new RuleTransformer(RemoveMislabelledBooks, AddToBooks)(xml) 
72. res4: scala.xml.Node = 
73. <library>
74.        <videos>
75.        <video type="dvd">Seven</video>
76.        <video type="blue-ray">The fifth element</video>
78.        </videos>
79.        <books>
80.        <book type="softcover">Memories of Ice</book>
81.        <book type="hardcover">Gardens of the moon</book></books>
82.        </library>