Why XML literals yield mutable objects in Scala?

Question

When declaring a node sequence in Scala as literals you yield a scala.xml.NodeBuffer which is mutable (it extends ArrayBuffer[scala.xml.Node] which in turn extends Seq[scala.xml.Node]):

scala> val xml = <a /><b />
xml: scala.xml.NodeBuffer = ArrayBuffer(<a></a>, <b></b>)

scala> xml += <c />
res46: xml.type = ArrayBuffer(<a></a>, <b></b>, <c></c>)

scala> xml
res47: scala.xml.NodeBuffer = ArrayBuffer(<a></a>, <b></b>, <c></c>)

This contradicts Scala's philosophy of using immutable objects and functional programming. Why collections are immutable by default, but XML literals (which are first class citizens) are not in this case?

However, is it possible to safely define an immutable node sequence by using XML literals?

Answer 1

It seems that immutable XML literals require a single root node (as does XML). scala.xml.Group allows you to do this, using a "pacifier" root node of <xml:group />.

This is eloquently described in the API docs as "A hack to group XML nodes in one node for output."

scala> val xml = <xml:group><a /><b /></xml:group>
xml: scala.xml.Group = <a></a><b></b>

When you combine this fragment with other XML, the group node disappears:

scala> <foo>{xml}</foo>
res1: scala.xml.Elem = <foo><a></a><b></b></foo>