Elliotte Rusty Harold writes

In XSLT 1.0 all output is XML. A transformation creates a result tree, which can always be serialized as either an XML document or a well-formed document fragment. In XSLT 2.0 and XQuery the output is not a result tree. Rather, it is a sequence. This sequence may contain XML; but it can also contain atomic values such as ints, doubles, gYears, dates, hexBinaries, and more; and there's no obvious or unique serialization for these things. For instance, what exactly are you supposed to do with an XQuery that generates a sequence containing a date, a document node, an int, and a parentless attribute? How do you serialize this construct? That a sequence has no particular connection to an XML document was very troubling to many attendees.

Looking at it now, I'm seeing that perhaps the flaw is in thinking of XQuery as like XSLT; that is, a tool to produce an XML document. It's not. It's a tool for producing collections of XML documents, XML nodes, and other non-XML things like ints. (I probably should have said it that way last night.) However, the specification does not define any concrete serialization or API for accessing and representing these non-XML collections. That's a pretty big hole left to implementers to fill.

The main benefits of XQuery are as a better way to retrieve data from one or more XML documents than previous methods (i.e. a better XPath) not as a way to transform one XML structure to another (i.e. XSLT). I assume that if Elliotte Rusty Harold isn't familiar with APIs that provided XPath as a standalone language such as the .NET Framework's XPathNavigator, the Oracle XDK, or Jaxen since all of these provide a way to get atomic values (number, string, or boolean) as well as nodes from querying an XML document.

Similrly, there is no well defined way to serialize the results of performing an arbitrary XPath on an XML document. The tough parts for implementers aren't atomic values or XML fragments as Elliotte Rusty Harrold describes both more mundane things like attribute values. For instance consider the following document

<test xmlns:e="http://www.example.com" e:id="1" />

queried using the following XPath expression


which returns the first attribute of the document element. How would one serialize these results? There are a bunch of options such as

  1. e:id="1"
  2. {http://www.example.com}id="1"
  3. @e:id="1"
  4. {xmlns:e="http://www.example.com"}id="1"

All of which I could argue are valid serializations of the attribute node returned by that query. By the way, the .NET Framework uses the first serialization if one calls XmlNode.OuterXml on the XmlAttribute object returned by executing that query on an XmlDocument object.

So what's my point? That the situation Elliotte Rusty Harrold bemoans as being unique to XQuery has always existed with XPath. Even more, as Oleg Tkachenko points out there is an  XSLT 2.0 and XQuery 1.0 Serialization draft recommendation which specifies how to serialize instances of the XPath 2.0/XQuery data model which even resolves the question about how one would serialize the results of the query above

It is a serialization error if an item in the sequence is an attribute node or a namespace node.

Short answer, you can't.


Comments are closed.