Signs on the Sand

Interesting finding on XQuery from Elliotte Rusty Harold:

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.

Hmmm, that's kinda confusing. Let's see. Formally speaking what XQuery produces is one(zero) or more instances of XPath 2.0 and XQuery 1.0 Data Model (DM), which then are subject to the serialization process, defined in XSLT 2.0 and XQuery 1.0 Serialization spec. The problem (typo?) is that XQuery spec says:

Serialization is the process of converting a set of nodes from the data model into a sequence of octets...

and thus doesn't mention what happens with items in the resulting DM, which are not nodes, but atomic values. I believe that's a mistake in XQuery spec, because XSLT 2.0 and XQuery 1.0 Serialization handles that pretty well - it defines serialzation of DM including all it can contain, particularly, atomic values are converted to their string representations.