QIODevice and QXmlQuery

Published Tuesday December 11th, 2007
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I have not yet seen an API for XQuery in which integrating the data model, atomic values, nodes and all, into the interfacing language has been a walk in the park.

At the top of the list of things people tend to ask on the forums around is “How do I get XML represented as a sequence of bytes in Java/C++ into my query?”, whose result is clear — a tree fragment for the query to operate on — but whose method for reaching is not that given if you ask me.

There is no “bytestream” type in XQuery. Should the user build the tree herself and then pass the tree to the query? Should the implementation in some voodoish way be instructed how to treat a string or custom type? Shouldn’t the query engine do it such that its scope of analysis is increased and its done the way it prefers it?

What I sense have been the problem with some solutions is that they mix the data, the bytestream, with interpretation.

In Qt this manifestate itself with that the content of a QIODevice should appear in a QXmlQuery. The way it’s now provided, is that when a QIODevice is bound to a variable using QXmlQuery::bindVariable(), the query sees a URI(an instance of xs:anyURI) which behind the scenes maps to the QIODevice the user bound. Hence, if the purpose is to build an XML document, one passes the URI to the builtin fn:doc() function.

I hope this is clean. Since it’s handled like any other URI, custom extensions stays at a minimum, error reporting is consistent, and the interpretation hasn’t been coupled with the data. For instance, later on I hope to merge in support for XInclude and XQuery Update, and in those cases the URI is again simply passed to for instance fn:put().

One can weight quite well on URIs and the abstraction the XPath Data Model provides, it seems.

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