[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: Lesson Learned: Use namespaces for both markup anddata
On Tue, 11 Aug 2009 16:06:05 -0400, Costello, Roger L. wrote: > When this QName value is taken out of context, > it does not have less information than a > non-QName value taken out of context. True enough, if you strip all context. You're expected to maintain the context of the element or attribute when interpreting any data in an XML document. If you aren't doing that, why use XML? CSV is much more compact. So, you have a labelled graph, and each information-bearing node has a label. You may want to look a little way up the ancestor axis (for instance, you may want to know what element an attribute is on before interpreting the content of the attribute). Or you may not, but ... if you're relying on that context, chances are good that you have a schema. The information content of a particular node is relatively well-localized; it may need no more than it's label (element or attribute name), or it may need slightly more information extracted from the ancestor axis, but the structure of XML imposes the rules, reasonably cleanly. You can cut a fragment (<example><object>value</object></example>) and--based on the namespaces required by the *structure* (example and object, here), paste it into some other XML safely (FSVO "safely"). Once you have namespaces in content, then you're pretty much forced to copy *all* in-scope namespaces for the example fragment, because you can't tell which ones you need. Your application is also forced to replicate the functionality of an XML processor. It must keep track not only of the expanded names of the elements/attributes that it processes, but also the prefix-to-identifier mappings that are in force for that document. "aquarium:tank" may equal "artillery:tank" or "tank", each of these QNames binding their prefixes to the same namespace identifier. Stick QNames in context and you then have to explain, to the application processors, that in this case, an artillery:tank is *just the same* as an aquarium:tank, because they're both bound to the http://www.gaz-guzzlers.com URI. Application developers will, with perfect reason, wish to beat you, and the first person who put QNames in content, and probably the authors of the namespaces in XML spec. Poor locality of data, information not enforced implicitly by the rules of the XML schema, and a data representation that misleadingly does not contain half of the significant information (assuming the local name carries half the value, a QName does not provide the other half, it provides a pointer that allows you to look that up, so long as you maintain the complete state at the point of parsing). QNames in content are a *terrible* idea. Amy! -- Amelia A. Lewis amyzing {at} talsever.com Confidence: a feeling peculiar to the stage just before full comprehension of the problem.
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