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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Enlightenment via avoiding the T-word
 In reviewing this thread, it seems to me that there is a certain word beginning with T the occurrence of which is strongly correlated with exuberant bursts of extrapolation and exegesis. So let me present an alternate reality which I claim to be internally consistent, consistent with all interesting real-world software implementations, and free of voodoo or magic: XML provides a method for textually encoding data objects; the encoding allows the components of the data objects (whether provided as elements or attributes) to be given labels. Let's call these labels "labels". The label syntax is given by the "Name" production from the XML recommendation. XML Namespaces allow the labels to be extended by the addition of a URI reference. Let's call these extended labels "ulabels". Because of the defaulting mechanism, ulabels cannot be distinguished syntactically from labels without processing their context. A large variety of software programs (and humans) process XML. In selecting which components to process, and what processing to do, they are observed to use a wide variety of input information. Most obvious is the label (or ulabel if provided), but other relevant information can include - the context of the component in the XML document - whether it has an attribute with a particular label - the value of an attribute with a particular label - some external operation based on the content of the component; i.e. treat it as a part number, look up the inventory in a database, and process it only if the count on hand is below a critical value. - entirely external information such as the time of day or the identity of the software's user One class of software application is called "validation", which consists in determining whether one or more components in an XML document (or the entire document) meet the constraints described in a declarative specification usually called a "schema". The original XML recommendation included the specification of a constraint language. This has supported the mistaken belief that validation is uniquely special and important among all the classes of applications which process XML. Historically, the only validation available was based on the DTD (an acronym we can't expand here). This ties constraints to elements *only* on the basis of their label, and to attributes based on the combination of their label and that of the element to which they are attached. This limitation, and an unfortunate choice of terminology in the XML recommendation, has supported the mistaken belief that labels are mystically tied one-to-one to validation constraints and other semantics. DTD validation has no support for the use of ulabels. Modern validation facilities such as XSDL, Schematron, and Relax allow the tying of constraints to components in a much more flexible way, including element context. They also provide good support for constraining combinations of components with labels, ulabels, or a mixture of the two. There is an open debate as to whether or not, in newly defined vocabularies, ulabels should be specified for all the elements. Those who feel that they should be are irritated by XSDL's apparent bias in the other direction. At the end of the day, labels are just labels. They are one of the stepping stones from content to semantics, but only one. If anyone wants to take issue with this, please try to do so without using the T-word. -Tim 
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