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Chapter 37. Toward XML > XML in Theory and Practice - Pg. 691

Toward XML Tip from 691 If your target market includes government agencies or contractors, there are probably existing SGML DTDs exactly suited to at least part of their problem space. If so, there are probably industry groups already translating those DTDs to XML. So it pays to do your homework before undertaking XML DTD development. An appropriate DTD might already be available. Although you don't have to know SGML to learn and use XML, it is fairly common to modify existing or legacy SGML DTDs into XML DTDs in the course of daily life as an XML expert. You will learn enough about this process to be able to do it with some confidence. Caution A caution is in order here. Many SGML DTDs are huge because the problems they address are large. You can't expect to waltz up to a 900-page SGML DTD that took a team of 20 experts five years to develop and convert it to XML in an afternoon. It might take a wee bit longer than that. It might take a year or more. XHTML XHTML redefines--as you already know--HTML as an XML application. This makes it more useful in an XML world. In particular, this means that the XHTML language is extensible, allowing users and groups of users to extend the language in powerful ways. Implications of this extensibility in- clude: · Single-edit-cycle documents that encapsulate databases, printed user manuals, and Web dis- play functionalities in one file. · Tremendous opportunities for extended e-commerce transactions and information exchange automation using standardized XML vocabularies. · An easy way to reuse or convert many of the millions of HTML Web pages in existence today, retaining compatibility with existing browsers while enabling new features for those using XML- enabled agents. · Allowing XML Web pages to be automatically validated, completely eliminating many common coding errors while retaining compatibility with legacy browsers. · Providing simple mechanisms that enable the Web to evolve to meet the needs of the diverse communities it encompasses without requiring proprietary or non-standard additions that make life difficult. XML in Theory and Practice XML is so logical you might wonder why it took so long to be invented. Part of the answer is that the basic concepts have been around for a long time but were only recently applied to computer data files. A component-based parts list, for example, is a trivial requirement for putting together any complex mechanical device. But the idea of extending this paper tool to the electronic one, and generalizing the concept so it could be used for anything made up of component parts, including non-physical objects, was a flash of insight very typical of human progress over the centuries. People organize almost everything into hierarchies. It's the only way to handle truly complex tasks, from buying supplies for the Department of Defense to building space shuttles. Any hierarchical structure can be described with XML, from the parts list that makes up an airliner to the corporate structure of IBM. But XML has limitations. It's not truly object-oriented, for example, so users with problem sets requiring a fully object- oriented (O-O) approach will have some trouble applying XML to their tasks. Initiatives are underway, however, to extend the domain of XML in object-oriented ways, so this limitation might be resolved eventually.