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<!DOCTYPE HTML PUBLIC "html.dtd"><HTML><HEAD><TITLE>Presenting XML:Resolution of the XML Specification:EarthWeb Inc.-</TITLE><META NAME="ROBOTS" CONTENT="NOINDEX, NOFOLLOW"><SCRIPT><!--function displayWindow(url, width, height) { var Win = window.open(url,"displayWindow",'width=' + width +',height=' + height + ',resizable=1,scrollbars=yes');}//--></SCRIPT></HEAD><BODY BGCOLOR="#FFFFFF" VLINK="#DD0000" TEXT="#000000" LINK="#DD0000" ALINK="#FF0000"><TD WIDTH="540" VALIGN="TOP"><!-- <CENTER><TABLE><TR><TD><FORM METHOD="GET" ACTION="http://search.itknowledge.com/excite/cgi-bin/AT-foldocsearch.cgi"><INPUT NAME="search" SIZE="20" VALUE=""><BR><CENTER><INPUT NAME="searchButton" TYPE="submit" VALUE="Glossary Search"></CENTER><INPUT NAME="source" TYPE="hidden" VALUE="local" CHECKED> <INPUT NAME="bltext" TYPE="hidden" VALUE="Back to Search"><INPUT NAME="sp" TYPE="hidden" VALUE="sp"></FORM></TD><TD><IMG SRC="http://www.itknowledge.com/images/dotclear.gif" WIDTH="15" HEIGHT="1"></TD><TD><FORM METHOD="POST" ACTION="http://search.itknowledge.com/excite/cgi-bin/AT-subscriptionsearch.cgi"><INPUT NAME="search" SIZE="20" VALUE=""><BR><CENTER><INPUT NAME="searchButton" TYPE="submit" VALUE=" Book Search "></CENTER><INPUT NAME="source" TYPE="hidden" VALUE="local" CHECKED> <INPUT NAME="backlink" TYPE="hidden" VALUE="http://search.itknowledge.com:80/excite/AT-subscriptionquery.html"><INPUT NAME="bltext" TYPE="hidden" VALUE="Back to Search"><INPUT NAME="sp" TYPE="hidden" VALUE="sp"></FORM></TD></TR></TABLE></CENTER> --><!-- ISBN=1575213346 //--><!-- TITLE=Presenting XML//--><!-- AUTHOR=Richard Light//--><!-- PUBLISHER=Macmillan Computer Publishing//--><!-- IMPRINT=Sams//--><!-- CHAPTER=17 //--><!-- PAGES=0309-0330 //--><!-- UNASSIGNED1 //--><!-- UNASSIGNED2 //--><P><CENTER><A HREF="0319-0321.html">Previous</A> | <A HREF="../ewtoc.html">Table of Contents</A> | <A HREF="0325-0327.html">Next</A></CENTER></P><A NAME="PAGENUM-322"><P>Page 322</P></A><P>and describing the results of parsing an SGML document. Initially there <BR>was simply no agreement between the two approaches, and this caused major <BR>problems.<P>Over the next few years a lot of heads must have done some seriousthinking and a lot of hard work; then, in 1995, the concept ofgroves (which was also incorporated into XML, as discussed in Chapter 7, "Physical Structures inXML Documents") first saw the light of day. With the publication of theproposal for a TC for DSSSL in November 1996, HyTime and DSSSL actuallystarted to agree with each other about certain essential points.</P><TABLE BGCOLOR="#FFFF99"><TR><TD>Note:</TD></TR><TR><TD><BLOCKQUOTE>These points of difference between DSSSL and HyTime werequite complicated and absolutely fundamental to their processing models.They involved a lot of extremely technical questions that sometimes seemed tothe outsider to be involved with trivia (such as the handling of white space). Atthe heart of it was the problem that DSSSL and HyTime could parse anSGML document and produce two quite different trees of its structure.Fortunately, we don't need to concern ourselves with that here!</BLOCKQUOTE></TD></TR></TABLE><H4><A NAME="ch17_ 19">Revising SGML</A></H4><P>Revision of the SGML standard did not seem to make much sense whilethe future form of HyTime and DSSSL was still being finalized. The HyTimeand DSSSL standards have now been revised. (Revision 2 of the HyTimestandard was approved in early May 1997 and should be published later this year;the DSSSL standard, distributed for voting by ISO members in August 1994and published in January 1996, was the subject of a TC in November 1996.)</P><P>With all this settled, and with the specter of XML looming large, itbecame obvious that the time for a revision of SGML had truly come and that ithad better happen quickly. Serious work has now started on the revision ofthe SGML standard, with the intention of publishing it by the end of 1997.</P><P>One of the most important changes is the incorporation of Appendix A ofthe HyTime standard into the body of the SGML standard. This includesthe following:</P><UL><LI> Lexicaltypes: Rules for identifying and describing the partsof SGML's language—delimiters, elements, and so on.<LI> Architecturalforms: Rules for creating and processing SGML<BR>elements.</UL><A NAME="PAGENUM-323"><P>PAGE 323</P></A><UL><LI> Property sets andgroves: Named sets of properties that can be recognized by applications. (See Chapter 7.)<LI> Formal systemidentifiers: The formal definition of thenotations used in system identifiers that can then be used to specify access tothe "storage objects" and the entities in them.</UL><P>While all this hard work was going on (and it really is a recurrent themethrough-out SGML's and XML's joint history), the World Wide Web had beenhappening, and HTML had become the "killer" SGML application. Thesechanges to the SGML standard had already been planned, and they are quiteradical and far-reaching changes.</P><P>With so much going on, it would surely seem like a small effort to adoptsome of the changes suggested by XML; however, the ISO standards procedurecan take years. (Two years is not an unreasonable length of time for passing adraft standard through a vote by all the member countries.) If ever a sayingtypified the Web, it is quite simply "the Web won't wait."</P><P>The answer is a Technical Corrigendum for the SGML standard that allowsa lot of the ISO's formal approval procedure to be either bypassed oraccelerated. The TC introduces changes that will give SGML some of XML'sstrengths and, on the other hand, will make XML an official "conforming" SGMLapplication.</P><P>The TC consists of two annexes to the SGMLstandard—Annex K: WebSGML Adaptations (a "normative" addition; in other words, not mandatory forcompliance with the SGML standard), and Annex L: Application Requirementsfor XML (an "informative" addition).</P><P>Annex L is really (in its version 1.0 form) nothing more than a pointer tothe XML syntax specification and a paraphrasing of the list of restrictionsimposed on SGML by XML contained in Appendix A of the XML specification.(These differences are listed and explained in Appendix A of this book.) In the restof this chapter, I concentrate on Annex K.</P><H4><A NAME="ch17_ 20">Annex K: WebSGML Adaptations</A></H4><P>Until Annex K is adopted and implemented into SGML, the existingSGML tools will be largely unable to handle XML instances. The XML Syntaxstandard includes its SGML declaration (in Appendix A of the standard),which makes most of XML's features valid in SGML, but not all of them. TheSGML "WebSGML Adaptations" basically lay out the instructions formodifying SGML</P><A NAME="PAGENUM-324"><P>Page 324</P></A><P> (mostly through changes to the SGML declaration), but they alsointroduce to SGML some of the basic concepts that make XML so powerful.</P><TABLE BGCOLOR="#FFFF99"><TR><TD>Caution:</TD></TR><TR><TD><BLOCKQUOTE>A word of caution before you go any further! The Technical<BR>Corrigendum as described here is only a proposal (project JTC1.18.15.1)<BR>that was submitted for ballot on June 9, 1997. The closing date for voting <BR>by the members of the working group (Working Group 8 of Subcommittee 18,<BR>which is a part of ISO/IEC Joint Technical Committee 1—even though <BR>there is only one joint TC) is September 22, 1997; only then can anyone <BR>outside the working group know what the future form of the TC will be.<BR>On May 8, 1997, the French and Norwegian representatives (Michel<BR>Biezunski and Steve Pepper, respectively) submitted a proposal for a<BR>phased introduction of the TC. Without guessing what the reaction to this <BR>proposal might be, I can say that it does give some indication that the<BR>future of the TC is not necessarily guaranteed.</BLOCKQUOTE></TD></TR></TABLE><P>You might be wondering if the changes might not even happen.Undoubtedly they will happen, but it's more a question of "when" than "what." Let'slook at what these changes are. I won't go into full details of all the changesbecause some of them require a pretty good knowledge of SGML to make themcomprehensible, and I won't even mention all of them because some are onlyof passing interest. (The proposal is freely available on the Internet if youwant to check out all the fine details. See Appendix B.) Instead, I'll concentrateon those changes most relevant to XML, and you'll see for yourself howXML and SGML seem to be doing their best to find some middle ground withSGML on the Web and XML as a recognized SGML application.</P><H4><A NAME="ch17_ 21">New Definitions</A></H4><P>The very first change to SGML—one that a lot of SGML diehards wouldprobably never have even dreamed of—is the adoption of XML's concept ofwell-formedness. Not to make it too obvious, though, SGML has adopted itsown terminology to define three different types of SGML documents:</P><UL><LI> Type-validdocuments: These are just the same old SGML documents they always were—that is, documents that have anSGML DTD associated with them to which they conform.<LI> Tag-validdocuments: These are documents that are fully taggedbut do not need a DTD. There may be a DTD, but the document canbe parsed with or without it.</UL><P><CENTER><A HREF="0319-0321.html">Previous</A> | <A HREF="../ewtoc.html">Table of Contents</A> | <A HREF="0325-0327.html">Next</A></CENTER></P></TD></TR></TABLE></BODY></HTML>⌨️ 快捷键说明
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