ch02_01.htm

Perl & XML. by Erik T. Ray and Jason McIntosh ISBN 0-596-00205-X First Edition, published April

"GML."</p> </blockquote>

<p>Goldfarb went on to lead a standards team at the
<a name="INDEX-61" /> <a name="INDEX-62" />American National
Standards Institute (ANSI) to make the power of GML available to the
world. Building on the GML and GenCode projects, the committee
produced the Standard Generalized Markup Language (SGML). Quickly
adopted by the U.S. Department of Defense and the Internal Revenue
Service, SGML proved to be a big success. It became an international
standard when ratified by the ISO in 1986. Since then, many
publishing and processing packages and tools have been developed.
</p>

<p>Generic coding was a breakthrough for digital content. Finally,
content could be described for what it was, instead of how to display
it. Something like this looks more like a database than a
word-processing file:
</p>

<blockquote><pre class="code">&lt;personnel-record&gt;
  &lt;name&gt;
    &lt;first&gt;Rita&lt;/first&gt;
    &lt;last&gt;Book&lt;/last&gt;
  &lt;/name&gt;
  &lt;birthday&gt;
    &lt;year&gt;1969&lt;/year&gt;
    &lt;month&gt;4&lt;/month&gt;
    &lt;day&gt;23&lt;/day&gt;
  &lt;/birthday&gt;
&lt;/personnel-record&gt;</pre></blockquote>

<p>Notice the lack of presentational information. You can format the
name any way you want: first name then last name, or last name first,
with a comma. You could format the date in American style (4/23/1969)
or European (23/4/1969) simply by specifying whether the
<tt class="literal">&lt;month&gt;</tt> or <tt class="literal">&lt;day&gt;</tt>
element should present its contents first. The document
doesn't dictate its use, which makes it useful as a
source document for multiple destinations.
</p>

<p>In spite of its revolutionary capabilities, SGML never really caught
on with small companies the way it did with the big ones. Software is
expensive and bulky. It takes a team of developers to set up and
configure a production environment around SGML. SGML feels
bureaucratic, confusing, and resource-heavy. Thus, SGML in its
original form was not ready to take the world by storm.
</p>

<p>"Oh really," you say.
"Then what about HTML? Isn't it
true that <a name="INDEX-63" />HTML is an application of
SGML?" HTML, that celebrity of the Internet, the
harbinger of hypertext and workhorse of the <a name="INDEX-64" />World
Wide Web, is indeed an application of SGML. By application, we mean
that it is a markup language derived with the rules of SGML. SGML
isn't a markup language, but a toolkit for designing
your own descriptive markup language. Besides HTML, languages for
encoding technical documentation, IRS forms, and battleship manuals
are in use.
</p>

<p>HTML is indeed successful, but it has limitations.
It's a very small language, and not very
descriptive. It is closer to troff in function than to
<a name="INDEX-65" />DocBook and other
SGML applications. It has tags like <tt class="literal">&lt;i&gt;</tt> and
<tt class="literal">&lt;b&gt;</tt> that change the font style without
saying why. Because HTML is so limited and at least partly
presentational, it doesn't represent an overwhelming
success for SGML, at least not in spirit. Instead of bringing the
power of generic coding to the people, it brought another one-trick
pony, in which you could display your content in a particular venue
and couldn't do much else with it.
</p>

<p>Thus, the standards folk decided to try again and see if they
couldn't arrive at a compromise between the
descriptive power of SGML and the simplicity of HTML. They came up
with the Extensible Markup Language (XML). The
"X" stands for
"extensible," pointing out the
first obvious difference from HTML, which is that some people think
that "X" is a cooler-sounding
letter than "E" when used in an
acronym. The second and more relevant difference is that your
documents don't have to be stuck in the anemic tag
set of HTML. You can extend the tag namespace to be as descriptive as
you want -- as descriptive, even, as SGML. Voil&agrave;! The
bridge is built.
</p>

<p>By all accounts, XML is a smashing success. It has lived up to the
hype and keeps on growing: XML-RPC, XHTML, SVG, and DocBook XML are
some of its products. It comes with several accessories, including
XSL for formatting, XSLT for transforming, XPath for searching, and
XLink for linking. Much of the standards work is under the auspices
of the <a name="INDEX-66" /> <a name="INDEX-67" />World Wide Web Consortium (W3C), an
organization whose members include Microsoft, Sun, IBM, and many
academic and public institutions.
</p>

<p>The W3C's mandate is to research and foster new
technology for the Internet. That's a rather broad
statement, but if you visit their site at <a href="http://www.w3.org/">http://www.w3.org/</a> you'll
see that they cover a lot of bases. The W3C doesn't
create, police, or license standards. Rather, they make
recommendations that developers are encouraged, but not required, to
follow.<a href="#FOOTNOTE-4">[4]</a>
</p><blockquote class="footnote"> <a name="FOOTNOTE-4" /><p>[4]When a trusted body like the W3C makes a
recommendation, it often has the effect of a law; many developers
begin to follow the recommendation upon its release, and developers
who hope to write software that is compatible with everyone
else's (which is the whole point behind standards
like XML) had better follow the recommendation as well.</p>
</blockquote>

<p>However, the system remains open enough to allow healthy dissent,
such as the recent and interesting case of XML Schema, a W3C standard
that has generated controversy and competition.
We'll examine this particular story further in <a href="ch03_01.htm">Chapter 3, "XML Basics: Reading and Writing"</a>. It's strong enough to be
taken seriously, but loose enough not to scare people away. The
recommendations are always available to the public.
</p>

<p>Every developer should have working knowledge of XML, since
it's the universal packing material for data, and so
many programs are all about crunching data. The rest of this chapter
gives a quick introduction<a name="INDEX-68" /> to XML<a name="INDEX-69" /> for developers.
</p>

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