8.4 Writing XML
Writing XML involves entering structured information that complies with a document type definition or schema. Even within Emacs, the XML support you receive varies. At the low end of the spectrum, there is plain vanilla Fundamental mode. It provides simply a screen where you type. Specialized modes like SGML mode provide support for entering tags, as we saw earlier in our discussion of HTML mode, a derivative of SGML mode. But neither of these approaches help you parse or validate XML (SGML mode has a command for validating, but it is tricky to set up correctly). More advanced Lisp packages, though currently not included in Emacs, are available to provide these functions. These add-on packages provide validation against DTDs or schemas, parsing capabilities, and, typically, an array of standard DTDs and schema definitions. In Emacs, these tools primarily work in conjunction with one of two major modes. psgml mode validates XML (and SGML) against DTDs. The newer nxml mode validates against RELAX NG schemas. We cover both of these options in this section. Before we go into detail on those modes, however, let's look briefly what Emacs has built-in with SGML mode.
8.4.1 Writing XML with SGML Mode
Emacs's own SGML mode provides support for entering tags. We covered much of this earlier under HTML mode, so we provide just one brief example here. Inserting, hiding, and showing tags are especially helpful features provided by SGML mode.
Let's look at a chapter on enumerated types by Java in a Nutshell author David Flanagan. This chapter uses the DocBook DTD.
Initial state:
Editing a document that uses the DocBook DTD (Mac OS X).
Note that Emacs displays XML on the mode line. XML mode in this context is a subset of SGML mode. Actually, despite this name, all the commands in this mode start with sgml, not xml. The menu of relevant commands is called SGML as well. Emacs doesn't pretend to have extensive XML support.
We want to insert a paragraph before the first paragraph.
Add a blank line following the title and type: C-c C-t
Emacs inserts an open angle bracket and prompts for the tag name (Mac OS X).
Type: para Enter
Emacs inserts opening and closing paragraph tags (Mac OS X).
Note that Emacs is not following our indentation style. We can correct it by moving to the beginning of the line and pressing Tab. See Table 8-4 earlier in this chapter for details on SGML mode commands.
8.4.2 TEI Emacs: XML Authoring for Linux and Windows
The Text Encoding Initiative (TEI) wanted an XML authoring environment for Emacs, so it created (the somewhat misleadingly named) TEI Emacs.[50] Despite its name, TEI Emacs does not include Emacs itself. Rather, it creates an authoring environment for writing XML using nxml mode or psgml mode. It incorporates XSLT tools, along with most of the standard DTDs, such as the three forms of XHTML DTDs (strict, frameset, and transitional), DocBook DTDs, and more. Naturally, the TEI's own DTDs and schemas are also included.
The active development of this tool and its careful packaging led us to describe this tool despite the fact that it is limited to Linux and Windows at this writing.[51] You should have Emacs 21.3 already installed before you install this tool. Installing TEI Emacs is trivial. The Windows version has an installer, and Linux users follow simple instructions at http://www.tei-c.org/Software/tei-emacs/, the web site for downloading TEI Emacs.
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We sincerely hope that this support will be extended to Mac OS X as well, providing developers and writers on that platform the benefits of this tool's capabilities. Meanwhile, Mac users may want to install nxml mode from http://thaiopensource.com/download/ and psgml mode from http://www.lysator.liu.se/projects/about_psgml.html.