Tag libraries in XSLT / XQuery?

[Erik Hennum <efhennum@gmail.com>]

Speculative XML Savants:

I'm wondering whether a moral equivalent to JSP tag libraries [1] would
be a useful addition to the XSLT / XQuery portfolio -- so that XML
documents could use tags

*  defined in an XSLT / XQuery module (instead of a Java class), and
*  executed by an XSLT / XQuery processor (instead of a JSP container).

The rationale for tag libraries (probably summarizing the familiar) is
to establish a division of responsibility and collaboration between

*  application developers who understand data access and manipulation, and
*  page designers who understand visual layout and style.

For an example, let's say an application developer creates a tag library
with an query:people tag that queries attendee records by year and sorts
the result list by first or last name.

A page designer could then create an XHTML page with the query:people tag:

<table class="attendees">
  <tr><th>Last Name</th>
    <th>First Name</th></tr>
<query:people select-year="2010" sort-by="last-name">
  <tr><td class="lname">{ $last-name  },</td>
    <td class="fname">{ $first-name }</td></tr></query:people>
</table>

When supplied with the tag page and the tag library, a processor would
generate an XHTML output page similar to the following:

<table class="attendees">
  <tr><th>Last Name</th>
    <th>First Name</th></tr>
<!-- query:people select-year="2010" sort-by="last-name" -->
  <tr><td class="lname">Crawford,</td>
    <td class="fname">Randy</td></tr>
  <tr><td class="lname">Franklin,</td>
    <td class="fname">Aretha</td></tr>
  <tr><td class="lname">Green,</td>
    <td class="fname">Albert</td></tr><!-- /query:people -->
</table>

Some potential scenarios for tags in XML processing environments:

*  An XQuery tag library could provide views on an XML store for
   integration and branding by HTML pages.

*  An XSLT tag library could transform articles into content sequences
   for layout as flows by XSL-FO book masters.

XSLT Simplified Stylesheet Modules [2] already have several of the
constructs needed for XML tag pages.  The legal XSLT statements include
equivalents (such as xsl:for-each and xsl:if) for important core JSTL
statements.  XSLT Attribute Value Templates [3] offer a parallel to the
EL expression language.

Simplified Stylesheet Modules would seem to need new constructs to:

*  Import the XSLT / XQuery library modules that implement the tags.
*  Execute value templates within element text (as with the $last-name
   and $first-name expressions in the example above).
*  Process tags as calls to library functions, passing the tag
   attributes as function parameters and the tag content as a
   callback (as with the attributes and content of the query:people tag).

A XSLT or XQuery tag library module would seem to need new constructs to:

*  Bind a tag name to its implementing function.
*  Process callbacks zero or more times as appropriate.

For experimentation, I put together a sketchy preprocessor [4] that
converts an XHTML tag page and an XSLT tag module to intermediate XSLT
files and then executes the intermediate XSLT files to produce an output
XHTML document (supporting callbacks through Dimitre Novatchev's
technique [5] for dynamic functions).

This preprocessor has significant limitations with respect to callbacks.
In particular, variables in scope at the definition of the callback
fragment aren't in scope during processing of the callback fragment.
One alternative might be to emit the tag implementation inline within
the converted tag page.

A really robust implementation of tag libraries may need native XSLT /
XQuery anonymous functions.  (Having just noticed Dynamic Function
Invocation [6] in the XPath 2.1 working draft, I'm wondering if that
would help.)

Anyway, that's the idea.  What do you think?  Wrongheaded?  Worth
pursuing?


Erik Hennum


Acknowledgments:  The thought was triggered by a talk about virtual
documents [7] by Vyacheslav Zholudev at the Balisage conference.


[1] http://en.wikipedia.org/wiki/JSTL
[2] http://www.w3.org/TR/xslt20/#simplified-stylesheet
[3] http://www.w3.org/TR/xslt20/#dt-attribute-value-template
[4] http://docs.google.com/leaf?id=0B_yrFnrTAZDGNGY3MjI2MTgtZGZjMS00ZDUwLTljNjgtNDg1NDdhNzY4NDE5&hl=en
    CAVEAT:  only an experiment, may change completely or be abandoned
[5] http://fxsl.sourceforge.net/
[6] http://www.w3.org/TR/xpath-21/#id-function-invoke
[7] http://www.balisage.net/Proceedings/vol5/html/Zholudev01/BalisageVol5-Zholudev01.html