[xsl] how to rearrange nodes based on a dependency graph?

[Gunther Schadow <gunther at aurora dot regenstrief dot org>]

Hi XSL listers,

I'm hung up on a puzzling problem and need your help. I am
relatively new to XSL but thanks to the pretty readable
specification and some example I was so far able to find
my way. Except when I wanted to be really clever, like here.

The purpose of what I'm doing is kind of what you might
know as "literal programming." It's writing a document
that explains the detail of some formal language utterance,
such as a program, or an XML schema or an IDL specification
or whatever have you. The document is to be written for
readability and may well have a different flow than what's
required for the formal language (program or xsd or whatever.)
For example, in describing a Pascal program, I might want to
discuss an outline of the main program first before I go
into the detail of the procedures. Yet in the program text,
the procedures need to come before the main program. The
point of course is that the text document should be the main
focus of development and maintenance, and the program text
should be generated from there. That's where XSLT comes
in and falls short? (you tell me if it does of if I fall
short :-)

Here is an example. This XML instance discusses a mocked up
up XML schema for defining some data.


<document>

Blah bla

<frag id='1' requires='2'>
    BEGIN
      DoSomethingUseful();
    END
</frag>

Blah blah

<frag id='2'>
   PROCEDURE DoSomethingUseful
   BEGIN
     ...
   END
</frag>

</document>




The <frag> tag has an 'id' (ID) attribute with matching
'requires' (IDREFS) attribute.

The XSL templates will go through the document and discard
everything, except when encountering a fragment. It will
then will hunt down the requires idrefs to find other
fragments that should come first. Of course, if a fragment has
already been emitted, it shouldn't appear again. And that't
really my big problem.

Here is my XSL so far:

<xsl:key name='fragkey' match='frag' use='@id' />

<xsl:template match='frag'>
   <xsl:for-each select='@requires'>
     <xsl:apply-templates select="key('fragkey',.)"/>
   </xsl:for-each>

   <xsl:for-each select='*'>
     <xsl:call-template name='copy-stuff'/>
   </xsl:for-each>
</xsl:template>

<xsl:template name='copy-stuff'>
   <xsl:copy>
     <xsl:for-each select='@*|*'>
       <xsl:call-template name='copy-stuff'/>
     </xsl:for-each>
   </xsl:copy>
</xsl:template>



This works nicely, except for the fact that fragment number 2
is emitted twice, first as the dependency of fragment 1 and
then because it appears as the next fragment in the document.
(Similarly this runs amok if there's a circular dependency.)

I tried to use a variable as a check-off list of fragments
already emitted, but that's not possible because XSL "variables"
are actually constants. When I try to write such a function
in LISP without using global variables and side effects, it
get's pretty difficult and the only way to survive here is
because I can examine return values. In XSL, I cannot examine
the output tree, or can I?

Any suggestions are appreciated. Of course I can dumb down
this system and instead of requirement links I could use
some kind of sequence number or forward linking and sort by
that etc. But I feel that the dependency graph is the most
appropriate way to represent this and if XSL is a complete
functional language it should have some way to deal with that.

regards,
-Gunther


-- 
Gunther Schadow, M.D., Ph.D.                    gschadow@regenstrief.org
Medical Information Scientist      Regenstrief Institute for Health Care
Adjunct Assistant Professor        Indiana University School of Medicine
tel:1(317)630-7960                         http://aurora.regenstrief.org



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