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XML-L  November 2010

XML-L November 2010

Subject:

Re: Somewhere between parsing and data binding?

From:

Peter Flynn <[log in to unmask]>

Reply-To:

General discussion of Extensible Markup Language <[log in to unmask]>

Date:

Wed, 24 Nov 2010 22:30:14 +0000

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text/plain

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text/plain (144 lines)

On 24/11/10 13:22, Peter Davis wrote:
> On Wed, Nov 24, 2010 at 01:40:58AM -0800, Peter Flynn wrote:
>> Quoting Peter Davis <[log in to unmask]>:
>>> I'm trying to build a prototype as a proof of concept for a workflow
>>> that requires parsing two XML files.  In coding this (C++), it occurred
>>> to me that having to conditionally test an element's name *after* the
>>> parser has already scanned it is redundant.
>>
>> I'm not clear what the difficulty is here: this would be the normal way 
>> of doing it. A parser checks for well-formedness, and then hands the 
>> resulting tree to the application so that the application can do  
>> whatever it's supposed to do.
> 
> Sorry I wasn't more clear.  I'll try to explain further.  I realize the
> normal order of business is to do a purely syntactic parse, and then
> hand off either events or a DOM tree to the application.  However, this
> results in the application's having to make a second pass over the same
> textual input in order to make sense of it.

Umm. I'm not sure it does, but I think it depends on how the parser is
integrated with the application. I don't think that a system where the
parse tree is just pointers back to the original text of the XML
document would be very efficient, but I may be wrong. In my ignorance of
the internals, I would guess that the parser hands the entire tree to
the application, consisting of every node it has identified, text and
all; and that any time the application needs an element and its content,
it gets it from the tree via whatever index the parser has provided, not
by re-reading the XML source text all over again. But this is largely
guesswork on my part: you may want to check this with someone who knows
the architecture of these things.

> A compiler doesn't usually
> make that clean a separation between syntactic and semantic analysis,
> because the two require a lot of the same steps.
> 
> With existing parsers, I have to write code like:
> 
> void HandleBeginElement(...)
> {
>     if (!strcmp(elementName, "document")
>     {
>         ... do document-y things ...
>     }
>     else if (!strcmp(elementName, "page")
>     {
>         ... do page-y things ...
>     }
>     ... etc.
> }

Oh dear. I think there is a misunderstanding going on here, either me
and C++ (not my language) or you and XML, or us both. This is a
procedural approach, and I stopped doing procedural stuff (except at the
trivial level) a long time ago, when XSLT replaced Omnimark.

> This is very cumbersome code, and not at all object-oriented.  Even with
> DOM parsing, I have to examine each element in the tree, and do some
> kind of cascading 'if' statement like the above to handle each case.

I think you might seriously want to re-examine XSLT and get well away
from C++ for this.

> So instead of setting up beginElement and endElement handlers, I want to
> setup a beginDocumentElement handler, and a beginPageElement handler,
> etc.  Since the parser is already scanning the input to determine when
> to call the callbacks (in the SAX case), it could almost as easily just
> call different callbacks for each interesting element type.
> 
> This is why I thought of it as somewhere between parsing and data binding.
> 
>> Ah. I am probably being dense here. Are you trying to locate or isolate 
>> one specific element from within each file (ie as opposed to needing to 
>> handle all the elements)?
> 
> No, I'm trying to handle all (or most) of the elements.  I just want the
> code to be straightforward.
> 
>> Can you explain a little more about what you are actually trying to do? I 
>> tend to build workflows at the scripting level rather than in a single 
>> language because the facilities that can be plugged into the pipeline are 
>> much more extensive.
> 
> In this particular case, I'm trying to convert XML to TeX or LaTeX for
> publishing.  I think the potential solution is pretty general, though.
> I've worked with XML on other solutions in the past, and run into this
> repeatedly. 

In that case you definitely need to use XSLT, IMHO. I use it all the
time for exactly this kind of application.

>> There are a number of tools which can be used to extract specific  
>> individual elements, but by design they are mostly limited to handing  
>> you the whole element, not the isolated start-tag or end-tag.
> 
> That would work, as long as I could continue to recurse into the element
> to fetch its children, grandchildren, etc. in a similar fashion.

XSLT does that.

>> XSLT is probably the most common, but I assume you have already looked  
>> at this. XQuery may also be useful if you are looking to identify an  
>> individual element.
> 
> XSLT is close to what I'm thinking of, but is not powerful enough.  For
> example, in the course of processing my document, I might run across
> something like <img src="..."/>.  So in processing that element for
> conversion, I want to actually retrieve that image, and inspect the file
> to determine what size it is, what resolution, etc.  I don't think
> there's a way to do that in XSLT.  I know there are some extended XSLT
> models like Saxon and Xalan, but I don't know how widely they're
> supported. 

Ah. I'd do this as a preparatory pass, and write the results to a file
that XSLT can read while processing. Yes, it does mean a pass through
the file, but dog is very fast, and so is ImageMagick's identify:

echo \<images\> >images.xml
for f in `dog --images myfile.html`; do
  wget -O - $f | identify -verbose - |\
  awk -v file=$f -F: 'BEGIN {print "<image name=\"" file "\""}
    /Resolution:/ {print "res=\"" $2 "\""}
    ...etc...}' >>images.xml
done
echo \</images\> >>images.xml

This will process very fast (you'll have a network delay no matter what
you do, unless all the files are local), andit gives you all the data
you need in a form that an XSLT script can use as a lookup table. If the
input is not HTML, or as an alternative, you could use
  lxgrep -w images '//graphic' filename.xml >images.xml
to extract the names, and then something like the above to run identify
over them.

Basically I'd separate the extraction of image data from the business of
converting XML to LaTeX.

> Ultimately, I'm going to be integrating with a fairly large body of C++
> code, so that's the language of choice.  I've been Googling for tools,
> but not finding much.

It's going to take a lot of reinventing of wheels. I generally prefer to
use existing wheels :-)

///Peter

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