Xml Matters #31: Sxml And Ssax

Manipulating XML in the Scheme Programming Language


David Mertz, Ph.D.
Parenthesizer, Gnosis Software, Inc.
October 2003

Previous installments have looked at XML libraries for various programming languages, and with various strengths and weaknesses. The Lisp family of languages remains popular, especially in teaching and among purists. The SSAX library for Scheme is an efficient pure-functional parser; SXML is a tree library (like DOM); and related tools SXSLT and SXPath have been created to work with these. This installment looks at the advantages of parsing in a strongly functional language, and compares SSAX with libraries for other languages.

Introduction

I am aware--as a factual matter--that there are connoisseurs of single-malt scotches. But given that I rarely drink alcohol at all; and when I do it in less concentrated amounts as beer or wine, I have trouble fully understanding the mindset or discourse of these connoisseurs.

I feel almost the same way about Lisp and Scheme programming. I can tell that it is an area filled with sophistication and intelligence, but somehow both the Polish (prefix) notation and endless parentheses, and the fervent semantic eshewal of a distinction between code and data, continue to feel alien to me. Nonetheless, I have enough of a fascination that I want to see how these languages approach XML processing.

What Is Sxml?

The starting point for the SSAX library for Scheme is a meme common among Lisp/Scheme enthusiasts: the observation that XML is semantically almost identical to the nested list-oriented data structures native to Lisp-like languages. Anything you can represent in XML can be straightforwardly represented as SXML--Scheme lists nesting the same data as the original XML. Moreover, Scheme comes with a rich library of list and tree manipulation functions, and a history of contemplating manipulation of those very structures. A natural fit, perhaps.

A good first step is to see SXML in its concrete form. Trees are the underlying abstraction--the Infoset--of XML; but the abstract information takes a specific semantic form. For example, the following is a starkly reduced (but still well-formed) version of another article I wrote recently:

An XML document with most XML features

$ cat example.xml
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/css"
      href="http://gnosis.cx/publish/programming/dW.css" ?>
<dw-document xmlns:dw="http://www.ibm.com/developerWorks/">
  <title>The Twisted Matrix Framework</title>
  <author name="David Mertz, Ph.D.">
    <bio>David thinks it's turtles all the way down...</bio>
  </author>
  <docbody>
    <dw:heading refname="" toc="yes">Introduction</dw:heading>
    <p>
      Sorting through Twisted Matrix is reminiscent of the old story
      about blind men and elephants. Twisted Matrix has many
      capabilities within it, and it takes a bit of a gestalt switch
      to <i>get</i> a good sense of why they are all there.
    </p>
  </docbody>
</dw-document>

Transformed to SXML, this article looks like:

An SXML document with most XML features

$ ./xml2sxml example.xml
(*TOP* (*PI* xml "version=\"1.0\" encoding=\"UTF-8\"")
       (*PI* xml-stylesheet
             "type=\"text/css\"
      href=\"http://gnosis.cx/publish/programming/dW.css\" ")
       (dw-document
         (title "The Twisted Matrix Framework")
         (author
           (@ (name "David Mertz, Ph.D."))
           (bio "David thinks it's turtles all the way down..."))
         (docbody
           (http://www.ibm.com/developerWorks/:heading
             (@ (toc "yes") (refname ""))
             "Introduction")
           (p "
      Sorting through Twisted Matrix is reminiscent of the old story
      about blind men and elephants. Twisted Matrix has many
      capabilities within it, and it takes a bit of a gestalt switch
      to "
              (i "get")
              " a good sense of why they are all there.
    "))))

There are a number of interesting differences between XML and SXML, but also a obvious and direct correspondence between every aspect. Some of the differences are relatively trivial--parentheses instead of angle brackets, for example. And others are ambivalent: for example, SSAX's creator, Oleg Kiselyov, thinks the reduction of closing tags to closing parentheses is a pure advantage in conciseness. However, the designers of XML went out of their way to remove the tag reduction options in SGML; maybe they were wrong, but explicit closing tags are not there because their possibility was overlooked.

In a number of ways, however, SXML corrects some awkwardness in XML, while not losing any information. In particular, the distinction between attributes and element contents feels arbitrary to most XML developers, and SXML removes it in an elegant way. An attribute collection is simply another nested list, but one that happens to start with the name @--a name that conveniently is not permitted in XML identifiers. Effectively, an SXML document is like an XML document that eshews attributes, but sometimes nests a <@> child inside other elements. Referring to children that happen to be named @ is no different in SXML is no different the filtering on any other tag name. Of interest is that both my gnosis.xml.objectify and RelaxNG stylesheets attempt a similar homogenization of attributes and elements.

Processing instructions and comments are also reduced to special "tag" names that are not available in XML: PI for the former, COMMENT for the latter. As with most of Lisp, there is just one basic data structure to represent everything.

Namespaces are also interesting in the SXML format. The full namespace URI simply becomes part of the tagname, when SXML is generated as above. However, and optional NAMESPACES tag can be used to abbreviate namespace references in essentially the same way as with XML. You need to either write SXML by hand, or enhance the conversion utility, to utilize this, however.

Working With The Ssax Library

From the description above, SXML seems like just another shortcut notation for XML, or which there are many, e.g. PYX, SOX, SLiP, XSLTXT, YAML (sort of), etc. The difference is that SXML is not merely (arguably) simpler to read and edit, but is already itself code in Scheme. No special parsers for the format are needed or even relevant.

As a first example of working with the SSAX library, let us take a look at the application xml2sxml that was utilized above:

xml2sxml conversion script

#!/sw/bin/guile -s
!#
(load "sxml-all.scm")
(pp (SSAX:XML->SXML (open-input-file (cadr (command-line))) '() ))

Not too much too it, huh? Of course, this relies on a collection of load functions that I put into my convenience module:

sxml-all.scm support file

(load "libs/pp.scm")
(load "libs/guile/common.scm")
(load "libs/guile/myenv.scm")
(load "libs/guile/parse-error.scm")
(load "libs/util.scm")
(load "libs/input-parse.scm")
(load "libs/look-for-str.scm")
(load "sxml/ssax.scm")
(define (port? x) (or (input-port? x) (output-port? x)))

I may not need all of those load functions every time, but this loads the complete collection of SSAX functions. The last line is an oddity--for whatever reason, the function port? that is used by SSAX is not available in the version of Guile that I installed on MacOSX using fink. However, the definition I added comes straight out of the manual for Guile. A different Scheme system would presumably not have this same issue.

The data structure produced by the function SSAX:XML->SXML is a regular list that you can work with using all the usual Scheme functions. E.g.:

guile> (load "sxml-all.scm")
guile> (define sxml
         (SSAX:XML->SXML (open-input-file "example.xml") `()))
guile> (list-ref sxml 1)
(*PI* xml "version=\"1.0\" encoding=\"UTF-8\"")
guile> (cadr (list-ref sxml 3))
(title "The Twisted Matrix Framework")
guile> (eq? (car (list-ref sxml 3)) `dw-document)
#t

Using Ssax

While an SXML representation is just a tree that can be manipulated and traversed with general Scheme techniques, the SSAX library provides a handy macro called SSAX:make-parser that works in a fashion similar to the SAX API in other programming languages. A number of tree-walking optimizations are built in to this macro, giving you linear, O(N), efficiencies in processing a given SXML structure; a naive approach could easily use more memory or CPU time.

Unlike the actual SAX API that you might have used in languages like C, C++, Python, Perl, and the like, SSAX is walks a tree rather than scans a linear bytestream. That is, SAX or expat simply look for opening and closing tags as events, and callback to the relevant handlers. If you want to keep track of the relative nesting and context in which a tag occurs, you need to maintain your own stack, or other data structure. In SSAX, in contrast, every node descends from a parent, passing and returning a seed Of course, this seed is itself essentially a data structure that you can modify in each NEW-LEVEL-SEED and FINISH-ELEMENT handler--but at least it is local rather than global.

To show off the working of SSAX I have enhanced an outline example that is available on the CVS directory for the SSAX library. I demonstrate displaying attributes and (abbreviated) CDATA content. This is most of the way toward writing an sxml2xml utility--one which oddly is not distributed as part of SSAX, not even as a direct function or macro. However, I do not handle proper escaping, nor processing instructions and a few other aspects.

outline conversion script

#!/sw/bin/guile -s
!#
(load "sxml-all.scm")

(define (format-attrs attrs)
  (if (and (pair? attrs) (pair? (car attrs)))
    (string-append " " (caar attrs) "='" (cdar attrs) "'"
      (if (> (length attrs) 1)
        (format-attrs (cdr attrs)) ""))
    ""))

(define (tag->string tag)
  (if (symbol? tag) tag (cdr tag)))

(define (outline xml-port)
  ((SSAX:make-parser
      NEW-LEVEL-SEED
      (lambda (elem-gi attrs namespaces expected-content seed)
        (display (string-append
          seed                  ; indent the element name
          "<"                   ; open brace
          (tag->string elem-gi) ; print the name of the element
          (format-attrs attrs)  ; display the attributes
          ">\n"))               ; closing brace, newline
        (string-append "  " seed)) ; advance the indent level

      FINISH-ELEMENT
      (lambda (elem-gi attributes namespaces parent-seed seed)
          parent-seed)          ; restore the indent level

      CHAR-DATA-HANDLER
      (lambda (s1 s2 seed)
        (if (> (string-length s1) 30)
          (display
            (string-append seed "|" (substring s1 0 30) "...\n")))
        seed)

      ) xml-port ""))

(display (call-with-input-file (cadr (command-line)) outline)))

The basics are a lot like a SAX class. The function outline is generated with the SSAX:make-parser macro which allows definition of several event types. The main ones are entering and leaving an element, and getting character data. A couple support functions help with the process.

The seed we use in outline is quite simple, it is just a string that gets longer as deeper branches of the tree are reached. Of course, you could pass around a whole list of encountered tags--e.g. for an XPATH-like analysis of what to do with a node. Our CDATA handler simply checks whether there is enough CDATA to bother displaying (at least 30 chars, arbitrarily chosen), then displays it at the same indent as the current element.

The NEW-LEVEL-SEED handler demonstrates a couple interesting aspects, mostly in the two support functions it employs. Not every tag is a simple symbol in our SSXML structure; specifically, a namespace qualified tag is a pair instead. The function tag->string checks which type a tag is, and only displays the local part of the name, not the namespace. You could do something different, of course, but this demonstrates the test needed.

The function format-attrs is probably more an example of recursive programming in Scheme generically than it is specific to SSAX. Still, tags can have none, one, or several attributes, and we need to return a string for each case. Probably a real Scheme programmer can point out an even more concise way to do this--I welcome comments in the discussion are for this article.

Let us look at the output, given the earlier XML document. By the way, warnings are generated for the unprocessed PIs, so I redirect STDERR to ignore those:

An outline display of example.xml

$ ./outline example.xml 2>/dev/null
<dw-document>
  <title>
  <author name='David Mertz, Ph.D.'>
    <bio>
      |David thinks it's turtles all ...
  <docbody>
    <heading refname='' toc='yes'>
    <p>
      |
      Sorting through Twisted...
      <i>
      | a good sense of why they are ...

What Else?

As well as its equivalents for SAX and DOM (i.e. the native Scheme nested lists), SSAX comes with its own SXPath and SXSLT components. This article does not have room for extensive discussion of these, but they are worth mentioning briefly.

Unfortunately, the SXPath and SXSLT functions and macros discussed in Oleg Kiselyov's document "XML, XPath, XSLT implementations as SXML, SXPPath and SXSLT" are not included in the SSAX distribution, at least not the one for Guile (other distributions are available for a number of Scheme systems). What can be easily downloaded only works with a few Scheme-systems, and versioning is unclear. Based just on the document mentioned, SXPath works much like XPath. For example, either of the following expressions expand to a selection function:

SXPath expressions, native and textual

((sxpath '(// TAF VALID @ Trange *text)) document)
((txpath "//TAF/VALID/@TRange/text()") document)

These undergo macro expansion to:

Full path selector function

((node-join
  (node-closure (node-typeof? 'TAF))
  (select-kids (node-typeof? 'VALID))
  (select-kids (node-typeof? '@))
  (select-kids (node-typeof? 'TRange))
  (select-kids (node-typeof? '*text*)))
document)

Of course, playing with this expanded form would allow programming arbitrary calculations inside an XPath-like selector--anything you can write in Scheme.

SXSLT is similar in concept. Stylesheets are written in Scheme form that is semantically similar to XSLT. But much as with the flexibility of HaXml, within any particular transformation rule, you can embed arbitrary extra code. Particular XSLT engines, of course, often come with foreign-function APIs to write extra capabilities in Javascript, VB, or other languages. But with SXSLT, the custom functions are written in the very same Scheme language as the transformation stylesheet elements.

Conclusion

I like the SSAX library quite a bit; and I suspect I will like it more as I become more comfortable with Lisp/Scheme.programming. It shares many of the advantages of other "native" XML libraries I have written about in other installments: gnosis.xml.objectify, REXML, XML::Grove, HaXml, and so on. There's a lot to be said for making XML into just another data object in whatever programming language you use.

That said, SSAX has a lot of rough edges. It is hard to figure out what to download, and what Scheme systems each part is available for. The documentation is somewhat inconsistent and incomplete--most of the documents are academic in focus, and do more to discuss abstract goals and concepts than on concrete usage and API. As a demonstration of what is possible in Scheme--using functional techniques--these papers are interesting; but it would be nice to have something that is easy to install and use, and just works.

Resources

The homepage for SXML is below. A number of overlapping documents are available on that site, mostly written by Oleg Kiselyov:

http://okmij.org/ftp/Scheme/xml.html

To download version of the SSAX library for various Scheme systems, see:

http://www196.pair.com/lisovsky/xml/ssax/

The SXPath extension appears to be available at the below page, but unfortunately not for Guile:

http://www196.pair.com/lisovsky/query/sxpath/

The XML Information Set (Infoset) is a W3C Recommendation that specifies the information content of XML documents; i.e. it indicates which features of a concrete document carry information, and which are incidental.

http://www.w3.org/TR/xml-infoset/

The version of Scheme I used for this article is the GNU project's Guile. Other versions, both commercial and free software exist as well, but Guile seems widespread--and it is the version that fink will install under MacOSX. The Guile home page is at:

http://www.gnu.org/software/guile/docs/index.html

A previous installment of this column discussed the XML library HaXml for the functional programming language Haskell. While Haskell is purer in its functional programming paradigm, many common motivations and designs went into HaXml and SXML.

http://www-106.ibm.com/developerworks/xml/library/x-matters14.html

Ed Dumbill wrote an article called "Exploring alternative syntaxes for XML" for developerWorks:

http://www-106.ibm.com/developerworks/xml/library/x-syntax.html

Scott Sweeney has also produced a nice visual summary of syntax varations of near-XML languages:

http://www.scottsweeney.com/static/projects/slip/XMLShorthandComparison.htm

About The Author

Picture of Author David Mertz once led the desperate life of scholarship. David may be reached at mertz@gnosis.cx; his life pored over at http://gnosis.cx/publish/. Suggestions and recommendations on this, past, or future, columns are welcomed. Check out David's new book Text Processing in Python.