Xml Matters #14: Those Awkward Limits Of Dom, Sax And Xslt

The HaXml functional programming model for XML


David Mertz, Ph.D.
Prestidigitator, Gnosis Software, Inc.
September 2001

The library HaXml creates native representation of XML documents as native recursive data structures in the functional language Haskell. HaXml brings with it a set of powerful higher order functions for operating on these "datafied" XML documents. Many of these HaXml techniques are far more elegant, compact, and powerful than one finds in familiar techniques like DOM, SAX, or XSLT.

Conventional Limitations

The most common techniques for manipulating XML documents are DOM, SAX and XSLT. These techniques have a distressing lack of unifying principles among them. Everything one might want to do with XML is available in one of the major approaches, but when what you want to do crosses the boundaries of what each technique does best, it is far from clear how to approach a problem. One is likely to wind up with a hodge-podge application in which various smaller transformations are chained together with heterogeneous techniques and tools.

DOM melds everything XML into an OOP framework, at a level of abstraction higher than that of any particular programming language. DOM is language-independent, and its "document object model" is provided by libraries in many general purpose programming languages. In a sense, DOM is the language--it specifies all the methods and arguments, and the underlying general purpose language is just a little glue. My utility xml_objectify, discussed in previous installments, provided a way of transforming XML documents into more native-feeling OOP-ified objects (in Python); largely in response to the somewhat artificial feel of DOM.

SAX is similar to DOM in its language-neutrality, but it substitutes an event-driven and procedural model for the OOP framework of DOM. SAX has the very nice feature that it is able to process XML documents as streams, acting upon each element and content as it is encountered. But with its event-driven philosophy comes SAX' disadvantage of having no real concept of the data structures represented by XML documents. One can build such structures in a concrete application, but even something so basic as a parent/child relationship must be represented in the vocabulary of the programming language that underlies a SAX library; SAX itself is blind to most of what is interesting about XML.

Finally, XSLT is the familiar technique that, in a sense, best matches the structure of XML. Perhaps reflecting this match, XSL documents are themselves XML document instances. XSLT is a special-purpose functional programming language that allows one to specify transformations of XML documents into other things (especially, but not only, into other XML documents). Aside from the somewhat annoying verboseness of XSLT, it is limited in its expressiveness--the things you can say are expressed rather clearly (and functionally, not procedurally), but one quickly bumps up against all the things that one simply cannot say in XSLT.

A Simple And Typical Task

Let me describe a quite realistic scenario, that shows weaknesses in the common techniques. XSLT is typically the most direct way to describe a transformation of an XML document into an output. For example, we might want to create an HTML representation of an XML document. Let us say we have an XML version of the I Ching that looks something like:

XML Version of the I Ching

<?xml version="1.0"?>
<IChing>
    <title>Some Hexagrams from the I Ching</title>
    <hexagram>
        <number>1</number>
        <name>Ch'ien / The Creative</name>
        <judgement>
            The Creative works sublime success,
            Furthering through perseverance.
        </judgement>
    </hexagram>
    <hexagram>
        <number>2</number>
        <name>K'un / The Receptive</name>
        <judgement>
            The Receptive brings about sublime success,
            Furthering through the perseverance of a mare.
        </judgement>
    </hexagram>
    <hexagram>
        <number>3</number>
        <name>Chun / Difficulty at the Beginning</name>
        <judgement>
            Difficulty at the Beginning works supreme success,
            Furthering through perseverance.
        </judgement>
    </hexagram>
</IChing>

To present this information in an HTML table, we might use something like the below XSLT instructions:

XSLT Instructions for an I Ching HTML Table

<xsl:stylesheet version="1.0"
                xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns="http://www.w3.org/TR/xhtml1/strict">
    <xsl:output method="html" indent="yes" encoding="UTF-8"/>
    <xsl:template match="IChing">
      <html>
        <head><title><xsl:value-of select="title"/></title></head>
        <body><table border="1"><xsl:apply-templates/></table></body>
      </html>
    </xsl:template>
    <xsl:template match="hexagram">
        <tr><xsl:apply-templates/></tr>
    </xsl:template>
    <xsl:template match="number">
        <td><xsl:apply-templates/></td>
    </xsl:template>
    <xsl:template match="name">
        <td><xsl:apply-templates/></td>
    </xsl:template>
    <xsl:template match="judgement">
        <td><xsl:apply-templates/></td>
    </xsl:template>
    <xsl:template match="*"></xsl:template>
</xsl:stylesheet>

This XSLT seems simple and direct: we just create a template to describe how we would like each element formatted. What could be easier? The problem comes as soon as we want to filter or compute something for the output--something that is not included in the few comparisons available to XSLT. For example, maybe we want (in a numerological spirit) to display only the even numbered hexagrams, or only the prime ones. With XSLT, we are out of luck for something this simple.

At this point, we might turn to DOM or SAX for the task. This will work, but we first have to simply throw away the work that went into earlier XSLT transformations. DOM or SAX are completely different models, and share no significant code or concepts with XSLT. For something as simple as my toy stylesheet, it hardly matters; but for large production-quality transformations, we might lose a lot.

Moreover, neither SAX nor DOM have particularly elegant or maintainable models for output. In order to output the simple (filtered) HTML table described, we just have to litter our application code with print statements or printf() functions (or whatever our general language uses). These output statements are themselves buried in conditional blocks that test for element types and other conditions. In such code, there is no way to tell at a glance how the output flows or make sure that a print "</tr>" is reached to correspond with every print "<tr>" that occurred earlier. There is certainly no guarantee given by imperative or OOP code that our output HTML or XML will be well-formed (let alone valid).

A Uniform Solution

What would be ideal for XML transformations would be a system that both let us express output declaratively (as XSLT does) and lets us include arbitrary filters and computations (as the implementation languages underlying DOM and SAX do). While we are at it, it couldn't hurt to be guaranteed the well-formedness, or even validity, of our output. And a compact and direct syntax would be nice too.

HaXml gives us everything requested in the previous paragraph. Actually, the power of HaXml is more general than was even asked for. Taking advantage of the higher order combinatorial functions that a functional programming language like Haskell allows, we can arbitrarily combine multiple filters in specifying output. In XSLT, each <xsl:template> acts as a sort of filter between the input and output. But the only real combination of filters created by an XSL file is a union on all the filters. In contrast, HaXml lets us specify much more fine grained chains of filters for each element of the output. Actually, much of the same can be achieved using XPATH with XSLT, but HaXml is much more concise, and strictly more powerful.

As well providing numerous combinators, HaXml allows the inclusion of arbitrary computations in the Haskell language as part of the filtering. An extra bonus is that output can be specified in a much more coherent block that allows readable intermixing of output terms with filtering conditions. The result is much more concise than XSLT, and has fewer punctuation symbols to make visual scanning difficult:

HaXml program to output an I Ching HTML Table

module Main where
import XmlLib
-- Concise XSLT-like specification of output
main = processXmlWith (hexagrams `o` tag "IChing")
hexagrams =
    html [
      hhead [htitle [keep /> tag "title" /> txt] ],
      hbody [htableBorder [rows `o` children `with` tag "hexagram"] ]
    ]
htableBorder = mkElemAttr "TABLE" [("BORDER",("1"!))]
rows f =
    let
      num = keep /> tag "number" /> txt
      nam = keep /> tag "name" /> txt
      jdg = keep /> tag "judgement" /> txt
    in
      if (condition (num f) (nam f) (jdg f))
      then hrow [hcol [num], hcol [nam], hcol [jdg]] f
      else []
condition num nam jdg = isPrime (makeInt num)

-- Supporting computations for rows condition
makeInt = stringToInt . unwrap      -- Turn [Content] into an Integer
unwrap [(CString b c)] = c          -- Turn [Content] into a String
stringToInt = revToInteger.reverse  -- Turn a String into an Integer
    where
    revToInteger = toInteger . revToInt
    revToInt []  = 0
    revToInt (d:ds) = digitToInt d + (10*(revToInt ds))
isPrime = ordSearch (sieve [2..])   -- ordered search of Sieve of Eratosthenes
    where
    ordSearch (x:xs) n
        | x < n     = ordSearch xs n
        | x == n    = True
        | otherwise = False
    sieve (x:xs) = x : sieve [y | y <- xs, y `mod` x > 0]

As with XSLT, definitions may occur in any order. The first twenty lines of the code specify the output format, with some definitions broken out into supporting functions, simply for readability. In Haskell syntax, a function is whatever thing occurs to the left of an equal sign, and the definition is to the right of the equal sign. The where and let clauses specify what we might call "inner functions" in other languages. The first lines are conceptually very much like the XSLT approach. But as an improvement, the HaXml version lets us define ad hoc filters at each point where we need it. For example, one filter is:

rows `o` children `with` tag "hexagram"

The `o` is somewhat fancifully called "Irish Composition", and can also be pronounced "of". rows is our very own filter that we have defined out of convenience; we are just as free to reuse rows as to use any of the standard filters, and in unlimited combination.

The latter half of the program contains a few computational functions. One nice example is a six line primality test that does a lot to illustrate the power and elegance of Haskell as a language. But as the program is structured, the function condition could equally well perform any tests it wanted on the Content of the <number>, <name> or <judgement> elements. "Content" is a special data type, so the first thing we need to do is unwrap the String contained in a Content. After that, we can convert it to an Integer, and test it (or do whatever else we want with it).

There are a lot more details to the HaXml library--and learning Haskell itself requires a learning curve for programmers accustomed to imperative and OOP styles of programming. But if one limits one's interest to just the capabilities one would find in XSLT, the top half of the example program is quite easy to expand upon (I would argue, with less of a learning curve than XSLT, while using a similar functional style). Not only is the syntax more readable than XSLT, but one holds in reserve learning how to do the sort of thing in the bottom half of the program (and much more).

Enforcing Validity

In the above example, an XML document was treated as a generic tree structure. For most purposes, doing this is the easiest and quickest approach. But HaXml provides something else that is totally unique. If one has a DTD for a document type one plans to work with, a set of native Haskell data structures can be generated from a DTD. From that point forward, applications can be written that utilize the native DTD in their manipulations. The generation mentioned involves several steps. The first thing is to create the data structure as a module, something like:

Create a HaXml data structure from a DTD

% DtdToHaskell MyFile.DTD MyFileDTD.hs

Once that is available, one can write an application that handles XML documents that must conform to the DTD. Such applications will generally contain at least the following lines:

Custom HaXml app for MyFile.DTD XML documents

import Xml2Haskell (readXml)
import MyFileDTD
[...]

From there, one can use all the higher order techniques Haskell provides for dealing with recursive data structures. The first thing, naturally, will probably be to readXml in order to work with a particular input XML document.

A reader can be forgiven for thinking at this point, "so what?" It would appear that this is no different from a DOM approach--or even SAX--where one can perfectly well work with structured data, or even validate against a DTD.

There is much more here than meets the eye. Haskell, unlike almost all other programming languages, is thoroughly type safe about data structures. It is quite simply impossible in Haskell to perform any computation on the generated data structure that would result in an invalid XML document. In contrast, the best one can do in languages like C/C++, Java, Perl, Smalltalk, Python is do a sanity check (validation) on the way in, and another one on the way out, and hope everything goes well. It might be possible in something like Eiffel to add enough contractual constraints on every "adder" and "deleter" to make sure DTD validity is maintained (or, with enough work, in all the mentioned languages), but doing so involves custom programming within every .addSpamTag() method. Moreover, all the DTD can do in the mentioned languages is provide a "cheat sheet" for an application programmer to look at.

With HaXml, the data structure generated programmatically from a DTD automatically includes every validity constraint. Mind you, just enforcing the constraints doesn't make an application programmer write correct code; but at least any bad code that would result in invalid documents is caught at compile time. The other caveat, of course, is that programming a custom application that enforces validity is just plain going to be more work than programming one that does not need to do this. But for "mission critical" requirements, HaXml could well provide the quickest and safest path to the rigorous goal.

Resources

Bijan Parsia has a very interesting related essay called, Functional Programming and XML at XML.com. Parsia makes the argument that functional programming styles are generally better suited to XML manipulation than are more familiar OOP techniques. He discusses HaXml and several other tools:

http://www.xml.com/lpt/a/2001/02/14/functional.html

A detailed discussion of HaXml was written by its original authors, Malcolm Wallace and Colin Runciman. Haskell and XML: Generic Combinators or Type-Based Translation may be found at the below URL. Its tone and level presume a greater familiarity with Haskell and functional programming than is requisite for this column, but Wallace and Runciman's paper thereby contains many details not addressed herein:

http://www.cs.york.ac.uk/fp/HaXml/icfp99.html

Information about Haskell, including several tutorials, numerous papers, and various compilers and interpreters can be found at the Haskell language website:

http://www.haskell.org/

I have myself written an introductory tutorial to Haskell, which is aimed at beginners:

http://gnosis.cx/publish/programming/Haskell.pdf

The files mentioned in this article may be found in the below archive:

http://gnosis.cx/download/xml_matters_14.zip

Some sample outputs of this transformations discussed in the article can be found at:

http://gnosis.cx/download/iching.html

The HaXml version at (whitespace and layout are not identical):

http://gnosis.cx/download/iching_hs.html

About The Author

Picture of Author David Mertz' tattoos provide a surprisingly deep insight into his programming and theoretical interests. One could take a look athttp://gnosis.cx/photos/tattoos.html for (strictly PG-13) insight. David may be reached at mertz@gnosis.cx; his life pored over athttp://gnosis.cx/publish/. Suggestions and recommendations on this, past, or future, columns are welcomed.