Haskell XML Toolbox 7.5

Contents

• Introduction
• Description
• Documentation
• Requirements
• Download
• Installation
• Change history
• Known problems and limitations
• Portability
• Related Work
• Feedback

Introduction

The Haskell XML Toolbox is a collection of tools for processing XML with Haskell. It is itself purely written in Haskell. The Haskell XML Toolbox is a project of the University of Applied Sciences Wedel, initiated by Uwe Schmidt.

The core component of the Haskell XML Toolbox is a validating XML-Parser that supports almost fully the Extensible Markup Language (XML) 1.0 (Second Edition).

Description

The Haskell XML Toolbox bases on the ideas of HaXml and HXML, but introduces a more general approach for processing XML with Haskell. The Haskell XML Toolbox uses a generic data model for representing XML documents, including the DTD subset and the document subset, in Haskell. This data model makes it possible to use filter functions as a uniform design of XML processing applications. The whole XML parser including the validator parts was implemented using this design. Libraries with filters and combinators are provided for processing the generic data model.

An new more flexible and typesaver API based on arrows instead of filters is included since version 5.2. There is a cookbook for using this arrow interface to build (nontrivial) applications. Manuel Ohlendorf has developed examples for processing RDF and has documented the development in his master thesis: A Cookbook for the Haskell XML Toolbox with Examples for Processing RDF Documents (the thesis as PDF)

Features:

• Unicode and UTF-8, US-ASCII and ISO-Laitin-1 support
• http: and file: protocol support
• http access via proxy
• wellformed document parsing, validation
• namespace support: namespace propagation and checking
• XPath support for selection of document parts
• liberal HTML parser for interpreting any text containing < ... > as HTML/XML
• liberal and lasy lightweight HTML/XML parser based on tagsoup
• Relax NG schema validator
• integrated XSLT transformer
• easy conversion between user defined data structures and XML by the use of pickler functions

Documentation

A (somewhat) gentle introduction to HXT is avalable in the Haskell Wiki.

The XSLT transformer has been developed by Tim Walkenhorst in this master thesis: Implementing an XSLT processor for the Haskell XML Toolbox. It's a rather complete implementation, but it's of course not a substitute for Xalan or other advanced XSLT systems. The XSLT module consists of less than 2000 lines of code. Compared with the more than 300,000 lines of Java for Xalan, this Haskell code can be viewed as one of the first formal specifications for XSLT.

Manuel Ohlendorfs master thesis, describing the arrow interface of the toolbox: A Cookbook for the Haskell XML Toolbox with Examples for Processing RDF Documents (the thesis as PDF). The source code of the example application is included in the doc/cookbook directory of the distribution.

The master's thesis "Design and Implementation of a validating XML parser in Haskell" by Martin Schmidt describes the design and motivation of the Haskell XML Toolbox (the thesis as HTML or PDF) and the development of the DTD validator module. The documentation in the thesis is a bit out of date, the modules and module names and some function names have been changed. For details the online haddock documentation should be used.

The description of the development of the XPath modules is described (in german) in Konzeption und Implementierung eines XPath-Moduls für die Haskell XML Toolbox (PDF-document).

The description of the internals of the Relax NG validator modules is described (in german) in Design und Entwicklung eines Relax NG Schema Validators auf Basis der Haskell XML Toolbox (PDF-document).

The Filter API Documentation generated with Haddock. For the new arrow interface there is a more user friendly Arrow API Documentation including only the basic types and the arrow descriptions.

Requirements

• ghc-6.8 or ghc-6.6 with cabal 1.2, HXT 7.5 works with ghc-6.6 and cabal < 1.2
• HTTP package
• tagsoup package
• Cabal of GNU make

Downloads

Haskell XML Toolbox 7.5, released 2008-03-01:

• This version works with ghc 6.8 and ghc 6.6 with cabal 1.2. For ghc 6.6 and cabal < 1.2 please use HXT 7.3.
• Includes sources for building a ghc package hxt with Cabal or make. This package contains a Haskell DOM, an XML parser, a HTML parser based on parsec, a lightweight HTML/XML parser based on tagsoup, a DTD validator, namespace processing functions, a Relax NG validator, an XPath expression evaluator, an XSLT transformer, and serialization/deserialisation of data to/from XML.
• HTTP access can be done with the Haskell HTTP module or with an external program curl The HTTP module must be installed before installing the toolbox. In the past the installation procedure for HTTP was a bit clumsy because of further dependencies on The Haskell Cryptographic Library and NewBinary package. The latest HTTP module from 2006-07-07 has removed these dependencies and consists only of one HTTP module (not HTTP and Browser). So there has only one package HTTP to be installed before installing HXT.
• Includes various examples, e.g. in example dir examples/arrows/hparser/ a validating parser, which can be used as a starting point for a HXT command line application.
• Provides a HUnit test HUnitTest for testing and demonstration of the available set of filters.
• Includes an arrow interface with type classes and overloading for a more flexible use of the filter technique.

A darcs repository is available under http://darcs.fh-wedel.de/hxt.

Installation

Before installing this version, install the Haskell HTTP module. For a quick install with Cabal execute the following commands in .../HXT-7.5

    make setup
    ./setup configure --ghc
    ./setup build
    ./setup install		# with root privileges
    

A quick test of the example programs:

    cd examples
    make all
    make test
    

Installation without Cabal with GNU make:

    make all
    make install               # with root privileges
    

Change History

• In Version 7.5
  • A module Text.XML.HXT.Arrow.XPathSimple for fast XPath selection for simple XPath queries added. If the XPath query only contains for navigaion the axis from the root down to the leaves, the query is evaluated by computing a simple arrow and applying the arrow. This gives a speedup for queries like /htm/body//h1/text() for extraction of the headline text of a HTML document.
  • Option added to control the selection of the parser (XML/HTML) by the mime type of the document.
  • New lasy and lightweight HTML/XML parser based on tagsoup library. Useful especially for converting XML into native Haskell data. (Example in examples/arrows/performance)
  • W3C XML Schema Datatype library for validating with RelaxNG extended. Currently supported Datatypes: Strings, URIs, QNames, binary string-encoded, decimal and all integer datatypes.
  • Regular expression pattern matcher for W3C XML Schema datatype patterns.
  • New arrow combinator 'mergeA' in Control.Arrow.ArrowList for combining the components of a tuple, resulting from applying arrows constructed with (&&&), (***) and similar combinators.
• In Version 7.4
  • Configuration changes for ghc-6.8
  • Error in DTD validation algorithm resulting in exponential growth of runtime during validating mixed content elements removed.
• In Version 7.3
  • Module Text.XML.HXT.Pickle for conversion between user defined data types and the HXT DOM structure extendet. The picklers are extended, such that a DTD can be derived from a pickler and can be checked for consistency. See Haddock doc (Text.XML.HXT.Pickle) and example directory examples/arrows/pickle for examples.
• In Version 7.2
  • New module Text.XML.HXT.XmlPickler for conversion between user defined data types and the HXT DOM structure. This enables the simple persistent storage and retrieval of arbitrary data with XML documents. See example directory examples/arrows/pickle for a none trivial example of these picklers. These functions are an adaptation of Andrew Kennedy's pickler combinators.
  • UTF-8 decoding done with UTF8 module from darcs. Decoding errors are detected and issued. US-ASCII decoding also checks encoding errors.
  • ISO-8859-X (X=2..11,13..16) deccoding of input documents implemented
  • Some bug fixes.
• In Version 7.1
  • Version control changed from CVS to darcs. darcs repository is available under http://darcs.fh-wedel.de/hxt/.
  • deepSeq and strict added for XmlTree and an arrow strictA for forcing the evaluation of a whole XML tree. This sometimes saves space when applied after document input, DTD processing and validation.
  • Typeable instances added for all DOM data types.
  • HTTP access via curl extended to handle automatic redirects
• In Version 7.0
  • New integrated XSLT transformer module. The example parser (examples/arrows/hparser) is extended to act as an XSLT transformer.
  • Errors concerning HTML parsing and implict closing of elements have been fixed.
  • Some minor changes of and additions to the arrow API.
  • New functions readString and readFromString for reading documents from Haskell strings the same way as reading external documents with readDocument.
• In Version 6.1
  • HXT 6.1. contains only changes in the cabal installation process for working with the Haskell HTTP module in tar archive http-20060707.tgz. This module no longer depends on The Haskell Cryptographic Library, and NewBinary package.
• In Version 6.0
  • The arrow interface has changed slightly, especially the handling of user defined states in the state and IO arrows has been simplified. This is the main reason for a 6.0 version.
  • The XPath arrows have been extended. There are arrows not only for selecting nodes via an XPath expression, but also for processing and modifying all nodes selected by an XPath expressions (see module Text.XML.HXT.Arrow.XmlNodeSet).
  • Separation of the API documentation into two documents, one for the old Filter API and a separate one for the Arrow API. The complete API documentation is still available.
  • DTD processing for the arrow part is done completely by arrow based routines.
  • Cabal config file and dependencies change to work with ghc 6.4.2.

Known problems and limitations

The parser has been tested with the XML Validation Suite form the W3C. The following problems have been encountered:

• Line numbers in XML parser do not always point to the correct position of the syntax error.
• Line numbers are not yet reported for validation constraint errors.
• The standalone document check is not yet implemented.
• The XSLT module does not support the complete XSLT standard.

Portability

Portability to Windows based systems has not been tested very intensively, but did work on an XP system with the Cygwin tools installed. Development was done under Linux with GHC 6.4 with the -Wall flag. No warnings were issued, when compiling the toolbox sources.

Haskell Modules and Libraries used in the toolbox

• Various GHC libraries including the Parsec parser combinator library.
• The HTTP and Browser Modules

Related work

• Malcolm Wallace and Colin Runciman wrote HaXml, a collection of utilities for using Haskell and XML together. The Haskell XML Toolbox is based on their idea of using filter combinators for processing XML with Haskell.
• Joe English wrote HXML - a non-validating XML parser in Haskell. His idea of validating XML by using derivatives of regular expressions was implemented in the validation functions of this software. Also his ideas and sources for navigateble trees are used in the hxpath modules.

Feedback

We are interested in hearing your feedback on our Haskell XML Toolbox, suggestions for improvements, comments and criticisms.

Mail address is hxmltoolbox@fh-wedel.de

The Haskell XML Toolbox is distributed under the MIT License.
Last modified: 2008-03-01