HaXml

HaXml is a collection of utilities for using Haskell and XML together. Its basic facilities include:

• a parser for XML,
• a separate error-correcting parser for HTML,
• an XML validator,
• pretty-printers for XML and HTML.

For processing XML documents, the following components are provided:

• Combinators is a combinator library for generic XML document processing, including transformation, editing, and generation.
• Haskell2Xml is a replacement class for Haskell's Show/Read classes: it allows you to read and write ordinary Haskell data as XML documents. The DrIFT tool (available from http://repetae.net/~john/computer/haskell/DrIFT/) can automatically derive this class for you.
• DtdToHaskell is a tool for translating any valid XML DTD into equivalent Haskell types.
• In conjunction with the Xml2Haskell class framework, this allows you to generate, edit, and transform documents as normal typed values in programs, and to read and write them as human-readable XML documents.
• Finally, Xtract is a grep-like tool for XML documents, loosely based on the XPath and XQL query languages. It can be used either from the command-line, or within your own code as part of the library.

Detailed documentation of the HaXml APIs is generated automatically by Haddock directly from the source code.

An introduction to HaXml for people who know more about XML than about Haskell can be found at IBM DeveloperWorks.

A paper describing and comparing the generic Combinators with the typed representation (DtdToHaskell/Xml2Haskell) is available here: (12 pages of double-column A4)

• icfp99.dvi (LaTeX dvi format)
• icfp99.ps.gz (PostScript format - gzipped)
• icfp99.html (HTML format)

Some additional info about using the various facilities is here:

• Combinators (HTML format)
• Haskell2Xml class (HTML format)
• DtdToHaskell tool (HTML format)
• Xtract tool (HTML format)

Known problems:

• To use -package HaXml interactively with GHCi, you need at least ghci-5.02.3.
• Haskell2Xml generates Parameter Entity Declarations in the internal subset of the DTD, which don't conform to the strict well-formedness conditions of XML. We think the constraint in question is spurious, and any reasonable XML tool ought to deal adequately with full PEs. Nevertheless, many standard XML processors reject these auto-generated DTDs. The solution is easy - just move the DTD into a separate file!
• DtdToHaskell generates the Haskell String type for DTD attributes that are of Tokenized or Notation Types in XML. This may not be entirely accurate.

Current version: HaXml-1.11, release date 2004.01.23
By HTTP: .tar.gz, .zip.
By FTP: ftp://ftp.cs.york.ac.uk/pub/haskell/HaXml/
FreeBSD port: http://freshports.org/textproc/haxml/

Development version: The development version of HaXml is available by anonymous CVS (write-access is also available to core Haskell developers). Read the instructions for getting ghc by CVS: http://haskell.org/ghc/docs/latest/html/building/sec-cvs.html and simply replace the project name fpconfig with HaXml.

Older versions:
By FTP: ftp://ftp.cs.york.ac.uk/pub/haskell/HaXml/

To install HaXml, you must have a Haskell compiler: ghc-5.02 or later, and/or nhc98-1.14/hmake-3.06 or later, and/or Hugs98 (Sept 2003) or later. Use

    ./configure [--prefix=...] [--buildwith=...]
    make
    make install

The latest version (1.11) has the following features and fixes:

• Fix a tiny but embarrassing bug in the previous fix for complex DTDs being translated by DtdToHaskell. It broke on a very simple DTD like
      <!ELEMENT A (B|C)>
  which became sequence
      data A = A B C
  instead of choice     data A = AB B | AC C
  

The previous version (1.10) has the following features and fixes:

• All being well, HaXml now works again for Hugs (versions ≥ September 2003). The library sources are installed using hugs-package, and the tools are installed as scripts for runhugs.
• Fixed the internal (and external) representations of XML character references.
• New combinators to `escape' and `unescape' reserved XML characters in text and attribute values has been contributed by George Russell. (e.g. to convert "<" into "&lt;".)
• Bugfixes to DtdToHaskell: A DTD content specification of the form
      <!ELEMENT A (B*,(C|(D?,E*)))>
  was incorrectly translated to the Haskell
      data A = A [B] (OneOf3 C (Maybe D) [E])
  but the new (correct) translation is
      data A = A [B] (OneOf2 C (Maybe D,[E]))
  

We are interested in hearing your feedback on these XML facilities - suggestions for improvements, comments, criticisms, bug reports. Please mail

• Malcolm.Wallace@cs.york.ac.uk (implementation & design)

Development of these XML libraries was originally funded by Canon Research Europe Ltd.. Subsequent maintenance and development has been partially supported by the EPSRC, and the University of York.

License: The library is Open Source, i.e., the bits we wrote are copyright to us, but freely licensed for your use, modification, and re-distribution, provided you don't restrict anyone else's use of it. HaXml is distributed under the Artistic License - see file LICENSE for more details. (If you don't like the licensing conditions, please contact us to discuss your requirements.)

• Joe English has written a more space-efficient parser for XML in Haskell, called hxml. What is more, it can be used as a simple drop-in replacement for the HaXml parser! Available here.
• Uwe Schmidt recently designed another Haskell XML Toolbox based on the ideas of HaXml and hxml.
• Some comparisons between functional language approaches to processing XML can be found in Bijan Parsia's article on xml.com
• Christian Lindig has written an XML parser in O'Caml: here.
• Andreas Neumann of the University of Trier has written a validating XML parser in Standard ML: here.
• Erik Meijer and Mark Shields have a design for a functional programming language that treats XML documents as basic data types: XMLambda.
• Benjamin Pierce and Haruo Hosoya have a different but similar design in XDuce, which is also implemented.
• Taking XDuce's approach further, is the very cool CDuce by Véronique Benzaken, Guiseppe Castagna, and Alain Frisch. The CDuce language does fully statically-typed transformation of XML documents, thus guaranteeing correctness, and what is more, it is also faster than the untyped XSLT!
• Ulf Wiger describes an Erlang toolkit for XML: XMerL
• There is a comprehensive reading list for XML and web programming in functional languages here.