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)
Some additional info about using the various facilities is here:
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-1.09, release date 2003.05.30
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. HaXml-1.09
has not been tested with recent versions of Hugs (we last tested at
HaXml-1.05, and would like some advice and help on how things work
with the newest versions of Hugs). Use
./configure [--prefix=...]
make
make install
to configure, build, and install HaXml as a package for your
compiler(s). Afterwards, to gain access to the HaXml libraries,
you only need to add the option -package HaXml to your
compiler commandline. Various stand-alone tools are also built -
DtdToHaskell, Xtract, Validate, MkOneOf - and copied to your final
preferred installation location, specified by the --prefix=...
option to configure.
The latest version (1.09) has the following fixes:
- ghc-6.0 is supported.
- hmake is no longer required to build HaXml, provided you
have ghc --make.
- A new combinator path has been added to
Text.XML.HaXml.Combinators, allowing queries to be expressed in the
style of XPath.
- Some of the signatures in Text.XML.HaXml.Xml2Haskell have changed,
in particular, there are new functions readXml and
showXml that convert to/from Strings, hGetXml and
hPutXml that use file Handles, and the old functions that
use filenames are renamed to fReadXml and fWriteXml.
- DtdToHaskell previously generated incorrect code for reading an XML
element given the following DTD fragment:
<!ELEMENT foo (a,b)+ >
- The parser had a fault when reading conditional sections nested inside an
IGNORE section in a DTD.
- In Text.XML.HaXml.Html.Generate, all functions now generate HTML tags
in lower-case rather than upper-case, to conform to the XHTML standard.
- DtdToHaskell now accepts NOTATION types for attributes. They are
treated just like enumeration types.
- If you give an output filename as a command-line argument to DtdToHaskell,
it now uses the filename as the basis for the generated module name.
- Fixed a configuration bug on the Cygwin platform with ghc-5.04.x.
- make install now places the executables (DtdToHaskell, Xtract,
Validate, MkOneOf, Canonicalise) into the directory specified by
./configure --prefix=...
The previous version (1.08) had the following new features:
- A new and highly useful function, Text.XML.HaXml.Validate.partialValidate,
does validation except for checking whether the root element type matches
that of the DTD's root element. This is just what you need in order to
validate any partial document or sub-document.
- The function Text.XML.HaXml.Html.Generate.htmlprint had a little
bug which caused it to loop infinitely if some text was longer
than 60 characters without a space.
- The Xtract parser and combinators are now included in the HaXml
library package, rather than existing solely for the Xtract
command-line tool.
- The code generated by DtdToHaskell had a minor cut-and-paste error.
- There were also some simple import naming problems in some of the
demonstration files in the examples/ directory.
- The file `script/echo.c' was missing from the distribution.
This only affected the configuration step on systems where
`echo -n' is not recognised (e.g. Solaris).
Complete Changelog
We are interested in hearing your feedback on these XML facilities -
suggestions for improvements, comments, criticisms, bug reports. Please mail
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.
- Ulf Wiger describes an Erlang toolkit for XML:
XMerL
- There is a comprehensive reading list for XML and web programming in
functional languages here.