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DtdToHaskell - a XML DTD to Haskell translator


DtdToHaskell [ dtdfile [ outfile ]]

Missing file arguments or dashes (-) indicate standard input or output respectively.


DtdToHaskell is a tool for translating any valid XML DTD into equivalent Haskell types.
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. It uses the
Text.XML.HaXml.XmlContent class as a framework for printing and parsing.

DtdToHaskell reads and parses a DTD from dtdfile (which may be either just a DTD, or a
full XML document containing an internal DTD). It generates into outfile a Haskell module
containing a collection of type definitions plus some class instance declarations for I/O.

In order to use the resulting module, you need to import it, and also to import
Text.XML.HaXml.XmlContent. To read and write XML files as values of the declared types,
use some of the convenience functions from XmlContent.

You will need to study the automatically-generated type declarations to write your own
code using them - but most things have pretty obvious parallels to the DTD structure.


The generated Haskell contains references to types like OneOf3 where there is a choice
between n (in this case 3) different tags. Currently, the module Text.XML.HaXml.OneOfN
defines these types up to n=20. If your DTD requires larger choices, then use the tool
MkOneOf to generate the extra size or range of sizes you need.

We mangle tag names and attribute names to ensure that they have the correct lexical form
in Haskell, but this means that (for instance) we can't distinguish Myname and myname,
which are different names in XML but translate to overlapping types in Haskell (and hence
probably won't compile).

Attribute names translate into named fields: but because Haskell doesn't allow different
types to have the same named field, this means your XML document which uses the same name
for similar attributes on different tags would crash and burn. We have fixed this by
incorporating the tagname into the named field in addition to the attribute name, e.g.
tagAttr instead of just attr. Uglier, but more portable.

XML namespaces. Currently, we just mangle the namespace identifier into any tag name
which uses it. Probably the right way to do it is to regard the namespace as a separate
imported module, and hence translate the namespace prefix into a module qualifier. Does
this sound about right? (It isn't implemented yet.)

External subset. Since HaXml release 1.00, we support the XML DTD external subset. This
means we can read and parse a whole bunch of files as part of the same DTD, and we respect
INCLUDE and IGNORE conditional sections. The sub-DTD files must be available locally - we
don't go looking for them on the web.

There are some fringe parts of the DTD we are not entirely sure about, such as Tokenised
Types and Notation Types. In particular, there is no validity checking of these external
references. If you find a problem, mail us:


The HaXml library and tools were written by and are copyright to
Copyright © 1998 – 2006 Malcolm Wallace and Colin Runciman

The library incorporates the module Text.ParserCombinators.HuttonMeijerWallace
Copyright © 1996 Graham Hutton and Erik Meijer

with modifications
Copyright © 1998 – 2000 Malcolm Wallace

and may also use or incorporate the module Text.PrettyPrint.HughesPJ
Copyright © 1996 – 1997 John Hughes and Simon Peyton Jones

The HaXml library is licensed under the terms of the GNU Lesser General Public Licence
(LGPL), with the following special exception:

As a relaxation of clause 6 of the LGPL, the copyright holders of this library give
permission to use, copy, link, modify, and distribute, binary-only object-code
versions of an executable linked with the Library, without requiring the supply of
any mechanism to modify or replace the Library and relink (clauses 6a, 6b, 6c, 6d,
6e), provided that all the other terms of clause 6 are complied with.

The HaXml tools Xtract, Validate, DtdToHaskell, and MkOneOf, are licensed under the terms
of the GNU General Public Licence (GPL).

This library and toolset is distributed in the hope that it will be useful, but WITHOUT
ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE. See the GNU Licences for more details.

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