ANN: Trang (multi-format schema converter)

[James Clark <jjc@jclark.com>]

I am happy to announce a new release of Trang, my multi-format schema
converter. Trang is written in Java, and available under a BSD-style
license.  In this release, I have added an input module for DTDs based
on my DTDinst program.  This implies that Trang can now convert
directly from DTDs to W3C XML Schema (XSD).  This may make it of
interest to people outside the RELAX NG community, which is why I am
announcing it here.

Although there are other DTD to XSD converters available, Trang has
some unique features.

- It is able to reliably turn parameter entities into the higher-level
semantic constructs available in XSD (simple types, groups, attribute
groups). It can do this even in the presence of arbitrarily deep
nesting of parameter entity references within parameter entity
declarations. At the same time, it accurately follows XML 1.0 rules on
parameter entity expansion, so that any valid XML 1.0 DTD can be
handled. If a parameter entity is used in a way that does not
correspond to any of these higher-level semantic constructs (for
example parameter entities used to allow a change of namespace prefix
in the DTD), then references to that parameter entity are simply
expanded.

- It supports namespaces, including DTDs that mix multiple namespaces.
It interprets DTDs in a namespace-aware way and can automatically
create additional files and move declarations between files so as to
create valid XML Schemas that accurately capture the intended
semantics of the DTD.

- It can create good-quality, idiomatic XSD, which takes advantage of
features such as substitution groups.

Trang is not limited to converting from DTD to XSD.  It supports the
following schema languages for XML:

- RELAX NG (XML syntax)
- RELAX NG compact syntax
- XML 1.0 DTDs
- W3C XML Schema

A schema written in any of the supported schema languages can be
converted into any of the other supported schema languages, except
that W3C XML Schema is supported for output only, not for input.

Trang is constructed around an RELAX NG object model designed to
support schema conversion. For each schema language supported for
input, there is an input module that can convert from the schema
language into this internal object model. Similarly, for each schema
language supported for output, there is an output module that can
convert from the internal object model in the schema language.

Trang aims to produce human-understandable schemas; it tries for a
translation that preserves all aspects of the input schema that may be
significant to a human reader, including the definitions, the way the
schema is divided into files, annotations and comments.

Trang has a command-line user interface. It has no graphical user
interface.

Trang can be downloaded from:

  http://www.thaiopensource.com/relaxng/trang.html

An example of its capabilities as a DTD to XSD converter is available
(temporarily) at

  http://www.thaiopensource.com/mathml/

This contains a dtd subdirectory containing the unmodified W3C MathML
DTD, and an xsd subdirectory containing the unmodified output from
Trang.

If you study this example, you may wonder why Trang turns the
PresInCont and Presentation parameter entities into groups rather than
abstract elements.  The answer is because, as written, the DTD uses
multiple inheritance:

<!ENTITY % PresInCont
     "%ptoken; | %petoken; |
      %plschema; | %peschema; | %pactions;" >
<!ENTITY % Presentation
     "%ptoken; | %petoken; | %pscreschema; |
      %plschema; | %peschema; | %pactions;">

If you change it to use single inheritance:

<!ENTITY % Presentation
     "%PresInCont; | %pscreschema;">

then Trang will use abstract elements.

James