A Plea for Schemas

[Rick Jelliffe]

 From: Tim Bray <tbray at textuality.com>

>Where I work, people aren't satisfied until there's running code that
>does useful stuff, and the schema-ware is an essential but fairly small
>component of getting there.

One of my hopes for new schema languages (especially those using XML
element syntax) is that, like any data with generic markup, there may be
useful and unexpected applications made possible.  The Xeena people were
nicely surpised that a DTD could be used to generate a structure-editor:
a better schema language will allow better such editors.

Already there is a spread of reasonable opinions on schemas:
    * they need to integrate into existing systems as much as possible
(the XML Schema approach?); or
    * they only need to provide information that generating and
receiving software do not already have (the SGML DTD approach?);
    * they need to be as complete as possible (the EXPRESS approach?);
or
    * they need to be simple and elegant (the XML DTD approach?);

A schema that provides enough information becomes useful as a
declarative programming tool. (For example, I have a working Schematron
that generates RDF based on tree-patterns: it is an automated external
markup tool, yet it is driven by a schema.)

Tim is of course correct that a schema is only part of the whole
picture, but a schema language can be judged on how readily it allows
useful systems and tools to be built.  That being so, it may be a strong
schema system is one which has some 'use' information as well as the
"pure" schemas: documentation, forms building, validation, style, etc.:
where the type information from the schema can directly be used for some
purpose.

DTDs provide simple stucture models but also have infoset contributions
with #FIXED attributes and default attribute values (i.e., a simple
transformation of the instance). This makes them 'impure' as a schema
language, but undoubtedly more useful.

Rick Jelliffe


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