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RE: XML and SGML (was XML & the future of tech writing)
Subject:RE: XML and SGML (was XML & the future of tech writing) From:"Simon North" <north -at- synopsys -dot- COM> To:"TECHWR-L" <techwr-l -at- lists -dot- raycomm -dot- com> Date:Fri, 2 Nov 2001 15:58:53 -0800
Rick Kirkham made some very good points. Despite the heat of discussion, I should probably
have been more precise. I was trying to avoid becoming too technical as this would most likely
lose a lot of people. This is the 'technically correct' version.
SGML is a set of rules for creating markup languages. However, SGML markup languages are all
characterised by being slanted towards the requirements of document processing applications.
XML is an application profile of SGML, and is also a (very short) set of rules for creating markup
languages. XML markup languages are very specifically targetted at the Internet environment, but
initially inherited some of SGML's document bias.
XML markup language declarations may be SGML compliant; SGML markup language
declarations are very rarely XML compliant. XML instances (XML "documents") may be SGML
complaint if they have a DTD (document type definition), if the DTD is available, and if they are
valid according to the declarations contained in that DTD. SGML documents must be valid, and
must have a DTD. XML instances are not required to be valid (although they must be well-
formed), and do not require a DTD. XML instances may use a Schema instead of a DTD. Schema
provide the data restraints (data types, values, lengths and so on) that are required for data
markup, making XML more suitable for data representation and use in programming contexts.
The core of XML, the XML language, is finished as such (it is a W3C recommendation, the W3C
does not publish standards). However, on their own, neither SGML nor XML markup languages
are very useful.
SGML relies on associated (ISO) standards such as HyTime and DSSSL.
XML relies on a wide range of other 'recommendations'. These recommendations include, XML
Base, XPath, XLink, XPointer, XML namespaces, XSLT, XSLT (FO), XMLQuery and XML
Schema. Ultimately, they will also include other recommendtions such as XML Encryption, XML
Signatures and XForms.
Most of the XML recommendations are still in draft form. In this sense, SGML can be considered
to be a mature "finished" technology and XML can be considered to be an immature, incomplete
work in progress.
Simon.
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