ISO (ISO/IEC JTC 1/SC34, to be precise) launched a work named "Document Schema Definition Language" (DSDL) in December 2001, which will be "a multipart International Standard defining a modular set of specifications for describing the document structures, datatypes, and data relationships in structured information resources." DSDL includes efforts to classify the different XML schema languages into "path-based," "grammar-based," and "object-oriented" schema languages, to define a generic semantic for simple datatypes, and to define a common framework in which these languages may be used.
Working from the specifications of existing schema languages (Schematron, RELAX NG, DTDs, and W3C XML Schema), DSDL should clarify the situation by documenting their differences and how they might fit together. More information about DSDL is available at http://www.jtc1.org/FTP/Public/SC34/DOCREG/0275.htm and http://www.dsdl.org.
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