[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Substitution Groups - Use 'em or lose 'em?
-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Marty Burns writes: > Should substitution groups be relied upon as an extension mechanism? For a number of architectures and extension paradigms, substitution groups are the cleanest and simplest implementation mechanism in W3C XML Schemas. Many major validators and tools support them. If there are vendors whose products do _not_ support them, please publish their names so that pressure can be brought to bear on them. Having said that, as became clear at the recent XML Schema 1.0 User Experiences workshop [1], many of the reports of interoperability problems wrt W3C XML Schema relate to schema-based data binding tools. Many data binding tools have problems with choice-groups, because of the difficulty of handling union types in the programming language being bound to. Since substitution groups are effectively a kind of choice group, support for substitution groups may also be a problem for such data binding tools. But, if the extension paradigm you're using is to include one or more abstract elements in content models as extension points, and no individual user ever declares more than one element as substitutable for such an extension point, no real choice is involved and data binding should be able to cope regardless of substrate. So, bottom line: there's no excuse for general-purpose schema processors ('parsers' in your message) to not support substitution groups, and all the major ones I'm aware of do so. Data binding tools, on the other hand, have some excuse for not supporting substitution groups in general, but if your use of them is restricted in such a way as to allow simple binding strategies to work, that _should_ be OK. ht [1] http://www.w3.org/2005/06/21-schema-workshop/chairs-report.html - -- Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh Half-time member of W3C Team 2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440 Fax: (44) 131 650-4587, e-mail: ht@i... URL: http://www.ltg.ed.ac.uk/~ht/ [mail really from me _always_ has this .sig -- mail without it is forged spam] -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.6 (GNU/Linux) iD8DBQFDFCVFkjnJixAXWBoRAk44AJ4zBr8bXfckERxSS3MmwdWJmiEURACffLm+ 6SyuzGhDjG6VZ6inkd8FyjM= =3Rsf -----END PGP SIGNATURE-----
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