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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Weak DTDs
 W. Eliot Kimber wrote: > Peter has run head-on into one of the fundamental problems with DTDs as > currently defined by SGML (and XML): we want them to describe *classes* of > documents when they actually describe *individual* documents (and are > incapable of defining classes of documents except in very weak ways). The argument that Eliot is making is that as SGML (and XML) are defined today, given an external declaration subset (the entity identified by the external identifier of doctype declarations), there is no (easy) way to guarantee that documents that reference it actually conform to it, unless those documents' doctype declarations do not include an internal subset. This is because entities, notations, elements, and attributes declared in a document's internal subset can radically alter the document's type: general entities may be redefined, completely unknown notations, element types, and attributes may be added, and parameter entities can be redefined such that notations, element types, and attributes declared in the external subset have completely different definitions. All of these modifications can be made completely without constraint. The only defenses DTD designers have against this all require the DTD to be even more rigid, as any opportunity for flexibility also opens up an opportunity for abuse. Moreover, even these defenses may not be enough. Disallowing the internal subset is not the answer, because it is still needed in order to describe document-level (as opposed to document type-level) characteristics, at least things like document-specific general entities, and configuration control parameter entities (that configure the DTD in predefined ways, through the use of marked sections). Architectures, IMO, are a step in the right direction, since they are immune to the kinds of haphazard modifications that make it difficult to recognize and process a class of documents, while still allowing the document-level flexibility needed by document authors. [Sean Mc Grath <digitome@i...> on Sat, 18 Oct 1997 09:48:39 +0100] > <Statement InvititationForTrouble=TRUE"> > HyTime allows parsing w.r.t. a meta-DTD via HyTime aware parsers. However, > I think there are many occasions when there is nothing "meta" involved. Just > a desire to parse w.r.t to an alternative schema. Not a meta-schema - just > a different schema. > </Statement> It is true that there is nothing "meta" about meta-DTDs. They should be called architectural DTDs instead, where "architectural" means "used via the SGML architecture mechanism defined in Annex A.3 of ISO/IEC 10744:1997", or "designed to be used architecturally", as in the case of the HyTime architecture's DTD. Architectural DTDs are just DTDs being used in a different way. And yes, architectural processing _is_ tantamount to parsing with respect to an alternative schema, only the architectural schema is better protected from the individual needs of documents, and individual documents are better protected from the generalized needs of the architectural schema. -peter -- Peter Newcomb TechnoTeacher, Inc. peter@p... peter@t... http://www.petes-house.rochester.ny.us http://www.techno.com xml-dev: A list for W3C XML Developers. To post, mailto:xml-dev@i... Archived as: http://www.lists.ic.ac.uk/hypermail/xml-dev/ To (un)subscribe, mailto:majordomo@i... the following message; (un)subscribe xml-dev To subscribe to the digests, mailto:majordomo@i... the following message; subscribe xml-dev-digest List coordinator, Henry Rzepa (mailto:rzepa@i...) 
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