[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Are we losing out because of grammars?
Rick Jelliffe wrote: > I believe propronents of various schema paradigms (grammars and rule-based > systems) need to justify that their paradigm is useful. When we only had > grammars, it was a moot point. Now we have Schematron and other rule-based > systems, there I think we can be bold enough to start to critique > grammars-as-schemas. (Of course, this is a two-way street.) I think the utility of grammar-based schemas has been well established. Just because there are some things they can't do doesn't mean the things they *can* do aren't useful. One of the biggest benefits of grammar-based schemata is that they are generative: there is an enumeration procedure as well as a decision procedure. This makes things like syntax-directed editors possible ("show me a list of all the elements that I can insert at this point in the document"), and makes it easier to verify transformations ("will this XSLT transform always produce a valid HTML document when fed an input document conforming to the specified source DTD?"). > Might we get > to higher-level schema languages faster by completely ditching the grammar > paradigm and treating schemas as systems of logical assertions which can be > queried? Well, yes; the existence of Schematron aptly demonstrates this point. But it's not an either-or proposition: grammar-based and rule-based schemas can work together quite nicely. [ ... earlier ...] > Lets contrast the grammar approach to a rule/path based approach (as in > Schematron). In Schematron we don't have any ambiguity problem, because we > don't have alternate paths for any location expression in the same way. But there *is* an ambiguity issue in Schematron -- the requirement that a node can match the context of at most one <rule> in a given <pattern>. As you mentioned, this doesn't cause a problem when trying to merge two schemas (just include all the <pattern>s from both schemas), but the question of whether all the <rules> within each of the original <pattern>s match disjoint node-sets remains. > XML does not have SGML's short refs and delimiter maps, so why does it now > need grammar-based content-modeling? XML doesn't need *any* kind of schema language, but lots of XML applications benefit greatly from them. Different kinds of applications call for different kinds of schemas; those based on tree-local and tree-regular grammars are still very useful for "document-centric" applications (HTML, DocBook, TEI, etc.). --Joe English jenglish@f...
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