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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Schemata are not just constraints
 From: Bullard, Claude L (Len) <clbullar@i...> > The site cited (ooooh) notes that ontology meant a > logic of existence. If we go that deep, we lose > ourselves in the ratholes of meaning ... As Martin mentioned, we can > look at the utility of substitution groups. I tend to think that much of the difficulty we have with schemas is precisely that of people not being aware of the ontological possibilities: that creating a schema language involves not just picking a set of use cases but creating a coherent ontological framework. What the hell does that mean (I hear Tim's voice) ? It means that when we create a schema language we adopt certain abstractions: Is there inheritance? Are there abstract and final things? Is a name simple, two-parts, three-parts or a full path? Are there types, do these form a type hierarchy, is it a tree or a graph, are the children restrictions or extensions, etc. In Schematron 1.5, the main abstractions are patterns, abstract rules and phases. I tend to think of a schema language as something that growing naturally or painfully out of four ingredients: the domain they are addressing (and the boundaries of the domain), the schema paradigm (e.g. regular expressions, path expressions, notation processing), the abstractions, and the XML language theories and tastes (including house rules) of the creators. For example, DTDs provide almost no abstractions: instead they provide PEs which shows the language taste of the creators (a minimalism that prefers to re-use a low-level mechanism used elsewhere rather than introduce new, higher-level abstractions.) While markup language theories and tastes can be fruitfully debated and discussed (e.g. my book), I am not all convinced that there is much point in having strong views on questions of ontological categories (in schema languages for markup languages) except for questions of completeness and inellectual congeniality. The same is pretty true of the schema paradigm. ...Because I think the proof of the pudding is in the eating (and will be based on the demographics and times of the eaters). We simply need more experimental schema languages--slicing the world into novel domains, exploring different schema paradigms (and in what may seem to be different domains), adopting different abstractions (ontological frameworks) and by people with different language inclinations or tastes (e.g., the difference between Schematron and XLinkit, which have very similar paradigms but very diffferent approaches to languages.) I think that the most interesting possibilities are for schema languages that meddle with our ideas of what domains are natural that have the best chances of being fresh: we tend to have very fixed ideas of what schema paradigms are useful (e.g., document people accept the grammar pardigm, database like the key constraint paradigm), similarly we tend to have fixed ideas about abstractions (OO people love objects, LISP people love eval), and people usually have strong convictions on good language design (e.g. PIs boo! -- elements hurrah!). What might be examples of a cross-domain schema? One might be a schema language which ties entity structure to markup declarations and naming conventions (i.e., a generalization of the XHTML Modularization system). Another might be to tie the language of a document to the characters used. Another might be to constrain the HTTP preferences (e.g. compression) for accessing a resource by some aspect of a topic map which points to it. Another might be to constrain parse trees according to a property set (e.g. like SGML GROVE plans). Another might be to constrain extended-XLink roles according to some grammar (e.g an architecture on the values of the role attributes) This may sound airy fairy, but it is practical. Why should XML Schemas have key constraints and uniqueness, but not multiple inheritance? Until we have a broad range of different experimental and commercial languages for constraining certain things in relation to other things, we cannot know what is powerful and convenient. Cheers Rick Jelliffe 
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