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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: XLink transformations
 Not asking would be a harsher problem. No, the schemas are not secretly presupposed. They may exist and are useful, but they do not of necessity exist. The existence of schemata and the usefulness of declaring links are related. There are more mathematically precise explanations than what follows (invariance under transforms). The importance of invariance under transforms is to determine if information lost by transform invalidates declared relationships. (Ie, did we snap the links?). There is a lot of fun stuff in the bits on manifolds... but... I think I can confuse you without all of that. :-) 1. XLink isn't an instance of XSLT. XLink is a declaration of a relationship, not a transform or stylesheet application. There is some cognitive overlap in that one way to make a link functional is to apply a transform. 2. Unlike SGML, XML does not automatically presuppose the existence of a declaration (eg, DTD or schema). In common practice, these probably exist because they are useful in several operational contexts. There are issues in which one declares a link which depends on the target being in a particular position of the tree. In that case, a validation is useful. One can more safely create persistent links given a schema, but again, it is left up to the designer to determine what safety is. The issue is how much information is declared versus how much is derived in the operational context of resolving a link or transform. 3. For a transform, one certainly does have a target in mind, but again, the existence of the schema is not predetermined since the target is an instance. When thinking about transforms, it is useful to keep the Champagne/Urbana concepts of down and uptranslation in mind. For example, a transform from an XML document to an HTML document is typically a downtranslation, meaning, information is lost in going to the target format. Translating in the other direction is typically an up translation, meaning information must be added. The reason to think about this is the namespace. If semantics are inferred by the namespace, that is, the namespace is a processing context, then ensuring the right names are used to create the correct context is at issue. In this case, the existence of the schema can determine the context and from it, one can get the information needed for the target context. 1. A schema declares an environment (a context). 2. An environment can be thought of as a space of potential objects (any legal context). 3. The schema declares the members for all of the possible operations that can be applied within that space. 4. Operations which change that space can add or subtract members from the environment or instance. If the environment is changed, the result is a loss or addition of potential members. 5. XLinks declare relationships that do not add or subtract objects from the space but can declare legal operations (constraints) over the objects within that space OR among different spaces by declaring what must exist in a given context for the link to be operationally valid. 6. Operations may be reversible (non-lossy) or may not be (lossy). (variant or invariant under transform). 7. XSLT declares a transform operation which may use environmental constraints. One use as stated is to determine by querying the declaration if the transform is valid in terms of the namespace context or by detemining if the transform operationally invalidates a link relationship. The math majors can now proceed to chew me out for mistakes here. :-) Len Bullard Intergraph Public Safety clbullar@i... http://fly.hiwaay.net/~cbullard/lensongs.ram Ekam sat.h, Vipraah bahudhaa vadanti. Daamyata. Datta. Dayadhvam.h From: Steve Boyce [mailto:SteveB@h...] This is: How is XLink anything other that a particular instance of XSLT? 
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