[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: 'is-a' Relationships in XML?
> > There seems to be no shortage of ways, thankfully, to express > 'has-a' relationships with XML (even without using a schema, > it seems there is an implicit 'has' relationship between > parent element and a child element or atttibute). There could > be said to be an implicit semantic 'has' relationship > declared between 'document' an 'date' when we write > <document><date>...</date>...</document>. Is there any way to > express the other key relationship of 'is-a' in XML? Is this > something a schema language can express? Can we say that > element (or even type) A 'is-an' element (or type) B? Any > plans to add this 'feature' to the XML technologies if it > isn't one already? It might be a key gap to fill. I might > want to somehow imply that my <invoice/> is a <document/>. > Are substitution groups (with, apparently, some inadequacies) > the only way to express such a relationship in XML? > is-a can be a relationship between types, or a relationship between an instance and a type. As a relationship between types (every invoice is a document), it can be represented in XSD using the concepts of derivation by restriction or by extension; substitution groups also play into this. As a relationship between an instance and a type, it is represented in the PSVI by instance properties including [element declaration] and [type definition]. Regards, Michael Kay http://www.saxonica.com/ http://twitter.com/michaelhkay
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