[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: Subtyping in XML
One has to be careful about stepping in dogma... Quoting from the apostle Fabian: "domains... are nothing but data types of arbitrary complexity." Gosh, I dunno. Classes seem to be types of arbitrary complexity, but I'm not ordained. The gist of the argument that classes are not domains is that there is no membership defined for a class. The way around this, of course, is to create a dictionary of class instances, each uniquely identified. Of all possible instances of classes, the ones in the collection are the members. In other words, you construct a table of instances, each instance is a proposition. I suppose one could use sparse arrays instead, and call the indices surrogate keys, but positions of instances must remain fixed. And, of course, OOP people use all types of collections, some without fixed identifiers, and get loosy-goosy with instances. But gee, ya know... my apps run fine without the formalism (QA would beg to differ, but they're professional critics). I wouldn't want to get in a knife fight with Data or Darwin (or even Mr. Pascal), but I do think the relational fascists tend to grind an argument to a brittle edge. -----Original Message----- From: John Schlesinger [mailto:jschlesinger@c...] Sent: Saturday, September 07, 2002 2:19 PM To: Jeff Lowery Cc: 'paul@p...'; 'ht@c...'; Xml-Dev (E-mail); 'Dare Obasanjo' Subject: RE: Subtyping in XML On Fri, 2002-09-06 at 13:23, Jeff Lowery wrote: No, but a normalized relational schema defines domains, represented by tables. Domains are the spittin' cousins of types. This is the standard wisdom, but is strongly contested by Chris Date and Hugh Darwen. They claim that equating Classes with Tables is a 'blunder' and that a Class really equates to a domain ('domain=class' not relvar). The argument for this is that a domain is the type of a column and that a set of columns, forming a table and therefore a relationship, is a set of true propositions about those types. From this follows all the good stuff in the relational model. The argument against this is that no-one seems to do this (even though the user-defined data types are now available in most databases, even if the encapsulating operators are not). Indeed, typical O/R mappings themselves commit the 'blunder'. Date and Darwen's idea is so counter-intuitive that I think they must be right. John Schlesinger SysCore Solutions
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