[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: Subtyping in XML
> > 1) Adding new value members to the value space of a type > can't be any good > > under any circumstances. It's possible to think up some > use for such a > > feature, but it would no doubt be better implemented using > one of the other > > mechanisms. > > If you mean simple type == datatype, then not only is it bad, > it's not > possible using W3C XML Schema. There is no derivation by extension > from simple types. I think that's the intent, anyway. I may be misremembering, but I thought there was a loophole or ambiguity in the spec that might allow one to add new enumeration values through simple type restriction? In any case it's bad, bad,bad. > Note here that W3C XML Schema extension is _always_ sequenced, in the > sense that any member of a type defined by extension _must_ contain a > member of the base type as a prefix. What you're saying is that base members come before extended members? That's fine, and it's not possible to extend (A, B, C[3]) with (A, B, C[3], C*) in XML Schema, although it is possible to extend it to (A, B, C[3], D, C*). It's that last case that I consider Not So Bad, but Paul seems to find objectionable. > This property does _not_ hold for substitution groups, where > type-based patterns rather than element-based ones are the only safe > way to go. Not quite following you here, sorry. > > > 2) "Content space" refers to all specified child elements > and attributes in > > a type definition (there's probably a term for this > already, but I've a > > peasouper of mental fog today). The big fork in the road > here is whether the > > model group of elements is sequenced or not. > > > > 2a) If it's unsequenced, then addition (via extension) of > new elements that > > were restricted in occurrence in the supertype would likely > break the bank. > > For sequenced content, it seems less of a worry, taking > Paul's XSLT example > > as a worse case. Frankly, if you're not blocking extension > of sequenced > > content, you have to accept what you're given. Does that > take too much > > discipline? I'm the wrong person to ask, but a schema > editor might support > > an autoblocking configuration for complextype extension. > > Note here that W3C XML Schema extension is _always_ sequenced, in the > sense that any member of a type defined by extension _must_ contain a > member of the base type as a prefix. > > This property does _not_ hold for substitution groups, where > type-based patterns rather than element-based ones are the only safe > way to go. > > ht > -- > Henry S. Thompson, HCRC Language Technology Group, > University of Edinburgh > W3C Fellow 1999--2002, part-time member of W3C Team > 2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) > 131 650-4440 > Fax: (44) 131 650-4587, e-mail: ht@c... > URL: http://www.ltg.ed.ac.uk/~ht/ > [mail really from me _always_ has this .sig -- mail without > it is forged spam] >
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