[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Rick Jelliffe quotable quote on the purpose of schemas
Michael Kay wrote: >>"The flaw with grammars is that they only allow to constraint >>content models in a declarative manner > > > There's nothing wrong with constraints being declarative Except that you will need 3000 tags to express all that people expect and will expect. The set of all declarative constraints is a subset of all constraints expressible (not necessary declarative). However, there is always a mean to turn a non-declarative constraint to a declarative one : just add the new tag that expresses what you want. <acme:the-number-of-cells-must-be-the-same-in-all-columns-of-the-table/> This is a curious declaration, but a declaration anyway. I can express it in an Active Schema without implementing this silly tag. I only argue that instead of making a "tag race", there is a better way to do things : allow to mix imperative instructions with declarative sentences, which is applyable not only for schemata, but for operating tag libs in general. If there is a component behind a tag, the hierarchy of the components expressed with an XML document is static, but can becomes dynamic if alternative statements are allowed. Of course, a new level of indirection is added, but a great number of tags may be saved. So, it's more simple. - in fact, they > definitely should be - the flaw is with the expressive power of a grammar as > a contraint language. > > This is all well known. And in fact, XML Schema itself has some > extra-grammatical constraints, namely uniqueness and referential > constraints. But there's still a camp that believes (wrongly, in my view) > that the expressive power of a schema language should be restricted for > performance reasons. > > Michael Kay > http://www.saxonica.com/ > -- Cordialement, /// (. .) --------ooO--(_)--Ooo-------- | Philippe Poulard | ----------------------------- http://reflex.gforge.inria.fr/ Have the RefleX !
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