RE: Quiz: what's the value space of the <Publisher> element?
> > Consider this element declaration: > > <xs:element name="Publisher" type="xs:string"/> > > What is Publisher's value space? Technically, an element does not have a value space. Only a type has a value space. The value space here is clearly the set of all (legal XML) strings. > > > The Publisher element is being used by Book: > > > <xs:element name="Book"> > <xs:complexType> > <xs:sequence> > <xs:element name="Title" type="xs:string"/> > <xs:element name="Author" type="xs:string"/> > <xs:element name="Date" type="xs:gYear"/> > <xs:element name="ISBN" type="xs:string"/> > <xs:element ref="Publisher" /> > </xs:sequence> > </xs:complexType> > </xs:element> > > > Okay, what is Publisher's value space? Again, wrong terminology. But the set of things that can appear within a Publisher that appears within a Book is potentially constrained both by the type of the Book element and by the type of the Publisher element. > > XML Schema 1.1 has a new element, the <assert> element. It > can impose additional constraints on the Publisher element. > For example, the <Book> element is nested within a > <BarnesAndNoble> element, which has an <assert> element that > constrains the Publisher element to a maximum string length > of 140 characters: > > > <xs:element name="BarnesAndNoble"> > <xs:complexType> > <xs:sequence> > <xs:element ref="Book" maxOccurs="unbounded" /> > </xs:sequence> > <xs:assert > test="not(Book[string-length(Publisher) gt 140])" /> > </xs:complexType> > </xs:element> > Nothing new here: you could also use key/keyref at the level of a Book to constrain what can appear in the Publisher element. > > What this means is that an element declaration cannot be > understood on its own, in isolation. > > To understand an element you must understand all possible > ancestors of the element. > Not really. The system is designed so that the set of valid Publisher elements is determined by the type of the Publisher element; but if you want to say that not all valid values of Publisher are allowed when the Publisher is a book publisher, then you can do so. For example, you might have some way of excluding magazine publishers. Regards, Michael Kay http://www.saxonica.com/ http://twitter.com/michaelhkay
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