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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: W3C Schema: Resistance is Futile, says Don Box
 On Mon, 2002-06-10 at 17:25, Aaron Skonnard wrote: > > Amelia A Lewis wrote: > > > > > In XML, typing specifies validation algorithms. > > > > Tim Bray: > > > > Hmm... well it also allows software to unpack XML instances into > native > > storage in a sensible way without being savvy to the internals of the > > applications that generated or will use the data. > > > > My question is, is there a need for this? -Tim Good question. Both you and John Cowan specify that as a functionality enabled by typing. I don't understand it, though. How does typing something as Notation, as NCName, as gMicrosecondEon, or as unsignedLong help me to unpack an XML instance into Java native storage without knowledge of semantics? I pick those primitive/built-in simple types specifically, because they don't map intuitively to any Java primitive that I can think of, which means that the mapper from XML type to language-native type is going to be highly dependent upon language, parser implementation, and other significantly complex locally-defined details. Using XSDL means that we have to agree on how to handle these types, even if gLunarcycleHour isn't relevant to what we're doing. More to the point, validation is a "lower level" than conversion to native type, if conversion to native type is a proper function of XML modules at all. > It also facilitates the reverse scenario: exposing strongly-typed data > (database, object graph, whatever...) as an Infoset, which can be > processed using any of XML's layered services (XPath, XSLT, XML Query, > etc.) in a natural way. The demand for such functionality seems to be > growing as businesses see it simplifying the server integration story. Umm, I'm working for an EAI vendor, as part of the SOAP team. This is usually regarded, by the folks I'm in regular contact with, as either a really pleasant opium dream, or a surprisingly thorny briar patch, however beautiful and fragrant its flowers. So I'll reiterate: the primary story for typing in XML is for validation. XML Schema, DTD, and Relax NG all provide mechanisms for structural validation (validation of the coarse-grained content of elements). The introduction of data types for "simple" (text nodes and attributes) nodes provides a way to validate the text, beyond existing parsed character data (the only valid type for text nodes in DTD), CDATA, Name, Token, NCName, QName, ID, IDREF, and the derived list types. Note that the attribute types are almost exclusively concerned with reference types (XML versions of pointers), and that at least two of them actually embed co-occurrence constraints (ID and IDRef), which cannot be modeled in any of the current schema languages, except by deus ex specificatia. So I'd suggest that XSDL's attempt to produce a universal system isn't a success, and that the first thing to begin with is to define the minimal role of typing in XML. Perhaps it's possible to move conversion from the application into XML (although I still don't quite understand how one is going to map, for instance, signedInt into perl or python without writing a specialized constraint handler of some sort, which effectively puts it back into the application), but I don't think that this should be the focus of simple type definitions. If it's supposed to be, then the type definition probably ought to include the target language or languages. And in any event, validation of primitive types is the first step to actually doing the conversions, and specifies the *XML content*, rather than per-language conversion APIs. Amy! -- Amelia A. Lewis amyzing@t... alicorn@m... A hundred thousand lemmings can't be wrong. This is a digitally signed message part 
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