Why XML data typing is hard

Michael Kay M.H.Kay at eng.icl.co.uk
Mon Nov 30 14:11:04 GMT 1998

>Then your example proposed range of values is inappropriate because "4,50"
>is a valid float from an I18N point of view.

"4,50" is a localized rendition of a float value. But in XML we should
encourage a rendition-independent encoding of information. (One reason for
having data types is that the default rendition can be determined from the
data type and the locale.) The encoding we choose for floats does not need
to be constrained by human conventions. Any of the notations used in
languages such as C, Java, and SQL would do nicely: and fortunately they are
all very similar.

In fact, I don't think defining data types in XML is harder than in any
other language. In many ways it's easier, because we don't have to worry
about defining operations, only valid states (and perhaps equivalence

In principle I'm quite happy with data types being defined as an optional
module above XML. My only concerns would be (a) that the XML family of
standards is developing a rather large collection of optional modules which
don't always work well together (as witness Namespaces and XSL), and (b)
that a bolted-on standard might be constrained not to extend the current DTD

Mike Kay

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