Michael Sperberg McQueen (CMSMcQ) has written a couple of interesting posts about datatypes in W3C’s XML Schema (XSDL). (The second is a response to a comment from John Cowan, and attempts to justify some of the seemingly arbitrary decisions made in the set of datatypes present in XSDL 1.0.) The posts are a discussion of one of the issues against XSDL 1.1 raised by Michael Kay:
Michael proposes: just specify that implementations may provide additional implementation-defined primitive types. In the nature of things, an implementation can do this however it wants. Some implementors will code up email dates and CSS lengths the same way they code the other primitives. Fine. Some implementors will expose the API that their existing primitive types use, so they choose, at the appropriate moment, to link in a set of extension types, or not. Some will allow users to provide implementations of extension types, using that API, and link them at run time. Some may provide extension syntax to allow users to describe new types in some usable way (DTLL, anyone?) without having to write code in Java or C or [name of language here].