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LC-93 disjoint datatypes

From: C. M. Sperberg-McQueen <cmsmcq@acm.org>
Date: Mon, 09 Oct 2000 20:00:47 -0600
Message-Id: <4.3.2.7.1.20001009195229.0254dd88@espanola.com>
To: "Vun Kannon, David" <dvunkannon@kpmg.com>
Cc: W3C XML Schema Comments list <www-xml-schema-comments@w3.org>
Dear Mr. Van Kunnon:

The W3C XML Schema Working Group has spent the last several months
working through the comments received from the public on the last-call
draft of the XML Schema specification.  We thank you for the comments
you made on our specification during our last-call comment period, and
want to make sure you know that all comments received during the
last-call comment period have been recorded in our last-call issues
list (http://www.w3.org/2000/05/12-xmlschema-lcissues).

Among other issues, you raised the point registered as issue LC93,
which suggests that XML Schema be modified to make it possible to
build a simple datatype out of other simple datatypes with disjoint
lexical spaces.

Ashok Malhotra replied in May that this is not possible (and indeed
it was not possible at that time).

More recently, however, after discussion of the issue you raised, in
conjunction with related issues raised by others, the WG agreed to
adopt a proposal to allow simple types to be constructed as the union
of other simple types.  There is no requirement that the lexical
spaces be disjoint: the value passed is tried against each member type
in turn, and the first one it matches is the one which applies.  (This
means that A union B will not behave in exactly the same way as B
union A, so schema authors are urged to exercise caution.)  This means
that it is possible to create union types for (for example) (a)
anything from this enumeration of strings, or (b) any string.  The
xsi:type attribute may be used in the instance as a discriminator to
force the value to be associated with a particular member type.

It would be helpful to us to know whether you are satisfied with the
decision taken by the WG on this issue, or wish your dissent from the
WG's decision to be recorded for consideration by the Director of
the W3C.

with best regards,

-C. M. Sperberg-McQueen
  World Wide Web Consortium
  Co-chair, W3C XML Schema WG
Received on Monday, 9 October 2000 16:19:37 GMT

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