- From: Michael Kay <mike@saxonica.com>
- Date: Thu, 28 May 2009 09:14:12 +0100
- To: "'Dag Hovland'" <dag.hovland@uib.no>, <xmlschema-dev@w3.org>
There are a number of "restrictions" in the xs:all facility, if by "restriction" you mean a facility that could have been added to the language but wasn't. The most obvious ones are: (a) maxOccurs is limited to the values 0 or 1 (b) the xs:all compositor cannot be combined with the sequence and choice compositors As a matter of historical research, it's always very difficult to give a reliable answer to the question "why did the WG decide X?". The minutes will usually only record the decision, and if you're lucky a very terse summary of some of the points that were made - it won't tell you why particular people voted in particular ways. It can be hard to know why a decision went a particular way even if you were at the meeting. However, this decision reflects a general tendency found in the working group to be cautious about offering features in the language that might prove (i) difficult to implement and (ii) useful to only a small number of users, while at the same time designing the syntax of the language so that it could easily be extended to remove the restrictions in a later release. Note that restriction (a) has been removed in XSD 1.1, restriction (b) has not. Regards, Michael Kay http://www.saxonica.com/ http://twitter.com/michaelhkay > -----Original Message----- > From: xmlschema-dev-request@w3.org > [mailto:xmlschema-dev-request@w3.org] On Behalf Of Dag Hovland > Sent: 28 May 2009 07:53 > To: xmlschema-dev@w3.org > Subject: Motivations for restricting the "all" group > > I have been trying to do some research into regular > expressions and the "all" group used in XML Schema, as mentioned in > > http://www.w3.org/TR/2001/REC-xmlschema-0-20010502/ > > It is clear that "all" is in some sense restricted, since it > can only combine elements, and must appear at top level. My > question is, what was the original inspiration for "all", > what is the "unrestricted" form of "all", and why was it > originally restricted, in the design of XML Schema? > > Some academic authors claim that "all" is a restricted form > of "interleaving", a known operator in regular language > theory, for which the membership problem is NP-complete. But > this is not clear to me, as interleaving means that the words > are shuffled in a way that does not seem to make sense for > natural languages. I believe that the "&" from SGML is a more > natural extensions, but I cannot find any reference to the > original motivation, or to the reasons for limiting "all". > > Thank you for any help, > > Dag Hovland >
Received on Thursday, 28 May 2009 08:14:49 UTC