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RE: Motivations for restricting the "all" group

From: Dag Hovland <dag.hovland@uib.no>
Date: Thu, 28 May 2009 11:06:39 +0200
Message-ID: <4A1E541F.3030404@uib.no>
To: xmlschema-dev@w3.org
Hi!

Thank you for the clear answer. I realize that finding the "reasons" are
probably hard. My intention was also to discover what is the
unrestricted form of xs:all. Is it the "&"-operator from SGML? Or is it
"interleaving" as described in regular language theory? Are there any
special problems in treating the full &-operator? I could not find much
information about this operator, or why it was/is seen as hard to treat.

Kind regards,

Dag Hovland


> 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 09:07:18 GMT

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