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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: RE: Concerned about the increasing reliance on XPath
 Hi Dimitre, > If you need to express *any* possible relationships > then most probably you need a full-fledged programming language and > analyzing/assessing programs is equivalent to the halting problem. You stated the problem perfectly: ... analyzing/assessing programs is equivalent to the halting problem That is, once XPath is introduced into an XML vocabulary then analysis/assessment becomes impossible. Often it is not necessary to express *any* possible relationship. For my "purchase XML vocabulary" it seems reasonable that one should be able to identify the relationships that are really needed. It is unlikely that *any* relationship is needed. By constraining the set of relationships -- using XML markup -- then the analysis/assessment problem is reduced from the halting problem (i.e., impossible) to something that is achievable. /Roger -----Original Message----- From: Dimitre Novatchev [mailto:dnovatchev@gmail.com] Sent: Monday, May 09, 2011 4:05 PM To: Costello, Roger L. Cc: xml-dev@lists.xml.org Subject: Re: RE: Concerned about the increasing reliance on XPath > Assessing the <Item> elements is easy; they just contain decimal values. Assessing the <Total> element is probably impossible since it can contain any arbitrary XPath expression. > > That's bad. > > XPath is fine if all you want to do is "execute" the XML vocabulary. But if you want to "assess/analyze" your XML vocabulary then XPath is not fine. This is true for any programming language. Why should the use of XPath be any different? If you need to express *any* possible relationships then most probably you need a full-fledged programming language and analyzing/assessing programs is equivalent to the halting problem. Please, reformulate, or otherwise this strikes the reader as another rediscovering the wheel. -- Cheers, Dimitre Novatchev --------------------------------------- Truly great madness cannot be achieved without significant intelligence. --------------------------------------- To invent, you need a good imagination and a pile of junk ------------------------------------- Never fight an inanimate object ------------------------------------- You've achieved success in your field when you don't know whether what you're doing is work or play ------------------------------------- Facts do not cease to exist because they are ignored. ------------------------------------- I finally figured out the only reason to be alive is to enjoy it. On Mon, May 9, 2011 at 12:18 PM, Costello, Roger L. <costello@mitre.org> wrote: > Hi Folks, > > Suppose you create an XML vocabulary for describing purchases: > > Purchase >   Item: decimal >   Total: XPath > > The value of <Total> is any XPath expression. > > Here's a sample instance document: > > <Purchase> >    <Item>10.00</Item> >    <Item>20.00</Item> >    <Total>sum(../Item)</Total> > </Purchase> > > You input that instance document into your "purchase processor" and it outputs: > >   Your purchases: >     Item: $10.00 >     Item: $20.00 >     Total: $30.00 > > The XPath expression in the <Total> element was evaluated by the "purchase processor." > > The <Total> element is powerful - the full power of XPath is available to it. To further illustrate its power, we could write an XPath expression to convert the sum of the Items to another currency: > >   <Total>sum(../Item) * 2.1034</Total> > > Or we could write an XPath expression that pulls in data from other documents to compute the total. > > Pretty powerful, aye? > > Now, write this tool: the input to the tool is a "purchase instance document", such as this: > > <Purchase> >    <Item>10.00</Item> >    <Item>20.00</Item> >    <Total>sum(../Item)</Total> > </Purchase> > > The tool assesses the instance document and outputs the results of the assessment. > > Ouch! > > Assessing the <Item> elements is easy; they just contain decimal values. Assessing the <Total> element is probably impossible since it can contain any arbitrary XPath expression. > > That's bad. > > XPath is fine if all you want to do is "execute" the XML vocabulary. But if you want to "assess/analyze" your  XML vocabulary then XPath is not fine. > > Contrast the above with this XML vocabulary: > > Purchase >   Item: decimal >   Total >     SumPrecedingItems >        Value: decimal > > Here's a sample instance document: > > <Purchase> >    <Item>10.00</Item> >    <Item>20.00</Item> >    <Total> >       <SumPrecedingItems> >         <Value>30.00</Value> >       </SumPrecedingItems> >    </Total> > </Purchase> > > The <Total> element is much less powerful - its content is an element that has the semantics "sum all the preceding <Item> elements." > > Now, write a tool in which you give it a "purchase instance document" and it assesses the instance document. > > Easy! > > Analysis of the XML vocabulary is possible (easy, in fact). > > Summary: if an XML vocabulary permits XPath expressions then analysis of the XML vocabulary becomes exceedingly difficult (or impossible). > > Comments? > > /Roger > > _______________________________________________________________________ > > XML-DEV is a publicly archived, unmoderated list hosted by OASIS > to support XML implementation and development. To minimize > spam in the archives, you must subscribe before posting. > > [Un]Subscribe/change address: http://www.oasis-open.org/mlmanage/ > Or unsubscribe: xml-dev-unsubscribe@lists.xml.org > subscribe: xml-dev-subscribe@lists.xml.org > List archive: http://lists.xml.org/archives/xml-dev/ > List Guidelines: http://www.oasis-open.org/maillists/guidelines.php > > 
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