Re: [xml-dev] RE: Concerned about the increasing reliance on XPath

[Dimitre Novatchev <dnovatchev@gmail.com>]

Also, it is absolutely not true that if an XPath expression is
expressed as XML (XQueryX), then its complexity is decreased --
breaking down a sentence of a PL to its syntax tree doesn't make this
sentence less complex.



-- 
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 2:09 PM, Dimitre Novatchev <dnovatchev@gmail.com> wrote:
> Roger,
>
>> 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.
>
> I still don't know what you mean by "analysis/assessment".
>
> If you know what exact relationships you want to express, then it
> might be possible to design a restricted language.
>
> Such a language would express only limited class of relationshops.
>
> I strongly suspect that there will be relationships that you'd want to
> express that cannot be done using this specific language only, and
> thus you'll need yet another language.
>
> So, it seems that the full power of a programming language will be
> necessary in the general case.
>
> If this isn't so, one must provide a proof (and a proof requires exact
> definitions) -- do you have one?
>
>
> --
> 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 1:20 PM, Costello, Roger L. <costello@mitre.org> wrote:
>> 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: [xml-dev] 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
>>>
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>>
>