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

[Dimitre Novatchev <dnovatchev@gmail.com>]

Thanks, Greg. You expressed better  what was said in my last message
in this thread:

> 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.

Dimitre Novatchev

On Mon, May 9, 2011 at 5:26 PM, Greg Hunt <greg@firmansyah.com> wrote:
> Roger,
> I thought that your original point was that the calculation should be
> encoded as XML to make it easy to manipulate. The halting problem
> arises from the complexity of the behaviour that is described, not
> from its encoding.  You seem to want your hypothetical analysis to be
> capable of anything (you did not seriously limit its scope in your
> example), in which case ecoding the instructions as XML doesn't get
> you much if anything over encoding them in some programming language
> (modulo the parsing required).
>
> Analysis becomes difficult when the behaviour that is described
> becomes sufficiently complex (for suitable values of "difficult" and
> "complex").  Your initial example seemed to suggest that you want to
> constrain the set of behaviours that the XPath could have.    If the
> same things are encoded as XPath or as some bespoke XML then the
> analysis of behaviour (after parsing) has the same degree of
> complexity because they are instructions for the same behaviours.
> Thats just a matter of defining a subset of the full language and
> enforcing that subset (I've seen examples of that).  Of course, if you
> have very few behaviours that you want to support (sum for example)
> then your XML element approach is easiest to implement, but as the
> number grows and the complexity of the operations grows the argument
> for picking up someone else's XPath implementation becomes stronger
> (for example avoiding mixing data and instructions, avoiding writing
> the code to implement the operations).
>
> Greg
>
> On Tue, May 10, 2011 at 6:20 AM, 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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>>
>
> _______________________________________________________________________
>
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>
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>



-- 
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.