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

["Costello, Roger L." <costello@mitre.org>]

Hi David,

> But do you have support to your assertion that xpath requires large
> computation for simple expressions  ?

If I write an application that will process an XML vocabulary and that can contain XPath then my application must implement the entire XPath specification.  Michael Kay's book on XPath 2.0 is 552 pages [1]. That's a lot of stuff for my application to implement.

Are you making a distinction when you say "xpath ... for simple expressions"? Note that, for example, the new XML Schema assert element (mostly) permits the entire XPath language.  It does not restrict it to a "simple expressions" subset.

/Roger

[1] http://www.amazon.com/XPath-2-0-Programmers-Reference-Programmer/dp/0764569104/ref=sr_1_3?ie=UTF8&qid=1304780709&sr=8-3 

-----Original Message-----
From: David Lee [mailto:dlee@calldei.com] 
Sent: Saturday, May 07, 2011 10:59 AM
To: Costello, Roger L.; xml-dev@lists.xml.org
Subject: RE: [xml-dev] Concerned about the increasing reliance on XPath

Contra-point.
Do you have any tangible evidence that using XPath with simple expressions
requires significant processing ?

Its true that it requires the support infrastructure - and the same can be
said of any level of computation that requires a lower layer (which is prety
much all ).
But do you have support to your assentation that xpath requires large
computation for simple expressions  ?


----------------------------------------
David A. Lee
dlee@calldei.com
http://www.xmlsh.org

-----Original Message-----
From: Costello, Roger L. [mailto:costello@mitre.org] 
Sent: Saturday, May 07, 2011 9:52 AM
To: xml-dev@lists.xml.org
Subject: [xml-dev] Concerned about the increasing reliance on XPath


Hi Folks,

XPath is a fabulous language. It is incredibly powerful. It is a large, rich
language.

I have observed in increasing usage of XPath.

For example, in XML Schema 1.1 the new assert element uses XPath to express
constraints:

    <assert test="XPath" />

XPath gives a lot of power to the assert element.

But it also means that a lot of power is needed to evaluate the assert
element. 

To evaluate that tiny, innocuous assert element you need to implement the
entire XPath language.

Suppose the assert element was simplified. The only kind of assertion that
can be made is, "The value of the first child element must be greater than
the value of the second child element." Here's how we might use it to
express the constraint between a meeting's start time and end time:

<greaterThan>
        <element name="start-time" type="dateTime" />
        <element name="end-time" type="dateTime" />
</greaterThan>

Very little power is needed to evaluate this "assertion".  The assertion is
expressed entirely in XML markup.

We've lost an enormous amount of power/expressivity. But we've gained in
reduced cost of evaluation/processing/coding. 

While XPath is nice, it is:

    - not XML
    - requires huge amounts of processing (i.e., coding) wherever it's used

I am concerned about the increasing reliance on XPath. 

My Position: In some cases it would be better to ratchet down capabilities
and use XML markup rather than XPath.

I know many people will disagree with my position. Let's have a good
discussion of the issue. Here's the issue:

Issue: Is the increasing reliance on XPath a positive or negative trend?

(Implicit in this "issue" is an assumption that there is indeed a growing
trend toward using XPath)

/Roger

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