RE: [xsl] Stepping through XPath in a visual XPath debugger

[Wendell Piez <wapiez@xxxxxxxxxxxxxxxx>]

Hi,

At 01:08 PM 7/31/2007, Mike wrote:
> > a) What is the most intuitive order in which to step-through
> > an XPath 1.0 expression visually?
>
> I really don't know the answer to this, but I think anything that presents a
> view of the evaluation as a sequential process is misleading and likely to
> be unhelpful. The concept should be to select parts of the expression and
> understand what contribution they make to the result - for example, being
> able to see which nodes a particular predicate is applied to, and which of
> them match.

While admitting, in sympathy with this caution, that it is an 
academic exercise to view the evaluation of an XPath expression 
sequentially, it might be helpful to see how we've learned to teach 
neophytes to interpret XPath 1.0 location paths:

1. The concept of a step, each step being evaluated from the context 
of the previous step
2. Anatomy of a step:
   2a Axis (required)
   2b Node test (required)
   2c Predicate(s) (optional)
3. Axes in detail
4. Node tests in detail
5. Predicates (as filters of sets of nodes)
6. Abbreviations (or, short vs long syntax)

It is of particular interest that XPath abbreviations sometimes have 
the effect of obscuring exactly what the axis and node tests are in a 
particular step. Accordingly, when we teach this subject at Mulberry, 
we lay some stress (how much depends on the class) on the long 
syntax, as it indicates explicitly just what is happening.

So I should think that a nice visualization of XPath would do, at 
least, the following:

1. Translate from short (abbreviated) syntax into long syntax
2. Discriminate between the steps in a path
3. Optionally, discriminate between the parts of each step
4. Show which nodes were selected in each step (perhaps identified in 
relation to the step's context)

Note that there have already been some worthy visualization tools for 
XPath, but nothing is a substitute for a concrete, meaningful 
example. While being able to visualize it generally is useful, it 
really clicks into place when students can see *why* particular 
XPaths are used in particular situations. This helps to dramatize 
which sorts of things matter (context nodes) and which ones don't 
(short or long syntax).

Cheers,
Wendell



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Wendell Piez                            mailto:wapiez@xxxxxxxxxxxxxxxx
Mulberry Technologies, Inc.                http://www.mulberrytech.com
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