Re: [xsl] grouping in XPATH with parenthesis

["G. Ken Holman" <gkholman@xxxxxxxxxxxxxxxxxxxx>]

At 2002-07-08 11:49 -0700, Dan Diebolt wrote:
> I recently saw an XPATH expression (in Dave Obasanjo's "Things to
> Know and Avoid When Querying XML Documents") that included parentheses
> used for grouping:

I wouldn't use the word "grouping" but rather "for containing a location 
path expression".  Sometimes, as in Dan's example, this is important in 
order to distinguish what would be construed as a location step expression 
to be a location path expression:

> (//*)[position()=1]/@on-loan

In the above "//*" is a location path expression because of the parens.

Consider taking out the parens:

  //*[position()=1]/@on-loan

This time the predicate is considered part of the second step "*" so as to 
address the on-load attribute of the first element child at every child 
level of the document tree.

Note that you cannot be fast and loose with parens, as addressing in a 
location step is proximity ordered while the addressing in a location path 
is document ordered, such that

   preceding-sibling::*[1]       (proximity order; addresses closest)

and

   (preceding-sibling::*)[1]     (document order; addresses furthest)

do not address the same node if there is more than one preceding 
sibling.  This is because the parens make the location step into a location 
path.

I have found parens very useful when starting off a location path 
expression with a union:

   (a | b | c)/d/e

Remember that in a multiple step location path, parens can only be used in 
the first step.

> I had to scratch my head wondering if I have ever seen parentheses
> used in this fashion. Is anyone familiar enough with this usage

Since day one of my XSLT/XPath training course I've underscored this with 
students because it helps to understand the nuances between location steps 
and paths and how proximity order differs from document order.

> to explain when parenthesis would be needed and where in the
> specs they are even allowed?

Production 15 in XPath.

I hope this helps.

................ Ken


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