I never got a direct answer to my question about the meaning of the
parentheses, but I think I've worked it out.
I find it easier to think of things visually (I always did well in geometry but
stumbled in algebra). What we're really working with is the notion of sets, so
I think of a Venn diagram. I picture this:
"x | y[1]"
as "The set of all x-things and the first y-thing."
and this:
"(x | y)[1]"
as "The first element of the set of all x-things and all y-things."
Is that about right?
--
Charles Knell
cknell(_at_)onebox(_dot_)com - email
-----Original Message-----
From: Wendell Piez <wapiez(_at_)mulberrytech(_dot_)com>
Sent: Tue, 15 Aug 2006 10:31:59 -0400
To: xsl-list(_at_)lists(_dot_)mulberrytech(_dot_)com
Subject: RE: [xsl] Selecting the first node set
At 01:00 PM 8/14/2006, Mike wrote:
One has to be willing to think with the
literal-mindedness of a machine to tell the difference
between /descendant-or-self::node()/child::Value[1] and
(/descendant-or-self::node()/child::Value)[1], and that can
take a bit of practice.
Yes. I'm not sure why people struggle with this though. It seems intuitive
enough that
x | y[1]
doesn't mean the same as
(x | y)[1]
so why is the "/" perceived so differently from the "|"? Is it that people
are somehow aware that it's a higher-order operator and therefore imagine it
doesn't obey normal precedence rules? Or is it the overloading of [], which
in its other role as a boolean filter is associative with "/"?
Possibly all of the above, and more, but none of it in specific. I
think it's really a case of brain overloading. When faced with what
appear to be complex expressions that remind them of middle-school
examinations, people's anxiety levels rise and their neural pathways
tighten up, leading to a negative-feedback-loop-driven brain shutdown
and a reversion to what "ought" to be. Some people learn to relax,
slow down and let the channels open, or they have learned from
practice not to be scared by mathematics, or they were never scared
in the first place.
It's a fascinating thing to observe. As a teacher, I try to convey
that all this persnicketiness is not only necessary (to understand
the machine, which doesn't suffer from anxiety), but it can be fun,
not scary, and that no one's hand will be slapped. (Of course, that
runs the risk of me being classed as the Other, at which point
teaching becomes even more of a challenge. :-)
But you've given me an idea. Maybe our poor school-scarred students
would be helped by examining
/ descendant-or-self::node() / child:element[1]
as a gloss on //element[1]
Thanks again Mike.
Cheers,
Wendell
Michael Kay
http://www.saxonica.com/
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