RE: [xsl] Streaming terminology: Climbing

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

Thank you very much Michael!

I really like this:

	A "stack" of ancestor data is maintained
	as the streaming processor works its way
	through the document.

I have a few follow-up questions, if I may.

You wrote:

	... this confuses the role of the context node
	and the current streaming position.

Would you elaborate on this please? I thought that the context node and the
current streaming position are one and the same, i.e., context node = current
streaming position?

You wrote:

	For example the construct

		descendant::x/parent::y

	is climbing, although it does not select ancestors
	of the context node. It's climbing because it reads
	information from the "stack" of ancestor data that
	is maintained as the streaming processor works its
	way through the document ...

Truthfully, my heart sank when I read that, as it reveals that I still do not
grasp what "climbing" means. I thought that climbing means "climbing up the
XML tree." Clearly descendant::x is not a construct that is climbing up the
XML tree. Are you saying that descendant::x/parent::y is climbing because
there is a portion of it that climbs up the XML tree, namely parent::y climbs
up the XML tree?

You finished the above sentence with

	... and when you're positioned in that stack, your ability
	to select downwards is constrained.

Would you elaborate on that please? It sounds like a very important concept.

Thanks!

/Roger

-----Original Message-----
From: Michael Kay [mailto:mike@xxxxxxxxxxxx]
Sent: Wednesday, January 29, 2014 6:50 AM
To: xsl-list@xxxxxxxxxxxxxxxxxxxxxx
Subject: Re: [xsl] Streaming terminology: Climbing

Yes, it's a very useful explanation.

I think your conceptual model that the processor is "positioned just after the
start tag" is probably helpful to many people but it's worth pointing out that
it's not in the spec and we try not to over-constrain implementations. There's
also a glitch in it in that we (deliberately) define has-children() to be an
inspection construct, and to evaluate this function the processor needs to
peek just beyond the end of the start tag. (Saxon doesn't yet implement
this).

There's also a minor error where you say:

A construct is climbing if, when executed, it yields the context node (self)
or ancestors of the context node.

Apart from the fact that attribute selection is also climbing (despite the
name), this confuses the role of the context node and the current streaming
position. For example the construct

descendant::x/parent::y

is climbing, although it does not select ancestors of the context node. It's
climbing because it reads information from the "stack" of ancestor data that
is maintained as the streaming processor works its way through the document,
and when you're positioned in that stack, your ability to select downwards is
constrained.

Incidentally, at one time we defined the spec so that as well as retaining
"the start tags of all ancestors" in this stack, we also retained some
positional information about the position of each ancestor relative to its
siblings, allowing match patterns such as match="para[1]". This has been
dropped from the spec, but it's partially implemented in Saxon, and I have yet
to decide whether to retain it in some form.

Michael Kay
Saxonica