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[xsl] Re: XSLT 3.0: What is the correct "Deep Skip" template rule code ?

2015-01-02 16:10:30
Thank you, Dr. Kay,

We made a conscious decision that deep-skip should work differently
for document nodes, because it was found that otherwise,
the effect of failing to supply an explicit template rule for the document 
node
was very bewildering. A built-in template rule for document nodes that skips 
the whole document
is simply not useful

I see the rationale behind this decision. I believe that this
explanation would be better included in the document.

it has to be overridden.

Some use cases come to mind immediately, where skipping document nodes
would be convenient -- such as when wanting to process only certain
documents from a collection (as matched by the pattern of a specific
template) and filter out all the rest -- without having to write a
template for this filtering.

-- 
Cheers,
Dimitre Novatchev



On Fri, Jan 2, 2015 at 2:00 PM, Michael Kay <mike(_at_)saxonica(_dot_)com> 
wrote:
We made a conscious decision that deep-skip should work differently for 
document nodes, because it was found that otherwise, the effect of failing to 
supply an explicit template rule for the document node was very bewildering. 
A built-in template rule for document nodes that skips the whole document is 
simply not useful; it has to be overridden.

The change was documented in item 27 of appendix K, though there is no link 
to the rationale used.

Michael Kay
Saxonica
mike(_at_)saxonica(_dot_)com
+44 (0) 118 946 5893




On 2 Jan 2015, at 21:07, Dimitre Novatchev <dnovatchev(_at_)gmail(_dot_)com> 
wrote:

Let me start with wishing to everyone a Happy and Safe New Year!


To continue with more routine, daily stuff :

In section "6.7.4 Built-in Templates: Deep Skip" of the 2nd Last Call
of the W3C "XSLT 3.0" specification
(http://www.w3.org/TR/2014/WD-xslt-30-20141002/#built-in-templates-deep-skip),
we read:

'The effect of processing a tree using a mode that specifies
on-no-match="deep-skip" is that where no explicit template rule is
specified for an element, that element and all its descendants are
ignored, and are not copied to the result tree.

The effect of choosing on-no-match="deep-skip" is as follows:

The built-in rule for document nodes is equivalent to calling
xsl:apply-templates with no select attribute, and with the mode
attribute set to #current. If the built-in rule was invoked with
parameters, those parameters are passed on in the implicit
xsl:apply-templates instruction.

In the case where there are no parameters, this is equivalent to the
following rule:

<xsl:template match="document-node()" mode="M">
 <xsl:apply-templates mode="#current"/>
</xsl:template>

The built-in rule for all items other than document nodes (that is,
for all other kinds of node, as well as atomic values and functions,
including maps) is to do nothing, that is, to return an empty sequence
(without applying templates to any children or ancestors).

This is equivalent to the following rule:

<xsl:template match="." mode="M"/> '


This definition raises some questions:

1. Why should there be inside a "deep skip" template, an
<xsl:apply-templates> instruction?  If the purpose is really "deep
skip", isn't this most directly accomplished by:

     <xsl:template match="document-node()" mode="M"/>

   And why pass parameters, if they wouldn't be used at all?

2. Why are there two separate "deep skip" built-in templates -- one
for matching document nodes and one for matching any item? Why not
just have a single template that matches any item (document node
included):

    <xsl:template match="." mode="M"/>



--
Cheers,
Dimitre Novatchev

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