Is xsl:if syntax sugar?

sambar · February 21st, 2007, 05:58 AM

a bit of theory after all the working class problems ;)

is xsl:if syntax sugar? can you replace

Code:

<xsl:if test="expr">
....
</xsl:if>

with

Code:

<xsl:apply-templates select="." mode="if_template"/>

<xsl:template match="*[expr]" mode="if_template">
....
</xsl:template>

???

is the test expression you can use in the square brackets in match somehow limited?

can you copy the content of xsl:if element to 'if_template' as it is or do you have to modify it(and maybe its context) to keep the semantics?

all this in xslt1.0

mhkay · February 21st, 2007, 06:42 AM

The context for evaluating expr in your original code and in its replacement is different. Consider test="position()=last()".

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

sambar · February 21st, 2007, 07:59 AM

thanks for superfast reply ... BUT

this is exactly why i'm asking. i thought when evaluating expr in if_template the node list contains just a single node(from apply-templates select=".")

but when i test this(with xalan)

Code:

    <xsl:template match="c">
        <xsl:apply-templates select="." mode="if_template" />
    </xsl:template>

    <xsl:template match="*[position()=last()]" mode="if_template">
        <xsl:value-of select="@atr" />
    </xsl:template>

    <xsl:template match="*|/" mode="if_template" />

on this source file

Code:

<a>
    <c atr="c1 atr" />
    <c atr="c2 atr" />
    <c atr="c3 atr" />
</a>

i get this result

Code:

c3 atr

my understanding of xslt is say limited ;) and i expected to see this

Code:

c1 atr
c2 atr
c3 atr

???

mhkay · February 21st, 2007, 08:39 AM

That's exactly the point I was making. The pattern

match="*[position()=last()]"

is defined to match any element that is capable of being selected, from some context node, by the XPath expression select="*[position()=last()]", that is, every element that is the last child of its parent. Namely c3.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

sambar · February 21st, 2007, 10:47 AM

sorry, but i still don't get it, even after consulting the xslt specification. there is some fundametal misunderstanding between me and xslt processing model.

can you please explain, where do i do the mistake in my assumptions? this is the stylesheet from above, this time with line numbers

Code:

1    <xsl:template match="c">
2        <xsl:apply-templates select="." mode="if_template" />
3    </xsl:template>

4    <xsl:template match="*[position()=last()]" mode="if_template">
5        <xsl:value-of select="@atr" />
6    </xsl:template>

7    <xsl:template match="*|/" mode="if_template" />

i think the input file is processed as follows

1)the template from line 1 is instantiated three times, for each c element.

2)each time the current node list contains all three c elements and if you test "position()=last()" on line 2, it would be "true" only the third time when the third element is the current node. But not because it's 'last child of its parent' as you state, but becase it's the last element in the current node list as xslt1.0 spec chapter 4 Expressions states - the context position comes from the position of the current node in the current node list. ???

2)APPARENTLY FALSE:say the line1 template is instantiated with the first c element as current node. apply-templates on line2 then evaluates the select and gets the first c element only - this becomes the new current node list. the current node becomes the only element in the node list.

3)APPARENTLY FALSE:template on line4 is matched, because current node list contains just one node and position()=last()=1.

from your previous post i also don't see, why should you get different result if you replace line 2 with <xsl:if test="position()=last()"> etc

thanks for your endless(i hope) patience ...

mhkay · February 21st, 2007, 11:23 AM

Please see section 5.3 Patterns in the XSLT 1.0 spec:

A pattern is defined to match a node if and only if there is possible context such that when the pattern is evaluated as an expression with that context, the node is a member of the resulting node-set. When a node is being matched, the possible contexts have a context node that is the node being matched or any ancestor of that node, and a context node list containing just the context node.

With the expression *[position()=last()] there is no node in the document such that this expression will select your c1 and c2 elements.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference

sambar · February 21st, 2007, 12:07 PM

ok, i've read 5.2 Patterns(http://www.w3.org/TR/xslt#patterns)
i see (spec section 4) context means

1)context node = current node
2)context position = position() = position of the current node in the current node list
3)context size = last() = size of the current node list

so if i sum up this context definition, section 5.2 and your last post, i see that there is no way that current node = c1 and current node list = c1(and so position()=last() holds)?

that means <apply-templates select="c[position()=1]"> doesn't change the current node list to c1 as i understood from section 5.1 Processing Model?

i know this is fundamental, but the specification doesn't define the content of current node list very clearly for me and the books i have just don't bother explaining the specification and instead invent their own description of the processing model, preferably without the term current node list.

i hope all this still relates to the subject.

sambar · February 25th, 2007, 02:24 PM

CONCLUSIONS

xsl:if is not syntax sugar.

why the above mentioned proposal doesn't work? the test expression in xsl:if evaluates in a context with current node list containing nodes selected in apply-templates *up the stack*. the match pattern containing a predicate [...] is evaluated in a context with "siblings of the context node that match the NodeTest as the context node list". if you use the select attribute in apply-templates to select anything else than all siblings(and nothing else) or if you use xsl:sort, these two contexts have different context node lists.

if you don't use @select, the lists are identical, but you can't use variables in patterns and so you can't use it in xsl:if test expr for the proposal to work.