I'm using Saxon-EE 9.3.0.4 for xsl transformations and found that when <xsl:result-document> is used inside <xsl:attribute> (as well as <xsl:namespace>, <xsl:processing-instruction>) instruction, processor
raises XTDE1480 error - Cannot switch to a final result destination while writing a temporary tree
<xsl:attribute name="a">
<xsl:result-document href="result.xml" > - error here
test
</xsl:result-document>
</xsl:attribute>
According to specification (Appendix D) permitted parents for <xsl:result-document> are:
any XSLT element whose content model is sequence constructor
any literal result element
Is this is bug in Saxon or limitations for <xsl:result-document> usage, which are not defined in specification?
Update:
I believe that this is not a good idea to use <xsl:result-document> inside <xsl:attribute>, but why error happens?
Thanks
In the spec, you can read :
The instructions in the initial template are evaluated in final output
state. An instruction is evaluated in the same output state as its
calling instruction, except that xsl:variable, xsl:param,
xsl:with-param, xsl:attribute, xsl:comment,
xsl:processing-instruction, xsl:namespace, xsl:value-of, xsl:function,
xsl:key, xsl:sort, and xsl:message always evaluate the instructions in
their contained sequence constructor in temporary output state.
The xsl:result-document write in a final tree result, and given the rule above, I understand that it is an error to try writting in a final tree result when inside the following elements
xsl:variable, xsl:param, xsl:with-param, xsl:attribute, xsl:comment, xsl:processing-instruction, xsl:namespace, xsl:value-of, xsl:function, xsl:key, xsl:sort, and xsl:message
Update in 2018: note that the XSLT 3.0 specification relaxes the restrictions. Instructions such as xsl:attribute and xsl:comment no longer set temporary output state. Recent Saxon versions (certainly 9.8) implement the revised rules.
Related
I'm starting to learn XSLT/XPath, and I copied the following from a study guide, making some modifications:
<xsl:variable name="fname" select="'polist.xml'"/>
<xsl:variable name="thePath" select="'/collection/doc'"/>
...
<xsl:value-of select="count(doc($fname)/collection/doc)"/>
It reports the number of doc elements in the XML file. The doc() function accepts the file name variable 'fname'. But if I try to do the same with the 'thePath' variable in the count() function, using $thePath instead of the "/collection/doc" text, I get an error.
Suggestions on whether/how to use the 'thePath' variable in the count() function? Is it possible? Thanks!
Learning from examples leaves you very exposed to this kind of problem: it's easy to build a completely incorrect mental model of how the examples actually work. That's why I always advise people to start by reading a good book that explains the concepts first.
In your case you've made a common mistake, which is to assume that variables work like macros, that is, that they represent fragments of XPath text that can be substituted into an expression. That's not the case: variables represent values, the result of evaluating an expression, and you can only use a variable in places where a literal value (like a number or string) could appear.
(I suspect it's the use of the $ sign that leads to this false impression. $ is often used to represent variables in macro-like languages, for example shell scripts).
In XPath 1.0 there's no direct way of achieving what you are trying to do. In practice people either use vendor extensions for this, or they construct a pipeline in which phase 1 generates an XSLT stylesheet and phase 2 executes it (that's easier in XSLT than in most other languages, because XSLT is XML and can therefore be easily manipulated using XSLT).
In 3.0 you can evaluate XPath expressions supplied in the form of a string using the xsl:evaluate instruction. But very often, the requirement can be met better using functions. We don't know what the real underlying requirement is here so it's hard to know whether that's true in this case.
An example use of xsl:evaluate in XSLT 3 would be e.g.
<xsl:evaluate xpath="'count(' || $thePath || ')'" context-item="doc($fname)"/>
Usually the programming languages allow you to declare a variable for example:
Dim test as integer <-- Visual Basic
and subsequently allow to increase the value in a for each cycle.
for test=0 to 3
print test
next
Can I create a similar structure in XSLT 1.0?
With 'xsl:variable', I declare a variable, but if I want to increase in a for-each as you do?
The closest is XSLT 2.0 with
<xsl:for-each select="0 to 3">
<xsl:value-of select="."/>
</xsl:for-each>
which processes the sequence of integers 0, 1, 2, 3.
As you see, it does not use a variable and increments that, as variables are immutable, you simply bind a value to them once.
With XSLT 1.0 you can process nodes or you can write a recursive, named templates where each recursive call passes on an incremented parameter value. Whether you actually need that depends on your requirements, if you are new to the declarative programming of XSLT then it is best that you define your problem by showing a sample of the XML input and the corresponding output you want to create, explaining how the input is mapped to the output.
<xsl:value-of select="$MyVar"/>
works but
<xsl:value-of select="MyDataPfath/$MyVar"/>
do not work.
What is wrong in my code?
From the look of it, what you are trying to achieve is 'dynamic evaluation'. XSLT does not support the dynamic evaluation of XPath by default, so you will need to make use of an extension function.
Depending on your XSLT processor, you might want to look at EXSLT extensions. In particular the dynamic module at http://www.exslt.org/dyn/index.html. This would allow to do something like this
<xsl:value-of select="dyn:evaluate('MyDataPfath/$MyVar')"/>
However, in your case, perhaps the $MyVar contains just a single element name. In which case you could change your command to the following, which would work without any extension functions
<xsl:value-of select="MyDataPfath/*[local-name() = $MyVar]"/>
Your code didn't fail, it did exactly what the specification says it should do. Which was different from what you were hoping/imagining that it might do.
Your hopes/imagination were based on a fundamental misunderstanding of the nature of variables in XPath. XPath variables are not macros. They don't work by textual substitution; they represent values. If the variable $E contains the string "X", then MyPath/$E means the same as MyPath/"X", which is illegal in XPath 1.0, and in XPath 2.0 returns as many instances of the string "X" as there are nodes in MyPath.
You probably intended MyPath/*[name()=$E]
it is not possible to get the value by using syntax 'MyDataPfath/$MyVar' in . it will not recognize the proper path.
suppose $MyVar has value 'Hi'. so it will be represented as 'MyDataPfath/"Hi"', this is not valid path, which you want to retrieve from the XML.
to remove this limitation, You can use name() or local-name() function, that can be used as follows:
or
I am new in XSLT so I'm little bit confused about the two tags,
<xsl:apply-templates name="nodes">
and
<xsl:call-template select="nodes">
So can you list out the difference between them?
<xsl:call-template> is a close equivalent to calling a function in a traditional programming language.
You can define functions in XSLT, like this simple one that outputs a string.
<xsl:template name="dosomething">
<xsl:text>A function that does something</xsl:text>
</xsl:template>
This function can be called via <xsl:call-template name="dosomething">.
<xsl:apply-templates> is a little different and in it is the real power of XSLT: It takes any number of XML nodes (whatever you define in the select attribute), processes each of them (not necessarily in any predefined order), somebody could say that apply-templates works like a loop, but this is not exactly the case, as the nodes may be processed in any order, even in parallel, and finds matching templates for them:
<!-- sample XML snippet -->
<xml>
<foo /><bar /><baz />
</xml>
<!-- sample XSLT snippet -->
<xsl:template match="xml">
<xsl:apply-templates select="*" /> <!-- three nodes selected here -->
</xsl:template>
<xsl:template match="foo"> <!-- will be called once -->
<xsl:text>foo element encountered</xsl:text>
</xsl:template>
<xsl:template match="*"> <!-- will be called twice -->
<xsl:text>other element countered</xsl:text>
</xsl:template>
This way you give up a little control to the XSLT processor - not you decide where the program flow goes, but the processor does by finding the most appropriate match for the node it's currently processing.
If multiple templates can match a node, the one with the more specific match expression wins. If more than one matching template with the same specificity exist, the one declared last wins.
You can concentrate more on developing templates and need less time to do "plumbing". Your programs will become more powerful and modularized, less deeply nested and faster (as XSLT processors are optimized for template matching).
A concept to understand with XSLT is that of the "current node". With <xsl:apply-templates> the current node moves on with every iteration, whereas <xsl:call-template> does not change the current node. I.e. the . within a called template refers to the same node as the . in the calling template. This is not the case with apply-templates.
This is the basic difference. There are some other aspects of templates that affect their behavior: Their mode and priority, the fact that templates can have both a name and a match. It also has an impact whether the template has been imported (<xsl:import>) or not. These are advanced uses and you can deal with them when you get there.
To add to the good answer by #Tomalak:
Here are some unmentioned and important differences:
xsl:apply-templates is much richer and deeper than xsl:call-templates and even from xsl:for-each, simply because we don't know what code will be applied on the nodes of
the selection -- in the general case this code will be different for
different nodes of the node-list.
The code that will be applied
can be written way after the xsl:apply templates was written and by
people that do not know the original author.
The FXSL library's implementation of higher-order functions (HOF) in XSLT wouldn't be possible if XSLT didn't have the <xsl:apply-templates> instruction.
Summary: Templates and the <xsl:apply-templates> instruction is how XSLT implements and deals with polymorphism.
Reference: See this whole thread: http://www.biglist.com/lists/lists.mulberrytech.com/xsl-list/archives/200411/msg00546.html
xsl:apply-templates is usually (but not necessarily) used to process all or a subset of children of the current node with all applicable templates. This supports the recursiveness of XSLT application which is matching the (possible) recursiveness of the processed XML.
xsl:call-template on the other hand is much more like a normal function call. You execute exactly one (named) template, usually with one or more parameters.
So I use xsl:apply-templates if I want to intercept the processing of an interesting node and (usually) inject something into the output stream. A typical (simplified) example would be
<xsl:template match="foo">
<bar>
<xsl:apply-templates/>
</bar>
</xsl:template>
whereas with xsl:call-template I typically solve problems like adding the text of some subnodes together, transforming select nodesets into text or other nodesets and the like - anything you would write a specialized, reusable function for.
Edit:
As an additional remark to your specific question text:
<xsl:call-template name="nodes"/>
This calls a template which is named 'nodes':
<xsl:template name="nodes">...</xsl:template>
This is a different semantic than:
<xsl:apply-templates select="nodes"/>
...which applies all templates to all children of your current XML node whose name is 'nodes'.
The functionality is indeed similar (apart from the calling semantics, where call-template requires a name attribute and a corresponding names template).
However, the parser will not execute the same way.
From MSDN:
Unlike <xsl:apply-templates>, <xsl:call-template> does not change the current node or the current node-list.
Consider this XML:
<people>
<person>
<firstName>Deane</firstName>
<lastName>Barker</lastName>
</person>
</people>
What if two XSLT templates match an element through different XPaths? I know that if the "match" element on two templates is identical (which should never happen, I don't think), the last template will fire.
However, consider this XSL:
<xsl:template match="person/firstName">
Template #1
</xsl:template>
<xsl:template match="firstName">
Template #2
</xsl:template>
The "firstName" element will match on either of these templates -- the first one as a child of "person" and the second one standalone.
I have tested this, and Template #1 executes, while Template #2 does not. What is the operative principle behind this? I can think of three things:
Specificity of XPath (highly doubtful)
Location in the XSLT file (also doubtful)
Some pre-emption of Template #2 by Template #1. Something happens during the execution of Template #1 that tells Template #2 not to execute.
Your first point is actually correct, there is a defined order described in https://www.w3.org/TR/1999/REC-xslt-19991116#conflict. According to the spec person/firstName has a priority of 0 while firstName has a priority of -0.5. You can also specify the priority yourself using the priority attribute on xsl:template.
I know that if the "match" element on
two templates is identical (which
should never happen, I don't think)
This can happen but would not be much point doing this and having two matching templates.
From the spec:
It is an error if this leaves more
than one matching template rule. An
XSLT processor may signal the error;
if it does not signal the error, it
must recover by choosing, from amongst
the matching template rules that are
left, the one that occurs last in the
stylesheet.
So in other words you may get an error or it will just use the last template in your XSLT depending on how the processor your are using has been written to handle this situation.
Note that the value of the match attribute is not an XPath expression (though it uses a subset of XPath syntax). It's an XSLT pattern. Absent explicit priority attributes, the choice comes down to which pattern has the highest default priority:
person/firstName has a default priority of .5
firstName has a default priority of 0
Thus, person/firstName wins.
A complete explanation of how conflict resolution works can be found here (although I recommend you study the entire chapter, "How XSLT Works"): Conflict Resolution for Template Rules
Consider this with the context in mind. The first one matches, and changes the context n (so the second does not match). The context is set to AFTER the first one is selected and processed so the visible element from that context no longer contains "firstname".
IF you want both to execute, then you can call them instead so that the context changes back to the top.
<xsl:template match="people">
<xsl:apply-templates select="person/firstname"/>
<xsl:apply-templates select="firstname"/>
</xsl:template>