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 <h2>&lt;xsl:variable&gt; / &lt;xsl:param&gt;</h2>

   <p align="right" size="2">
 <a href="#content">(top)</a>
 </p>
 <h3>Contents</h3>
   <ul>
     <li>
 <a href="#functionality">Functionality</a>
 </li>
     <li>
 <a href="#implementation">Implementation</a>
 </li>
   </ul>


   <a name="functionality">‌</a>
   <p align="right" size="2">
 <a href="#content">(top)</a>
 </p>
 <h3>Functionality</h3>

   <p>Variables in XSLT are not really variables, as their values cannot be
   changed. They resemble constants from conventional programming languages. The
   only way in which a variable can be changed is by declaring it inside a
   for-each loop, in which case its value will be updated for every iteration.
   Top-level variables (variables that are direct child nodes of the
   <code>&lt;xsl:stylesheet&gt;</code> element) can never be changed.</p>

   <blockquote class="source">
 <pre>
     &lt;xsl:for-each select="/foo/bar"&gt;
       &lt;xsl:variable name="some-bar" select="."/&gt;
       &lt;xsl:value-of select="$some-bar"/&gt;
     &lt;/xsl:for-each&gt;</pre>
 </blockquote>

   <p>Parameters are assigned a value either from the process that invoked
   the stylesheet (top-level parameter), or from a
   <code>&lt;xsl:with-param&gt;</code> or from a default value (in which case it
   behaves as if it was a variable).</p>

   <blockquote class="source">
 <pre>
     &lt;xsl:template match="/"&gt;
       &lt;xsl:call-template name="blob"&gt;
         &lt;xsl:with-param name="par" select="'some-value'"/&gt;
       &lt;/xsl:call-template&gt;
     &lt;/xsl:template&gt;

     &lt;xsl:template name="blob"&gt;
       &lt;xsl:param name="par" select="'default-value'"/&gt;
       &lt;xsl:value-of select="$param"/&gt;
     &lt;/xsl:template&gt;</pre>
 </blockquote>


   <a name="implementation">‌</a>
   <p align="right" size="2">
 <a href="#content">(top)</a>
 </p>
 <h3>Implementation</h3>

     <p>Variables and parameters share a common base class
     <code>VariableBase</code> that contains a lot of common methods. This class
     handles both global and local variables/parameters.</p>

     <p align="right" size="2">
 <a href="#content">(top)</a>
 </p>
 <h4>Top-level parameters and variables</h4>

     <p>All top-level (ie. global) parameters and variables are stored inside
     fields in the translet class. Variables are stored as objects or basic
     data types (such as boolean, char, int, etc.) while parameters have to be
     "boxed" inside an object. This is because parameters are also stored as
     objects inside the translet. The <code>addParameter()</code> method of the
     <code>AbstractTranslet</code> class stores the parameter in a Hashtable
     (the Hashtable maps the parameter name to the parameter value). The
     "boxing" of the parameter's value is done by the class that handles the
     parameters type. This class is a subclass of
     <code>org.apache.xalan.xsltc.compiler.util.Type</code>.</p>

     <p>Note that all top-level parameters and variables from all imported and
     included stylesheets will be placed as direct children of the top-level
     stylesheet in the AST. This done to make global variables truly global and
     not just global in the stylesheet where it was declared.</p>


     <p align="right" size="2">
 <a href="#content">(top)</a>
 </p>
 <h4>Local parameters and variables</h4>

     <p>Local variables that are accessible from a given syntax tree node will
     first be put on the JVM's stack and stored in a local variable slot. This
     makes the variable or parameter accessible from all code within that
     method. But, in some special cases, the code that is compiled to handle an
     element/expression within the variable scope is not put inside the same
     method as the actual variable. This is the case for some predicates.
     All syntax-tree nodes implement the <code>isClosureBoundary()</code> method
     to indicate if its child an ancestor nodes will end up in a different method
     then itself. This method is used by the <code>Variable</code> and
     <code>Param</code> classes to determine if the variable or parameter will
     "escape" the variable frame.</p>

     <blockquote class="source">
 <pre>
     &lt;xsl:for-each select="/foo/bar/baz"&gt;
         &lt;xsl:variable name="pos" select="3"/&gt;
         &lt;xsl:apply-templates select="/foo/bar[$pos]"/&gt;
     &lt;/xsl:for-each&gt;</pre>
 </blockquote>

     <p>The predicate in this stylesheet fragment is compiled into a separate
     auxiliary class that implements the <code>Filter</code> interface. It will
     therefore not have access to the variable "pos" in the current stack frame.
     A common technique for cases like this is to use a <b>"closure"</b>. A
     closure is a record that contains references to all variables that are in
     scope for a certain part of the compiled scope. This is done in a very
     simple manner in XSLTC. All variables or parameters that can "escape" the
     stack are passed to the translet via its <code>addVariable()</code> method.
     They can then later be retrieved by the <code>getVariable()</code> method.
     </p>

     <p>
 <b>Important note 1:</b> A predicate does not always result in a
     auxiliary class. In some cases we optimize the code by using tailored
     iterators and goodies like that instead. We may want to update the
     predicate code to check if an auxiliary class will be generated before
     returning true or false from the <code>isClosureBoundary()</code>
     method.</p>

     <p>
 <b>Important note 2:</b> There could be other closure boundaries
     that we have not yet discovered or considered. This could be, for instance,
     sort records and other auxiliary classes:</p>

     <blockquote class="source">
 <pre>
     &lt;xsl:variable name="sort-order" select="'decending'"/&gt;
     &lt;xsl:for-each select="/foo/bar/baz"&gt;
         &lt;xsl:sort select="@name" order="$sort-order"/&gt;
         &lt;xsl:value-of select="."/&gt;
     &lt;/xsl:for-each&gt;</pre>
 </blockquote>

     <p>I would not be surprised if this fails. A fix could be to implement the
     <code>isClosureBoundary()</code> in the <code>Sort</code> class and have the
     method return 'true' in all cases.</p>


     <p align="right" size="2">
 <a href="#content">(top)</a>
 </p>
 <h4>Parameter and variable references</h4>

     <p>A parameter or variable reference does the oposite of a parameter or
     variable. The value is read from either a global field, a local variable
     slot or from a call to <code>getVariable()</code> /
     <code>getParameter()</code>. The chosen method depends is we're dealing with
     a parameter or a variable, a global or a local, an escaping variable or not.
     </p>

     <p>The XPath parser identifies all variable references and instanciates
     either a <code>VariableRef</code> or a <code>ParameterRef</code>. The XPath
     parser calls the parser's <code>lookupVariable</code> method in an initial
     attempt to find the variable/parameter instance. If that fails, it goes on
     to call the symbol table's <code>lookupName()</code> method. If that also
     fails this means that either:</p>

     <ul>
       <li>a variable or parameter with the given name does not exist</li>
       <li>the variable will be declared at a later stage
       (but within the same scope)</li>
     </ul>

     <p>The XPath parser creates an instance of the <code>UnresolvedRef</code>
     class. This class attempts to locate the variable after the whole AST has
     been built, when the <code>typeCheck()</code> method is called. If this
     fails an error is reported and the compilation stops. Otherwise the class
     creates a <code>VariableRef</code> or a <code>ParameterRef</code> instance
     and lets that handle the reference.</p>


     <p align="right" size="2">
 <a href="#content">(top)</a>
 </p>
 <h4>Forward references</h4>

     <p>XSLTC allows for forward references to global variables and parameters.
     You can even reference variables in not-yet included/imported stylesheets.
     In most cases, this is handled by changing the order of top-level elements.
     (Variables are placed first so that they are handled before any includes
     or imports).  But when a variable contains references to other variables,
     then this requires some extra code in the <code>Stylesheet</code> and
     <code>VariableBase</code> classes. The <code>VariableBase</code> has a
     method that returns a vector containing all variables that are referenced
     in the variable definition.</p>

     <blockquote class="source">
 <pre>
     &lt;xsl:variable name="C" select="$A &lt; $B"/&gt;
     &lt;xsl:variable name="A" select="1"/&gt;
     &lt;xsl:variable name="B" select="2"/&gt;</pre>
 </blockquote>

     <p>In this case, the <code>getDependencies()</code> method for variable
     C will return the variables A and B. The stylesheet has a method called
     <code>resolveReferences</code> that will order the variables accordingly
     so that the variable values are computed in the desired order. This method
     will issue an error message and terminate the compilation if there are
     circular variable/parameter dependencies.</p>


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	<div id="content">
	<h2><xsl:variable> / <xsl:param></h2>

	<p align="right" size="2">
	<a href="#content">(top)</a>
	</p>
	<h3>Contents</h3>
	<ul>
	<li>
	<a href="#functionality">Functionality</a>
	</li>
	<li>
	<a href="#implementation">Implementation</a>
	</li>
	</ul>


	<a name="functionality">‌</a>
	<p align="right" size="2">
	<a href="#content">(top)</a>
	</p>
	<h3>Functionality</h3>

	<p>Variables in XSLT are not really variables, as their values cannot be
	changed. They resemble constants from conventional programming languages. The
	only way in which a variable can be changed is by declaring it inside a
	for-each loop, in which case its value will be updated for every iteration.
	Top-level variables (variables that are direct child nodes of the
	<code><xsl:stylesheet></code> element) can never be changed.</p>

	<blockquote class="source">
	<pre>
	<xsl:for-each select="/foo/bar">
	<xsl:variable name="some-bar" select="."/>
	<xsl:value-of select="$some-bar"/>
	</xsl:for-each></pre>
	</blockquote>

	<p>Parameters are assigned a value either from the process that invoked
	the stylesheet (top-level parameter), or from a
	<code><xsl:with-param></code> or from a default value (in which case it
	behaves as if it was a variable).</p>

	<blockquote class="source">
	<pre>
	<xsl:template match="/">
	<xsl:call-template name="blob">
	<xsl:with-param name="par" select="'some-value'"/>
	</xsl:call-template>
	</xsl:template>

	<xsl:template name="blob">
	<xsl:param name="par" select="'default-value'"/>
	<xsl:value-of select="$param"/>
	</xsl:template></pre>
	</blockquote>



	<a name="implementation">‌</a>
	<p align="right" size="2">
	<a href="#content">(top)</a>
	</p>
	<h3>Implementation</h3>

	<p>Variables and parameters share a common base class
	<code>VariableBase</code> that contains a lot of common methods. This class
	handles both global and local variables/parameters.</p>

	<p align="right" size="2">
	<a href="#content">(top)</a>
	</p>
	<h4>Top-level parameters and variables</h4>

	<p>All top-level (ie. global) parameters and variables are stored inside
	fields in the translet class. Variables are stored as objects or basic
	data types (such as boolean, char, int, etc.) while parameters have to be
	"boxed" inside an object. This is because parameters are also stored as
	objects inside the translet. The <code>addParameter()</code> method of the
	<code>AbstractTranslet</code> class stores the parameter in a Hashtable
	(the Hashtable maps the parameter name to the parameter value). The
	"boxing" of the parameter's value is done by the class that handles the
	parameters type. This class is a subclass of
	<code>org.apache.xalan.xsltc.compiler.util.Type</code>.</p>

	<p>Note that all top-level parameters and variables from all imported and
	included stylesheets will be placed as direct children of the top-level
	stylesheet in the AST. This done to make global variables truly global and
	not just global in the stylesheet where it was declared.</p>



	<p align="right" size="2">
	<a href="#content">(top)</a>
	</p>
	<h4>Local parameters and variables</h4>

	<p>Local variables that are accessible from a given syntax tree node will
	first be put on the JVM's stack and stored in a local variable slot. This
	makes the variable or parameter accessible from all code within that
	method. But, in some special cases, the code that is compiled to handle an
	element/expression within the variable scope is not put inside the same
	method as the actual variable. This is the case for some predicates.
	All syntax-tree nodes implement the <code>isClosureBoundary()</code> method
	to indicate if its child an ancestor nodes will end up in a different method
	then itself. This method is used by the <code>Variable</code> and
	<code>Param</code> classes to determine if the variable or parameter will
	"escape" the variable frame.</p>

	<blockquote class="source">
	<pre>
	<xsl:for-each select="/foo/bar/baz">
	<xsl:variable name="pos" select="3"/>
	<xsl:apply-templates select="/foo/bar[$pos]"/>
	</xsl:for-each></pre>
	</blockquote>

	<p>The predicate in this stylesheet fragment is compiled into a separate
	auxiliary class that implements the <code>Filter</code> interface. It will
	therefore not have access to the variable "pos" in the current stack frame.
	A common technique for cases like this is to use a <b>"closure"</b>. A
	closure is a record that contains references to all variables that are in
	scope for a certain part of the compiled scope. This is done in a very
	simple manner in XSLTC. All variables or parameters that can "escape" the
	stack are passed to the translet via its <code>addVariable()</code> method.
	They can then later be retrieved by the <code>getVariable()</code> method.
	</p>

	<p>
	<b>Important note 1:</b> A predicate does not always result in a
	auxiliary class. In some cases we optimize the code by using tailored
	iterators and goodies like that instead. We may want to update the
	predicate code to check if an auxiliary class will be generated before
	returning true or false from the <code>isClosureBoundary()</code>
	method.</p>

	<p>
	<b>Important note 2:</b> There could be other closure boundaries
	that we have not yet discovered or considered. This could be, for instance,
	sort records and other auxiliary classes:</p>

	<blockquote class="source">
	<pre>
	<xsl:variable name="sort-order" select="'decending'"/>
	<xsl:for-each select="/foo/bar/baz">
	<xsl:sort select="@name" order="$sort-order"/>
	<xsl:value-of select="."/>
	</xsl:for-each></pre>
	</blockquote>

	<p>I would not be surprised if this fails. A fix could be to implement the
	<code>isClosureBoundary()</code> in the <code>Sort</code> class and have the
	method return 'true' in all cases.</p>



	<p align="right" size="2">
	<a href="#content">(top)</a>
	</p>
	<h4>Parameter and variable references</h4>

	<p>A parameter or variable reference does the oposite of a parameter or
	variable. The value is read from either a global field, a local variable
	slot or from a call to <code>getVariable()</code> /
	<code>getParameter()</code>. The chosen method depends is we're dealing with
	a parameter or a variable, a global or a local, an escaping variable or not.
	</p>

	<p>The XPath parser identifies all variable references and instanciates
	either a <code>VariableRef</code> or a <code>ParameterRef</code>. The XPath
	parser calls the parser's <code>lookupVariable</code> method in an initial
	attempt to find the variable/parameter instance. If that fails, it goes on
	to call the symbol table's <code>lookupName()</code> method. If that also
	fails this means that either:</p>

	<ul>
	<li>a variable or parameter with the given name does not exist</li>
	<li>the variable will be declared at a later stage
	(but within the same scope)</li>
	</ul>

	<p>The XPath parser creates an instance of the <code>UnresolvedRef</code>
	class. This class attempts to locate the variable after the whole AST has
	been built, when the <code>typeCheck()</code> method is called. If this
	fails an error is reported and the compilation stops. Otherwise the class
	creates a <code>VariableRef</code> or a <code>ParameterRef</code> instance
	and lets that handle the reference.</p>



	<p align="right" size="2">
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	<h4>Forward references</h4>

	<p>XSLTC allows for forward references to global variables and parameters.
	You can even reference variables in not-yet included/imported stylesheets.
	In most cases, this is handled by changing the order of top-level elements.
	(Variables are placed first so that they are handled before any includes
	or imports). But when a variable contains references to other variables,
	then this requires some extra code in the <code>Stylesheet</code> and
	<code>VariableBase</code> classes. The <code>VariableBase</code> has a
	method that returns a vector containing all variables that are referenced
	in the variable definition.</p>

	<blockquote class="source">
	<pre>
	<xsl:variable name="C" select="$A < $B"/>
	<xsl:variable name="A" select="1"/>
	<xsl:variable name="B" select="2"/></pre>
	</blockquote>

	<p>In this case, the <code>getDependencies()</code> method for variable
	C will return the variables A and B. The stylesheet has a method called
	<code>resolveReferences</code> that will order the variables accordingly
	so that the variable values are computed in the desired order. This method
	will issue an error message and terminate the compilation if there are
	circular variable/parameter dependencies.</p>





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