(14) Why Variables?

• Reuse of values in different locations
  • texts required for the transformation
  • facilitates better separation of structure and content

  <xsl:value-of select="$email-prefix"/> <!- - $email-prefix = 'You have ' - ->
  <xsl:value-of select="count(//message)"/>
  <xsl:value-of select="$email-suffix"/> <!- - $email-suffix = ' e-mail messages.' - ->

• Using the correct context is essential
  • variables cannot be updated
  • if they need to be updated, they have to be re-created
• Why are they called variables if they are constants?
  • their value varies in different invocations of the context
  • they are computed at runtime (dynamic constants) rather than statically

(15) Scope and Extent

• Variables can be global or local
  • global variables are visible in all templates
  • local variables are visible in their context (i.e., at following-sibling::*/descendant-or-self::*)
  • local variables are allowed to shadow global (not local) variables
• Variable values may be assigned using the select attribute
  • The XPath's result is the value of the variable
• Variables can contain arbitrary XPath code
  • the code is executed in the same way as when constructing the result tree
  • the result tree fragment is the value of the variable
  • it can be used as a string (value-of) or as a tree (copy-of)

(16) Using Variables

 <xsl:variable name="name" select="'value'"/>
 <xsl:template match="/">
  <xsl:choose>
   <xsl:when test="$name = 'value'">
    <xsl:variable name="result" select="'ok'"/>
   </xsl:when>
   <xsl:otherwise>
    <xsl:variable name="result" select="'problem'"/>
   </xsl:otherwise>
  </xsl:choose>
  <xsl:value-of select="$result"/>
 </xsl:template>

 <xsl:variable name="name" select="'value'"/>
 <xsl:template match="/">
  <xsl:variable name="result">
   <xsl:choose>
    <xsl:when test="$name = 'value'">
     <xsl:value-of select="'ok'"/>
    </xsl:when>
    <xsl:otherwise>
     <xsl:value-of select="'problem'"/>
    </xsl:otherwise>
   </xsl:choose>
  </xsl:variable>
  <xsl:value-of select="$result"/>
 </xsl:template>

(18) Parameters vs. Variables

• Parameters are variables with additional semantics
  • they are passed to their scope from the outside
  • they are available within the scope like a variable (scopes are stylesheets and templates)
  • like variables, they cannot be updated (and only global parameters can be shadowed)
• XSLT does not check proper parameter passing
  • if a declared parameter is not passed, it gets a default value (specified or '')
  • if a passed parameter is not declared, it is ignored
  • like variables, parameters have no type (any value can be passed)
  • XSLT's robustness makes it hard to spot programming errors

(19) Stylesheet Parameters

• Passed to the stylesheet when calling the stylesheet
  • the exact way of specifying the parameters depend on the processor and the environment
  • the passed values are available in the same way as global variables
  • parameter checking has to be done by hand

 <xsl:param name="number" select="0"/>
 <xsl:variable name="number-param">
  <xsl:if test="string(number($number)) = 'NaN'">
   <xsl:message terminate="yes">"number" must be a proper number!</xsl:message>
  </xsl:if>
  <xsl:value-of select="$number"/>
 </xsl:variable>
 <xsl:template match="/">
  <xsl:value-of select="10 * $number-param"/>
 </xsl:template>

(20) Template Parameters

• Parameters can be passed to templates
  • works with apply-templates and call-template
  • with-param elements list the passed parameters
  • parameter matching is done by name (there is no particular order to parameters)
• Templates can be programmed as parametrized components
  • checking the signature has to be done by hand
  • with-param elements list the passed parameters
  • parameter matching is done by name (there is no particular order to parameters)
• Parametrized template calls need a lot of markup
  • XSLT's XML syntax makes the code hard to read

main param start = 1 ; param count = 10 ; {
	loop (0) };
loop param counter ; {
	print $start + $counter ;
	if ( $counter < $count - 1) then 
		loop ($counter + 1) ; }

(21) Parameter Passing

 <xsl:param name="start" select="1"/>
 <xsl:param name="count" select="10"/>
 <xsl:template match="/">
  <xsl:call-template name="loop">
   <xsl:with-param name="counter" select="0"/>
  </xsl:call-template>
 </xsl:template>
 <xsl:template name="loop">
  <xsl:param name="counter"/>
  <xsl:message>
   <xsl:value-of select="$start + $counter"/>
  </xsl:message>
  <xsl:if test="$counter &lt; $count - 1">
   <xsl:call-template name="loop">
    <xsl:with-param name="counter" select="$counter + 1"/>
   </xsl:call-template>
  </xsl:if>
 </xsl:template>

(22) Message Facility

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:param name="message" select="'plain'"/>
    <!-- messaging style: 'silent', 'plain', or 'prefixed' -->
    <xsl:template name="message">
        <xsl:param name="level" select="'fatal'"/>
        <!-- message level: 'warning', 'error', or 'fatal' -->
        <xsl:param name="text" select="'no text given'"/>
        <xsl:choose>
            <xsl:when test="$message = 'silent'"/>
            <xsl:when test="$level = 'warning'">
                <xsl:call-template name="print_message">
                    <xsl:with-param name="level" select="'warning'"/>
                    <xsl:with-param name="terminate" select="'no'"/>
                    <xsl:with-param name="text" select="$text"/>
                </xsl:call-template>
            </xsl:when>
            <xsl:when test="$level = 'error'">
                <xsl:call-template name="print_message">
                    <xsl:with-param name="level" select="'error'"/>
                    <xsl:with-param name="terminate" select="'no'"/>
                    <xsl:with-param name="text" select="$text"/>
                </xsl:call-template>
            </xsl:when>
            <xsl:when test="$level = 'fatal'">
                <xsl:call-template name="print_message">
                    <xsl:with-param name="level" select="'fatal'"/>
                    <xsl:with-param name="terminate" select="'yes'"/>
                    <xsl:with-param name="text" select="$text"/>
                </xsl:call-template>
            </xsl:when>
            <xsl:otherwise>
                <xsl:call-template name="print_message">
                    <xsl:with-param name="level" select="'fatal'"/>
                    <xsl:with-param name="terminate" select="'no'"/>
                    <xsl:with-param name="text" select="concat('fatal:unknown level for message [', $text, ']')"/>
                </xsl:call-template>
            </xsl:otherwise>
        </xsl:choose>
    </xsl:template>
    <xsl:template name="print_message">
        <!-- this template should never be called from outside; for printing messages, call the "message" template. -->
        <xsl:param name="level"/>
        <xsl:param name="terminate"/>
        <xsl:param name="text"/>
        <xsl:choose>
            <xsl:when test="$message = 'plain'">
                <xsl:message terminate="{$terminate}">
                    <xsl:value-of select="$text"/>
                </xsl:message>
            </xsl:when>
            <xsl:otherwise>
                <xsl:message terminate="{$terminate}">
                    <xsl:value-of select="concat($level, ':', $text)"/>
                </xsl:message>
            </xsl:otherwise>
        </xsl:choose>
    </xsl:template>
</xsl:stylesheet>