As of XSLT 2.0, and as-far-as-I-know-correct-me-if-I’m-wrong, there’s no native mechanism in the language for exception handling. (Update: although that’s still true, I should have looked at 2.1 or Saxon’s extension. Though I’m still going with this method because I don’t have 2.1 and I’m not using the EE version of Saxon.)
I have some stylesheets that attempt to do some processing on specified chunks of an input document, copying everything else unaltered. There are rare exceptional conditions that I can’t easily detect before I start producing output for a given chunk. These are rare enough that when I encounter them, all I want to do is cancel processing on this chunk and emit it unaltered. Some sort of exception handling is in order, but XSLT doesn’t help very much.
Here’s an example of the sort of scenario I’m talking about. Here’s an input document:
[xml]
<doc>
<block>some text, just copy.</block>
<!– the following table should have B substituted for a –>
<table>
<tr><td>a</td><td>b</td><td>c</td></tr>
<tr><td>b</td><td>a</td><td>c</td></tr>
<tr><td>b</td><td>c</td><td>a</td></tr>
</table>
<block>some more text, just copy.</block>
<!– the following table should be copied unaltered because of the presence of an x –>
<table>
<tr><td>a</td><td>b</td><td>c</td></tr>
<tr><td>b</td><td>a</td><td>x</td></tr>
<tr><td>b</td><td>c</td><td>a</td></tr>
</table>
</doc>
[/xml]
I want to look through each table and replace all cell values ‘a’ with ‘B’. However, if there’s an ‘x’ somewhere in the table, I want to just copy the table unmodified. I know that in this case, I could just do a tr/td[.='x'] test on the table to discover this condition. In the real case, though, it’s not so easy to test ahead of time for the condition.
Here’s some XSLT that doesn’t account for the exception:
[xslt]
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="table">
<xsl:copy>
<xsl:copy-of select="@*"/>
<xsl:apply-templates mode="inner"/>
</xsl:copy>
</xsl:template>
<xsl:template mode="inner" match="td">
<xsl:copy>
<xsl:copy-of select="@*"/>
<xsl:choose>
<xsl:when test=". = ‘a’">
<xsl:value-of select="’B’"/>
</xsl:when>
<xsl:otherwise>
<xsl:value-of select="."/>
</xsl:otherwise>
</xsl:choose>
</xsl:copy>
</xsl:template>
<xsl:template mode="inner" match="@*|node()" priority="-10">
<xsl:copy>
<xsl:apply-templates mode="inner" select="@*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="@*|node()" priority="-10">
<xsl:copy>
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
[/xslt]
The output of that is:
[xml]
<?xml version="1.0" encoding="UTF-8"?><doc>
<block>some text, just copy.</block>
<!– the following table should have B substituted for a –>
<table>
<tr><td>B</td><td>b</td><td>c</td></tr>
<tr><td>b</td><td>B</td><td>c</td></tr>
<tr><td>b</td><td>c</td><td>B</td></tr>
</table>
<block>some more text, just copy.</block>
<!– the following table should be copied unaltered because of the presence of an x –>
<table>
<tr><td>B</td><td>b</td><td>c</td></tr>
<tr><td>b</td><td>B</td><td>x</td></tr>
<tr><td>b</td><td>c</td><td>B</td></tr>
</table>
</doc>
[/xml]
(it did the substitutions in the second table, which I don’t want.)
My current solution is to do this:
- Emit each table into a variable instead of directly into the output
- If the exception occurs, emit an <EXCEPTION/> tag
- After each table is processed, look through the variable for the <EXCEPTION/> tag.
- If the exception happened, copy the original table, else copy the contents of the variable.
Here’s the modified code and output:
[xslt]
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="table">
<xsl:variable name="result">
<xsl:copy>
<xsl:copy-of select="@*"/>
<xsl:apply-templates mode="inner"/>
</xsl:copy>
</xsl:variable>
<xsl:choose>
<xsl:when test="$result//EXCEPTION">
<xsl:copy-of select="."/>
</xsl:when>
<xsl:otherwise>
<xsl:copy-of select="$result"/>
</xsl:otherwise>
</xsl:choose>
</xsl:template>
<xsl:template mode="inner" match="td">
<xsl:copy>
<xsl:copy-of select="@*"/>
<xsl:choose>
<xsl:when test=". = ‘a’">
<xsl:value-of select="’B’"/>
</xsl:when>
<xsl:when test=". = ‘x’">
<EXCEPTION/>
</xsl:when>
<xsl:otherwise>
<xsl:value-of select="."/>
</xsl:otherwise>
</xsl:choose>
</xsl:copy>
</xsl:template>
<xsl:template mode="inner" match="@*|node()" priority="-10">
<xsl:copy>
<xsl:apply-templates mode="inner" select="@*|node()"/>
</xsl:copy>
</xsl:template>
<xsl:template match="@*|node()" priority="-10">
<xsl:copy>
<xsl:apply-templates select="@*|node()"/>
</xsl:copy>
</xsl:template>
</xsl:stylesheet>
[/xslt]
[xml]
<?xml version="1.0" encoding="UTF-8"?><doc>
<block>some text, just copy.</block>
<!– the following table should have B substituted for a –>
<table>
<tr><td>B</td><td>b</td><td>c</td></tr>
<tr><td>b</td><td>B</td><td>c</td></tr>
<tr><td>b</td><td>c</td><td>B</td></tr>
</table>
<block>some more text, just copy.</block>
<!– the following table should be copied unaltered because of the presence of an x –>
<table>
<tr><td>a</td><td>b</td><td>c</td></tr>
<tr><td>b</td><td>a</td><td>x</td></tr>
<tr><td>b</td><td>c</td><td>a</td></tr>
</table>
</doc>
[/xml]
It works, but I’m still wondering if there’s a better approach…