{"id":523,"date":"2010-09-23T16:17:28","date_gmt":"2010-09-23T22:17:28","guid":{"rendered":"http:\/\/www.ourada.org\/blog\/?p=523"},"modified":"2010-09-23T16:17:28","modified_gmt":"2010-09-23T22:17:28","slug":"xslt-exceptions","status":"publish","type":"post","link":"https:\/\/www.ourada.org\/blog\/archives\/523","title":{"rendered":"XSLT exceptions"},"content":{"rendered":"

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:<\/strong> although that’s still true, I should have looked at 2.1<\/a> or Saxon’s extension<\/a>. 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.)<\/p>\n

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.<\/p>\n

Here’s an example of the sort of scenario I’m talking about. Here’s an input document:
\n[xml]
\n<doc>
\n\t<block>some text, just copy.<\/block>
\n\t<!– the following table should have B substituted for a –>
\n\t<table>
\n\t\t<tr><td>a<\/td><td>b<\/td><td>c<\/td><\/tr>
\n\t\t<tr><td>b<\/td><td>a<\/td><td>c<\/td><\/tr>
\n\t\t<tr><td>b<\/td><td>c<\/td><td>a<\/td><\/tr>
\n\t<\/table>
\n\t<block>some more text, just copy.<\/block>
\n\t<!– the following table should be copied unaltered because of the presence of an x –>
\n\t<table>
\n\t\t<tr><td>a<\/td><td>b<\/td><td>c<\/td><\/tr>
\n\t\t<tr><td>b<\/td><td>a<\/td><td>x<\/td><\/tr>
\n\t\t<tr><td>b<\/td><td>c<\/td><td>a<\/td><\/tr>
\n\t<\/table>
\n<\/doc>
\n[\/xml]<\/p>\n

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']<\/code> 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.<\/p>\n

Here’s some XSLT that doesn’t account for the exception:
\n[xslt]
\n<?xml version="1.0" encoding="UTF-8"?>
\n<xsl:stylesheet version="2.0"
\n\txmlns:xsl="http:\/\/www.w3.org\/1999\/XSL\/Transform"><\/p>\n

\t<xsl:template match="table">
\n\t\t<xsl:copy>
\n\t\t\t<xsl:copy-of select="@*"\/>
\n\t\t\t<xsl:apply-templates mode="inner"\/>
\n\t\t<\/xsl:copy>
\n\t<\/xsl:template><\/p>\n

\t<xsl:template mode="inner" match="td">
\n\t\t<xsl:copy>
\n\t\t\t<xsl:copy-of select="@*"\/>
\n\t\t\t<xsl:choose>
\n\t\t\t\t<xsl:when test=". = ‘a’">
\n\t\t\t\t\t<xsl:value-of select="’B’"\/>
\n\t\t\t\t<\/xsl:when>
\n\t\t\t\t<xsl:otherwise>
\n\t\t\t\t\t<xsl:value-of select="."\/>
\n\t\t\t\t<\/xsl:otherwise>
\n\t\t\t<\/xsl:choose>
\n\t\t<\/xsl:copy>
\n\t<\/xsl:template><\/p>\n

\t<xsl:template mode="inner" match="@*|node()" priority="-10">
\n\t\t<xsl:copy>
\n\t\t\t<xsl:apply-templates mode="inner" select="@*|node()"\/>
\n\t\t<\/xsl:copy>
\n\t<\/xsl:template><\/p>\n

\t<xsl:template match="@*|node()" priority="-10">
\n\t\t<xsl:copy>
\n\t\t\t<xsl:apply-templates select="@*|node()"\/>
\n\t\t<\/xsl:copy>
\n\t<\/xsl:template>
\n<\/xsl:stylesheet>
\n[\/xslt]<\/p>\n

The output of that is:
\n[xml]
\n<?xml version="1.0" encoding="UTF-8"?><doc>
\n <block>some text, just copy.<\/block>
\n <!– the following table should have B substituted for a –>
\n <table>
\n <tr><td>B<\/td><td>b<\/td><td>c<\/td><\/tr>
\n <tr><td>b<\/td><td>B<\/td><td>c<\/td><\/tr>
\n <tr><td>b<\/td><td>c<\/td><td>B<\/td><\/tr>
\n <\/table>
\n <block>some more text, just copy.<\/block>
\n <!– the following table should be copied unaltered because of the presence of an x –>
\n <table>
\n <tr><td>B<\/td><td>b<\/td><td>c<\/td><\/tr>
\n <tr><td>b<\/td><td>B<\/td><td>x<\/td><\/tr>
\n <tr><td>b<\/td><td>c<\/td><td>B<\/td><\/tr>
\n <\/table>
\n<\/doc>
\n[\/xml]<\/p>\n

(it did the substitutions in the second table, which I don’t want.)<\/p>\n

My current solution is to do this:<\/p>\n

    \n
  1. Emit each table into a variable instead of directly into the output<\/li>\n
  2. If the exception occurs, emit an <EXCEPTION\/> tag<\/li>\n
  3. After each table is processed, look through the variable for the <EXCEPTION\/> tag.<\/li>\n
  4. If the exception happened, copy the original table, else copy the contents of the variable.<\/li>\n<\/ol>\n

    Here’s the modified code and output:
    \n[xslt]
    \n<?xml version="1.0" encoding="UTF-8"?>
    \n<xsl:stylesheet version="2.0"
    \n xmlns:xsl="http:\/\/www.w3.org\/1999\/XSL\/Transform"><\/p>\n

    <xsl:template match="table">
    \n <xsl:variable name="result">
    \n <xsl:copy>
    \n <xsl:copy-of select="@*"\/>
    \n <xsl:apply-templates mode="inner"\/>
    \n <\/xsl:copy>
    \n <\/xsl:variable>
    \n <xsl:choose>
    \n <xsl:when test="$result\/\/EXCEPTION">
    \n <xsl:copy-of select="."\/>
    \n <\/xsl:when>
    \n <xsl:otherwise>
    \n <xsl:copy-of select="$result"\/>
    \n <\/xsl:otherwise>
    \n <\/xsl:choose>
    \n <\/xsl:template><\/p>\n

    <xsl:template mode="inner" match="td">
    \n <xsl:copy>
    \n <xsl:copy-of select="@*"\/>
    \n <xsl:choose>
    \n <xsl:when test=". = ‘a’">
    \n <xsl:value-of select="’B’"\/>
    \n <\/xsl:when>
    \n <xsl:when test=". = ‘x’">
    \n <EXCEPTION\/>
    \n <\/xsl:when>
    \n <xsl:otherwise>
    \n <xsl:value-of select="."\/>
    \n <\/xsl:otherwise>
    \n <\/xsl:choose>
    \n <\/xsl:copy>
    \n <\/xsl:template><\/p>\n

    <xsl:template mode="inner" match="@*|node()" priority="-10">
    \n <xsl:copy>
    \n <xsl:apply-templates mode="inner" select="@*|node()"\/>
    \n <\/xsl:copy>
    \n <\/xsl:template><\/p>\n

    <xsl:template match="@*|node()" priority="-10">
    \n <xsl:copy>
    \n <xsl:apply-templates select="@*|node()"\/>
    \n <\/xsl:copy>
    \n <\/xsl:template>
    \n<\/xsl:stylesheet>
    \n[\/xslt]<\/p>\n

    [xml]
    \n<?xml version="1.0" encoding="UTF-8"?><doc>
    \n <block>some text, just copy.<\/block>
    \n <!– the following table should have B substituted for a –>
    \n <table>
    \n <tr><td>B<\/td><td>b<\/td><td>c<\/td><\/tr>
    \n <tr><td>b<\/td><td>B<\/td><td>c<\/td><\/tr>
    \n <tr><td>b<\/td><td>c<\/td><td>B<\/td><\/tr>
    \n <\/table>
    \n <block>some more text, just copy.<\/block>
    \n <!– the following table should be copied unaltered because of the presence of an x –>
    \n <table>
    \n <tr><td>a<\/td><td>b<\/td><td>c<\/td><\/tr>
    \n <tr><td>b<\/td><td>a<\/td><td>x<\/td><\/tr>
    \n <tr><td>b<\/td><td>c<\/td><td>a<\/td><\/tr>
    \n <\/table>
    \n<\/doc>
    \n[\/xml]<\/p>\n

    It works, but I’m still wondering if there’s a better approach…<\/p>\n","protected":false},"excerpt":{"rendered":"

    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 […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[3],"tags":[],"class_list":["post-523","post","type-post","status-publish","format-standard","hentry","category-general","author-admin"],"_links":{"self":[{"href":"https:\/\/www.ourada.org\/blog\/wp-json\/wp\/v2\/posts\/523","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.ourada.org\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.ourada.org\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.ourada.org\/blog\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.ourada.org\/blog\/wp-json\/wp\/v2\/comments?post=523"}],"version-history":[{"count":0,"href":"https:\/\/www.ourada.org\/blog\/wp-json\/wp\/v2\/posts\/523\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.ourada.org\/blog\/wp-json\/wp\/v2\/media?parent=523"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.ourada.org\/blog\/wp-json\/wp\/v2\/categories?post=523"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.ourada.org\/blog\/wp-json\/wp\/v2\/tags?post=523"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}