The Doctor What

I just (re-)discovered this bit of stupidity in XPath/XSL. The equal operator will demote node-sets, result-trees, etc. into to strings when compared with a string.

Check out this example:

XML File:

<?xml version="1.0" encoding="utf-8"?>
<nodes>
  <node>one fish</node>
  <node>two fish</node>
  <node>red fish</node>
  <node>blue fish</node>
</nodes>

XSL File:

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet
    version   = "1.0"
    xmlns:xsl = "http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="xml" indent="yes"/>
  <xsl:template match="/">
    <red-fish>
      <xsl:value-of select="//node = 'red fish'"/>
    </red-fish>
  </xsl:template>
</xsl:stylesheet>

The output:

<?xml version="1.0"?>
<red-fish>true</red-fish>

This is ridiculous. How can you be sure that the equal operator is returning what you want if it silently promotes/demotes things?  Reading through the section on booleans in the XPath documentation explains how equality works:

If one object to be compared is a node-set and the other is a string, then the comparison will be true if and only if there is a node in the node-set such that the result of performing the comparison on the string-value of the node and the other string is true.

It’s almost like it was designed by people who don’t program… or maybe a comittee.

This is bad because equality tests will return true in unexpected places.  It also means that the designers of XPath could ignore things like set operations. A map(), reduce(), etc. would all be very handy.  As would some way to write functions.  EXSLT helps with this somewhat, but not much.

Ciao!