Performance observation.

The goal is to create a namespace node if the value of xsi:type is a QName.

The code here simulates the situation

https://xsltfiddle.liberty-development.net/a9HjZj

The problem is that this fails if the select evaluates to the empty string.

 <xsl:template match="@xsi:type">
         <xsl:namespace name="{substring-before(.,':')}"
select="$schemas/thing/@targetNamespace"/>
         <xsl:next-match/>
   </xsl:template>

Now in the real code the select is actually a key retrieval so looks more
like this

   <xsl:template match="@xsi:type">
         <xsl:namespace name="{substring-before(.,':')}"
select="key('myKey',.,$schemas)/@targetNamespace"/>
         <xsl:next-match/>
   </xsl:template>

So in order to avoid failure I tried this

     <xsl:template match="@xsi:type[
key('myKey',.,$schemas)/@targetNamespace => normalize-space()]">
         <xsl:namespace name="{substring-before(.,':')}"
select="key('myKey',.,$schemas)/@targetNamespace"/>
         <xsl:next-match/>
   </xsl:template>

now that works but quadrupled the execution time of the stylesheet.

Doing it with try catch

      <xsl:template match="@xsi:type">
         <xsl:try>
             <xsl:namespace name="{substring-before(.,':')}"
select="key('myKey',.,$schemas)/@targetNamespace"/>
             <xsl:catch/>
         </xsl:try>
         <xsl:next-match/>
   </xsl:template>

is a teenie bit quicker.

Overall execution time is still tolerable so posting this as an observation.

For the  XSL list

Is there a way to code

   <xsl:template match="@xsi:type[ key('myKey',.,$schemas)/@targetNamespace
=> normalize-space()]">
         <xsl:namespace name="{substring-before(.,':')}"
select="key('myKey',.,$schemas)/@targetNamespace"/>
         <xsl:next-match/>
   </xsl:template>

without repeating the code for the key access thats on the template rule
predicate inside the template rule.

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