XSLT - Selecting unique elements when multiple files - xslt

Question

I can not figure out how to make this work using two files with xsltproc. cooking.xml is opened using document() and menu.xml is passed in on the command line. I can select the recipes without issue, what I can not figure out is how to get a unique list of ingredients. When I use the preceding-sibling function on my list of ingredients it behaves like this {[shell, beef, lettuce, tomato, cheese], [eggs, cheese]}. Why does a select like "cooking/recipe[#name = $menu]/ingredients" create a disjoint set that I can't use preceding-sibling on?
This is a contrived example from a larger system.
File cooking.xml
<?xml version="1.0" encoding="UTF-8"?>
<cooking xmlns="https://cooking.com/2022/cooking">
<recipe name="tacos">
<ingredient name="shell"/>
<ingredient name="beef"/>
<ingredient name="lettuce"/>
<ingredient name="tomato"/>
<ingredient name="cheese"/>
</recipe>
<recipe name="hamburger">
<ingredient name="bun"/>
<ingredient name="beef"/>
<ingredient name="lettuce"/>
<ingredient name="tomato"/>
</recipe>
<recipe name="omelet">
<ingredient name="eggs"/>
<ingredient name="cheese"/>
</recipe>
<recipe name="soup">
<ingredient name="chicken"/>
<ingredient name="stock"/>
</recipe>
</cooking>
File menu.xml
<?xml version="1.0" encoding="UTF-8"?>
<cooking xmlns="https://cooking.com/2022/cooking">
<recipe name="tacos"/>
<recipe name="omelet"/>
</cooking>
File shop.xsl
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:set="http://exslt.org/sets"
xmlns:cook="https://cooking.com/2022/cooking"
extension-element-prefixes="set">
<xsl:output method="xml" encoding="UTF-8" indent="yes"/>
<xsl:key name="rcp" match="recipe" use="#name" />
<xsl:template match="cooking">
<output>
<xsl:variable name="menu" select="recipe/#name" />
<!-- switch context to target document in order to use key -->
<xsl:for-each select="document('cooking.xml')">
<xsl:for-each select="set:distinct(key('rcp', $menu)/ingredient/#name)">
<ingredient name="{.}"/>
</xsl:for-each>
</xsl:for-each>
</output>
</xsl:template>
</xsl:stylesheet>
xsltproc shop.xsl menu.xml >ingredients.xml
<?xml version="1.0" encoding="UTF-8"?>
<output xmlns:cook="https://cooking.com/2022/cooking"/>
Desired output:
<?xml version="1.0" encoding="UTF-8"?>
<cooking xmlns:cook="https://cooking.com/2022/cooking">
<ingredient name="shell"/>
<ingredient name="beef"/>
<ingredient name="lettuce"/>
<ingredient name="tomato"/>
<ingredient name="cheese"/>
<ingredient name="eggs"/>
</cooking>

Answer 1

Why does a select like "cooking/recipe[#name = $menu]/ingredients" create a disjoint set that I can't use preceding-sibling on?
Because the preceding-sibling axis is evaluated in the context of the input tree, not in the context of your selection. It would be different if you had copied your selection to a variable.
Either way, using the preceding-sibling axis is not a good method to select unique values. If you're using xsltproc (i.e. libxslt) then you could do something like:
XSLT 1.0 + EXSLT set:distinct-values()
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:set="http://exslt.org/sets"
extension-element-prefixes="set">
<xsl:output method="xml" encoding="UTF-8" indent="yes"/>
<xsl:template match="/cooking">
<output>
<xsl:for-each select="set:distinct(document('cooking.xml')/cooking/recipe[#name = current()/recipe/#name]/ingredient/#name)">
<ingredient name="{.}"/>
</xsl:for-each>
</output>
</xsl:template>
</xsl:stylesheet>
or perhaps a bit more efficiently:
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:set="http://exslt.org/sets"
extension-element-prefixes="set">
<xsl:output method="xml" encoding="UTF-8" indent="yes"/>
<xsl:key name="rcp" match="recipe" use="#name" />
<xsl:template match="/cooking">
<output>
<xsl:variable name="menu" select="recipe/#name" />
<!-- switch context to target document in order to use key -->
<xsl:for-each select="document('cooking.xml')">
<xsl:for-each select="set:distinct(key('rcp', $menu)/ingredient/#name)">
<ingredient name="{.}"/>
</xsl:for-each>
</xsl:for-each>
</output>
</xsl:template>
</xsl:stylesheet>
to get:
Result
<?xml version="1.0" encoding="UTF-8"?>
<output>
<ingredient name="shell"/>
<ingredient name="beef"/>
<ingredient name="lettuce"/>
<ingredient name="tomato"/>
<ingredient name="cheese"/>
<ingredient name="eggs"/>
</output>
But for this cooking.xml must a well-formed XML document.