hello world xml -> json template - xslt-3.0

Question

I'm trying to transform an xml input to a json output. My XSLT 1.0 is pretty proficient my XSLT 2.0/3.0 not so.
I thought I'd start with a hello world style template and build from there.
My belief is that you can simply create an output as map/array data structure and then some magic will map that into the desired output, so this is my first attempt (I've not defined an input, because any old xml will do in this example, it ignores it):
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
exclude-result-prefixes="xs"
version="3.0">
<xsl:output method="json" encoding="UTF-8" indent="yes"/>
<xsl:template match="/">
<xsl:variable name="foo">
<map xmlns="http://www.w3.org/2005/xpath-functions">
<string key='desc'>Distances between several cities, in kilometers.</string>
<string key='updated'>2014-02-04T18:50:45</string>
<boolean key="uptodate">true</boolean>
<null key="author"/>
<map key='cities'>
<array key="Brussels">
<map>
<string key="to">London</string>
<number key="distance">322</number>
</map>
<map>
<string key="to">Paris</string>
<number key="distance">265</number>
</map>
<map>
<string key="to">Amsterdam</string>
<number key="distance">173</number>
</map>
</array>
</map>
</map>
</xsl:variable>
<xsl:value-of select="xml-to-json($foo)"/>
</xsl:template>
</xsl:stylesheet>
this almost works but I get a string output...(the '"' chars exist in the output file includeing all the escaping, so not a valid json output).
"{\"desc\":\"Distances between several cities, in kilometers.\",\"updated\":\"2014-02-04T18:50:45\",\"uptodate\":true,\"author\":null,\"cities\":{\"Brussels\":[{\"to\":\"London\",\"distance\":322},{\"to\":\"Paris\",\"distance\":265},{\"to\":\"Amsterdam\",\"distance\":173}]}}"
If there are any basic guides to do this, then please let me know, the web is awash with odd examples, out of date instruction based on XSLT 1.0/2.0 or hard to understand pdfs discussing more in depth scenarios.

Answer 1

The function you use already gives you a string with the JSON (see https://www.w3.org/TR/xpath-functions-31/#func-xml-to-json) so if you want to write that to a file just use <xsl:output method="text"/>.
The json output method mainly makes sense if you construct XDM/XPath 3.1 maps/arrays and want to serialize them as JSON.
For your sample I would also use <xsl:template name="xsl:initial-template"> instead of <xsl:template match="/">, then you don't need to provide any dummy input XML at all but can just start with that default named template using e.g. -it from the command line or callTemplate(null, ..) from the API.