pattern for generating JSON output - xslt-3.0

Question

I'm trying to map XML to JSON using XSLT 3.0
my broad plan is to take the input, map it to some elements in memory, and then map that to 'map's and 'array's to by applying templates and then letting the XSLT serialise that as JSON.
Here is my initial effort:
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
exclude-result-prefixes="xs"
xmlns:map="http://www.w3.org/2005/xpath-functions/map"
version="3.0">
<xsl:output method="json" encoding="UTF-8" indent="yes"/>
<xsl:template match="/">
<xsl:variable name="dsl" as="element()">
<epg lastBuildDate="10/4/2019 9:46:00 AM">
</epg>
</xsl:variable>
<xsl:variable name="output">
<xsl:apply-templates select="$dsl" mode="interpret"/>
</xsl:variable>
<xsl:sequence select="$output"/>
</xsl:template>
<xsl:template match="epg" mode="interpret">
<xsl:sequence select="map {
'lastBuildDate' : #lastBuildDate
}"/>
</xsl:template>
</xsl:stylesheet>
sadly I get
Cannot add a map to an XDM node tree
in the 'interpret' template.

Answer 1

The error occurs with
<xsl:variable name="output">
<xsl:apply-templates select="$dsl" mode="interpret"/>
</xsl:variable>
as that is an implicit use of
<xsl:variable name="output">
<xsl:document>
<xsl:apply-templates select="$dsl" mode="interpret"/>
</xsl:document>
</xsl:variable>
While maps can contain nodes as values, nodes can't contain maps.
For that simple example it should work to use
<xsl:variable name="output" as="item()*">
<xsl:apply-templates select="$dsl" mode="interpret"/>
</xsl:variable>
or even
<xsl:variable name="output" as="item()">
<xsl:apply-templates select="$dsl" mode="interpret"/>
</xsl:variable>
or
<xsl:variable name="output" as="map(*)">
<xsl:apply-templates select="$dsl" mode="interpret"/>
</xsl:variable>

Answer 2

this works, I'll mark the other answer as correct though there is a 2nd issue where you have to 'cast' the attribute to a string
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
exclude-result-prefixes="xs"
xmlns:map="http://www.w3.org/2005/xpath-functions/map"
version="3.0">
<xsl:output method="json" encoding="UTF-8" indent="yes"/>
<xsl:template match="/">
<xsl:variable name="dsl" as="element()">
<epg lastBuildDate="10/4/2019 9:46:00 AM">
</epg>
</xsl:variable>
<xsl:variable name="output" as="map(*)*">
<xsl:apply-templates select="$dsl" mode="interpret"/>
</xsl:variable>
<xsl:sequence select="$output"/>
</xsl:template>
<xsl:template match="epg" as="map(*)" mode="interpret">
<xsl:sequence select="map {
'lastBuildDate' : #lastBuildDate => xs:string()
}"/>
</xsl:template>
</xsl:stylesheet>