Duplicates in a map - xslt

Question

I currently have an XSLT function that loads key=value pairs from a text file into a map.
<xsl:function name="myns:loadMapping" as="map(*)">
<xsl:variable name="mapping" as="map(xs:string, xs:string)">
<xsl:map>
<xsl:for-each select="unparsed-text-lines($inputFile,$fileEncoding)">
<!-- Takes only lines which are in the form abc=xyz and are not comments (does not start with #) -->
<xsl:if test="contains(.,'=') and not(starts-with(.,'#'))">
<xsl:map-entry key="substring-before(.,'=')" select="substring-after(.,'=')"/>
</xsl:if>
</xsl:for-each>
</xsl:map>
</xsl:variable>
<xsl:sequence select="$mapping"/>
</xsl:function>
The function works fine unless the user tries to load a file containing duplicates, in which case the XSLT transform fails with an error (expected behaviour):
Error evaluating (map:merge(...)) on line xyz column xy of xyz.xsl:
XTDE3365: Duplicate key in constructed map: {keyInError}
Is there a way I could catch this case and keep the transformation from aborting, something like this :
<xsl:function name="myns:loadMapping" as="map(*)">
<xsl:variable name="mapping" as="map(xs:string, xs:string)">
<xsl:map>
<xsl:for-each select="unparsed-text-lines($inputFile,$fileEncoding)">
<!-- Takes only lines which are in the form abc=xyz and are not comments (does not start with #) -->
<xsl:if test="contains(.,'=') and not(starts-with(.,'#'))">
<xsl:choose>
<xsl:when test="...map contains key...">
<xsl:message>Map already contains key. Please check input file.</xsl:message>
</xsl:when>
<xsl:otherwise>
<xsl:map-entry key="substring-before(.,'=')" select="substring-after(.,'=')"/>
</xsl:otherwise>
</xsl:choose>
</xsl:if>
</xsl:for-each>
</xsl:map>
</xsl:variable>
<xsl:sequence select="$mapping"/>
</xsl:function>
I see that there is something implemented for a future XSLT 4.0 release (Saxon - Controlling duplicates on xsl:map) but I would like to stick to XSLT 3.0 for the time being.
Thanks.

Answer 1

To add to Martin Honnen's suggestions, you could use xsl:iterate instead of xsl:for-each, passing the map as a parameter, which would allow you to inspect the map before adding another entry to it.
<xsl:iterate select="...">
<xsl:param name="map" select="map{}"/>
<xsl:choose>
<xsl:when test="map:contains($map, ...)">...</xsl:when>
<xsl:otherwise>
<xsl:next-iteration>
<xsl:with-param name="map" select="map:put($map, ..., ...)"/>

Answer 2

Well, both map:merge in XPath 3.1 or of course grouping with e.g.
<xsl:for-each-group select="unparsed-text-lines($inputFile,$fileEncoding)[contains(.,'=') and not(starts-with(.,'#'))]" group-by="substring-before(., '=')">
<xsl:map-entry key="current-grouping-key()" select="substring-after(., '=')"/>
<xsl:if test="current-group()[2]">
<xsl:message>..</xsl:message>
</xsl:if>
</xsl:for-each-group>
allow you more control than your approach without having to wait for XSLT 4 or trying to use experimental extensions.