passing a result set from a user defined function into the max() function - max

Question

first time poster long time browser to bear with me if I'm not clear. I'm quite new to xslt.
I'm trying to write a function which passes a list of cleansed date values to the max() function. Following is my input document:
<dates>
<date>1990-09-02Z</date>
<date>1990-09-03Z</date>
<date>1990-09-04Z</date>
<date>1990-09-05Z</date>
<date>1990-09-06Z</date>
</dates>
As you can see, the string values have a trailing 'Z'. If I try to pass these directly to max() using a nested substring() function
<xsl:template match="/dates">
<xsl:value-of select="max(xs:date(substring(//date,1,10)))"/>
</xsl:template>
I get this error:
A sequence of more than one item is not allowed as the first argument of fn:substring() ("1990-09-02Z", "1990-09-03Z")
so I've included an xsl:function declaration into my stylesheet which now looks like this:
<?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"
xmlns:test="http://www.blah.blah/funct"
version="3.0">
<xsl:function name="test:funct" visibility="public">
<xsl:param name="input"/>
<xsl:sequence>
<xsl:for-each select="$input">
<xsl:value-of select="xs:date(substring(.,1,10))"/>
</xsl:for-each>
</xsl:sequence>
</xsl:function>
<xsl:template match="/dates">
<xsl:value-of select="max(test:funct(//date))"/>
</xsl:template>
</xsl:stylesheet>
However, now I'm getting the following error
Failure converting {1990-09-02} to a number
I thought max() could handle dates? I'm quite confused about what's being passed into the max() function and why it's not working. the output I'm looking for is 1990-09-06
I try to read the w3org specification docs but the terms are too technical for me so not making sense of it. Appreciate any help you can offer.
By the way, processing engine I'm using is Saxon-PE 9.8.0.12
edit: my ultimate goal is to have a stylesheet with a list of functions which I can include within other xsl stylesheets, so ultimately the solution has to be a function. In this specific case a function which produces a list of cleansed dates which can then be passed to max().

Answer 1

As you have tagged that as XSLT 3, I would suggest to start with basic XPath 2/3 where you can simply write
//date/xs:date(substring(., 1, 10))
i.e. you can use function calls in the last step of your path to extract the substring and construct an xs:date: https://xsltfiddle.liberty-development.net/6rexjii
So that expression //date/xs:date(substring(., 1, 10)) gives you a sequence of xs:date values, you can then use the max function on them:
max(//date/xs:date(substring(., 1, 10)))
https://xsltfiddle.liberty-development.net/6rexjii/1
As for writing a user-defined function to have that last step done, I would write a function where the input is an xs:string and which returns an xs:date:
<xsl:function name="mf:date" as="xs:date">
<xsl:param name="input" as="xs:string"/>
<xsl:sequence select="xs:date(substring($input, 1, 10))"/>
</xsl:function>
Then you can call it as max(//date/mf:date(.)): https://xsltfiddle.liberty-development.net/6rexjii/2
If you really wanted to write a function to process a sequence of input items to return a sequence of xs:dates then use
<xsl:function name="mf:dates" as="xs:date*">
<xsl:param name="input" as="xs:string*"/>
<xsl:sequence select="$input ! xs:date(substring(., 1, 10))"/>
</xsl:function>
and call it with
<xsl:value-of select="max(mf:dates(//date))"/>
https://xsltfiddle.liberty-development.net/6rexjii/3
As a syntax alternative, in XPath 3.1 you can use the arrow operator =>:
<xsl:value-of select="//date => mf:dates() => max()"/>
https://xsltfiddle.liberty-development.net/6rexjii/4