Implementing product() in XSLT - xslt

Question

XPath 2.0 (and 3.0) has convenient aggregate functions sum() and avg(), but nothing that returns the product of a sequence of numeric atomic values.
Although implementing such a function is trivial in a language that allows assignment statements, it seems not to be that easy in XSLT. The only way I've found to get an aggregate product is to use recursion:
<xsl:function name="fn:products">
<xsl:param name="input" />
<xsl:param name="total" />
<xsl:if test="exists($input)">
<xsl:variable name="x" select="$input[1] * $total"/>
<xsl:sequence select="$x"/>
<xsl:sequence select="fn:products(subsequence($input,2),$x)"/>
</xsl:if>
</xsl:function>
But the above is actually my adaptation of a function that appears on p.994 of Michael Kay's book XSLT 2.0 and XPath 2.0 4th Edition. The original function outputs a sequence of running balances by recursively adding a sequence of numbers.
My version just multiplies the numbers, instead of adding them. The result is then a sequence of "running products". For example, fn:products((4, 0.75, 10, 0.7), 1) returns the sequence 4, 3, 30, 21.
Since I'm only interested in the product of all numbers, I just take the last item of the output with a filter expression: fn:products((4, 0.75, 10, 0.7), 1)[last()].
It works. But just for the sake of making good code, I wonder if there is a way to make the function to just directly return the last item as the aggregate product (i.e. just get 21, in the example above).
I tried a couple of things, but they just broke the function. Is there a way to achieve this? Is it possible also to implement it in a better, leaner way (the original function was not meant to return a singleton sequence)?

Answer 1

If you use one of the fancy processors, you can use dyn:evaluate.
<?xml version="1.0" encoding="UTF-8"?>
<test>
<num>4</num>
<num>0.75</num>
<num>10</num>
<num>0.7</num>
</test>
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:dyn="http://exslt.org/dyn/dyn.xml"
extension-element-prefixes="dyn"
>
<xsl:template match="/">
<xsl:variable name="run">
<xsl:for-each select="test/num"><xsl:value-of select="."/><xsl:if test="position() != last()"> * </xsl:if></xsl:for-each>
</xsl:variable>
<xsl:value-of select="dyn:evaluate(string($run))"/>
</xsl:template >
</xsl:stylesheet>