Saxon

Reading the spec about the streamability of the fold-left function at https://www.w3.org/TR/xslt-30/#streamability-fn-fold-left it suggests in my interpretation that the example given there (fold-left(/*/transaction, 0, function($x as xs:decimal, $y as xs:decimal) as xs:decimal {$x+$y}) should be streamable.

However, Saxon 9.8.0.12 EE and Saxon 9.9.1.1 EE do reject such an expression as not being streamable

Error on line 12 of sheet1-spec-example.xsl:
  XTSE3430: Template rule is not streamable
  * Operand {element()/transaction} of {fn:fold-left(...)} selects streamed nodes in a
  context that allows arbitrary navigation (line 13)
Template rule is not streamable
  * Operand {element()/transaction} of {fn:fold-left(...)} selects streamed nodes in a context that allows arbitrary navigation (line 13)

Looking at the test cases for streaming and fold-left in the test suite I find a comment in there (https://github.com/w3c/xslt30-test/blob/master/tests/strm/sf-fold-left/sf-fold-left-A.xsl)

TODO: fold-left is defined in the current XSLT 3.0 draft to have the operand usage N (Navigation) making it non-streamable. This should be fixed; it should have type-determined usage based on the type of the first argument of the function provided in the third argument, if this is known. In the meantime, we explicitly atomize the sequence to be processed

and test cases seem to use e.g. fold-left(/*/transaction/data(), 0, function($x as xs:decimal, $y as xs:decimal) as xs:decimal {$x+$y}).

As the final spec seems to have been fixed according to the "TODO" note it looks like the test cases and Saxon's implementation have not been adjusted.

Minimal test case I used:

<?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:math="http://www.w3.org/2005/xpath-functions/math"
    exclude-result-prefixes="xs math"
    version="3.0">
    
    <xsl:mode streamable="yes"/>
    
    <xsl:output method="text"/>
    
    <xsl:template match="/">
        <xsl:sequence select="fold-left(/*/transaction, 0, function($x as xs:decimal, $y as xs:decimal) as xs:decimal {$x+$y})"/>
    </xsl:template>
    
</xsl:stylesheet>