Thank you for your as-always clear explanation Michael.

I work more and more with strong datatype in XSLT2, which make the code stronger, I'll do always declare a datatype pour stylesheet params.. and as I always use Saxon I know how it behaves now.
Really usefull tip for boolean parameters !

After testing this, let me just add that the if the supply parameter cannot be casted to the declared type, saxon raise an error.
I don't know how other xslt processors behaves in regard to this point, and sorry that my question is a bit "implementation dependant".

  <xsl:variable name="arg-used" as="xs:whatever?"
                select="if( $arg castable as xs:whatever )
                        then xs:whatever( $arg ) else ()"/>

Thank you for your response Ken, but as said Michael my question was about "always supplied parameters".

But this make me think about another question : if no parameters is supplied, I'm use to test it againts empty string with
<xsl:if test="$foo=''"/>
and it usually works.
Is an empty sequence equal to an emty string?
(maybe the equal operator tries to cast both part to the same type ?

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