I worked on a similar implementation for calling SQL stored procedures. To
do that I had to support mutiple output parameters as well as multiple
inputs. I think this is useful functionality to have in general...

First, using unabbreviated form and seriously pumping up xsl:call-template
syntax (I am sorry to admit that I lost track of whether the function
proposal is still being considered on top of xsl:call-template, or as a
separate entity)

<xsl:template match="*">

  <xsl:bound-variable name="ret"/>
  <xsl:call-function name="f">
   <xsl:with-param name="input1">5</xsl:with-param>
   <xsl:bind-output-variable name="output1" bound-name="r"/>
  </xsl:call-function>

  <xsl:value-of select="$ret"/>

</xsl:template>

<xsl:function name="f">
  <xsl:param name="input1"/>
  <xsl:output-variable name="output1">
    <xsl:value-of select="$input1"/>
  </xsl:output-variable>
</xsl:template>

I'm sure better naming conventions could be chosen, but I hope the idea is
clear.

In abbreviated form, someone suggested var := value syntax for input
parameters; I think this is a great idea, and for output parameters, var =:
bound-name makes sense...

xf:call('f', 'input1 := 5', 'ret =: output1')

For return value, in my simple case (SQL cannot return nodesets), I just
took one of the output variables and said I'll use it as return value
(another , but I don't have a preference on how to do it...

Just a thought.

- Eugene

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list