[xsl] Splitting an +attribute+ string

[ben senior <ben@xxxxxxxxxxxxx>]

I wonder if anybody could quickly profer some advice on this.
I may well be wrong, but does xsl treat attribute strings different to 
normal text nodes when it comes to the "contains()" function. I may well 
also be gibbering. I've got my mind into a muddle on this one.

Problem :  To take an attribute in the form of a comma delineated string 
(e.g. <gui onShow="apple,pear,banana" onHide="black,white,grey"/>) and 
to covert the attributes into javascript array syntax (i.e. 
['apple','pear','banana'] etc.).

Strategy : "Just" replace the commas with ',' and top and tail the 
output string with [' and ']

Code  :
<!--To convert onShow-->
<xsl:call-template name="commaDelineate"><xsl:with-param name="str" 
select="@onShow"/></xsl:call-template>

<xsl:template name="commaDelineate">
<xsl:param name="str"/>
   <xsl:when test="contains($str,',')">
     <xsl:value-of select="substring-before($str,',')"/>','
     <xsl:call-template name="commaDelineate">
       <xsl:with-param name="str" select="substring-after($str,',')"/>
     </xsl:call-template>
   </xsl:when>
</xsl:template>

Frustration : The errors seem to be generated around the "when" 
statement. Unfortunately I can't  provide any error output because this 
is being done inside a PHP script.

Many thanks for reading this...  if anyone has any advice about +why+ 
this isn't working, and where I ought to go, I'd be grateful. Thanks.

Ben