Yes, that seems like the most elegant solution, since for my purposes an empty 
string and a null attribute are synonymous. Thanks!

On Tue, 26 Apr 2016, Martin Honnen martin.honnen@xxxxxx wrote:

> On 26.04.2016 20:39, David Sewell dsewell@xxxxxxxxxxxx wrote:
>> (Following assumes XSLT 2.0)
>> 
>> I don't use the @group-adjacent form of <xsl:for-each-group> often
>> enough to be a master of it. I just ran into the problem that an empty
>> sequence is not allowed as the result of evaluating the group-adjacent
>> expression. Given data like
>> 
>> <contributor role="author">Joe Jones</contributor>
>> <contributor role="author">Mary Mitchell</contributor>
>> <contributor role="editor">Betty Berkeley</contributor>
>> <contributor role="editor">Peter Parker</contributor>
>> 
>> my first instinct was to use
>>   <xsl:for-each-group select="contributor" group-adjacent="@role">
>> 
>> but I got a run-time error because the @role is not always present in my
>> data. So I revised to
>>
>>   <xsl:for-each-group select="contributor" group-adjacent="if (@role)
>>     then string(@role) else 'false'">
>> 
>> which give the desired results. But is there a better general strategy,
>> maybe using boolean false() as the grouping value, and then being sure
>> to cast current-grouping-key() as a string whenever a string comparison
>> is needed?
>
>
> I would be inclined to simply use
>
>   group-adjacent="string(@role)"
>
> unless you could have both <el role="">...</el> and <el>...</el> and
> would need to distinguish them.
>
>
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-- 
David Sewell, Editorial and Technical Manager
ROTUNDA, The University of Virginia Press
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Email: dsewell@xxxxxxxxxxxx   Tel: +1 434 924 9973
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