[xquery-talk] when to use document { }?

[Howard Katz]

 > Does this make the document {} constructor clearer?

It does. I understand the difference; I was so focused on the serialized
result that I wasn't thinking about any further downstream processing.

Thanks to yourself, Mike, and Martin for reminding me that other clients
might be consuming the product of an XQuery transformation.

Howard

 > -----Original Message-----
 > From: talk-bounces at xquery.com 
 > [mailto:talk-bounces at xquery.com] On Behalf Of Jens Teubner
 > Sent: Tuesday, July 26, 2005 3:42 AM
 > To: talk at xquery.com
 > Subject: Re: [xquery-talk] when to use document { }?
 > 
 > On Mon, Jul 25, 2005 at 05:33:52PM -0700, Howard Katz wrote:
 > 
 > > I just came across an example of a query being used to generate an 
 > > xhtml document where the query is wrapped in a document 
 > constructor  :
 > > 
 > > [...]
 > > 
 > > What is the document constructor doing in this case? The 
 > query would 
 > > generate perfectly good xhtml without it, would it not? I 
 > know that 
 > > Saxon automatically applies document construction to any document 
 > > being serialized (unless you turn it off), but I've seen lots of 
 > > perfectly valid xml/xhtml generated without its use in other 
 > > environments. What's the difference between the two cases?
 > 
 > Well, the *serialized output* of that query may be the same 
 > with or without the document constructor, depending on your 
 > query engine.
 > 
 > However, in the XQuery data model, the application of the 
 > document {} constructor makes a big difference. Consider, 
 > e.g. the queries
 > 
 >   let $a := <a/>
 >     return $a/a
 > 
 > and
 > 
 >   let $a := document { <a/> }
 >     return $a/a
 > 
 > In the former case, $a is bound to the element a. If you do 
 > a child step on that (empty) element, you will get an empty result.
 > 
 > In the latter case, $a will be bound to a document node that 
 > sits on top of the (empty) element <a/>. If you now do a 
 > child step from $a, you will happliy get the <a/> element.
 > 
 > Does this make the document {} constructor clearer?
 > 
 > Jens
 > 
 > --
 > Jens Teubner
 > Technische Universitaet Muenchen, Department of Informatics
 > D-85748 Garching, Germany
 > Tel: +49 89 289-17259     Fax: +49 89 289-17263
 > 
 > Unix is user friendly - it's just picky about it's friends.
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