RE: null call of document function

Sorry, but advising on application design is much harder than advising on
coding or debugging, and I really don't like doing it without knowing a lot
more than this about your requirements and constraints. And it then becomes
a consultancy assignment rather than a simple question+answer on a forum.

It's easy to spot when someone is doing things the wrong way, it's sometimes
much harder to identify the right way.

Michael Kay
http://www.saxonica.com/
 

> -----Original Message-----
> From: Steve [mailto:subsume@xxxxxxxxx] 
> Sent: 08 April 2007 01:47
> To: xsl-list@xxxxxxxxxxxxxxxxxxxxxx
> Subject: Re:  null call of document function
> 
> Maybe you could give me some additional guidance? My 
> situation is this:
> 
> I have a sql table 'alerts' which contains system messages to 
> be delivered to the user.
> 
> A call to my xsl template "getAlerts" calls a script which 
> serves these alerts up as XML and then clears them from the table.
> 
> In some cases I want a file my user is viewing to generate an alert.
> My idea was to use the document function to insert the alert, 
> and then later in the script it would be available for the 
> getAlerts template.
> 
> Are you suggesting it would be wiser to insert this Alert in 
> my processing script pre-transform? Some other suggestion?
> 
> -Steve
> 
> On 4/7/07, Michael Kay <mike@xxxxxxxxxxxx> wrote:
> > > What if I want to make a call to the document() function 
> but I don't 
> > > expect any value?
> >
> > That presumably means you expect the call to have side-effects.
> >
> > A call on the document function is analagous to an HTTP GET 
> request, 
> > and it's considered bad form for GET requests to have side effects:
> >
> > http://www.w3.org/2001/tag/doc/whenToUseGet.html
> >
> > At the XSLT level, calling document() and expecting 
> side-effects has 
> > all the problems associated with extensions functions that 
> have side 
> > effects, notably that the call is liable to be optimized 
> away if the 
> > result isn't used. In fact it's worse than extension 
> functions, because:
> >
> > (a) with extension functions, the optimizer might if you're 
> lucky take 
> > into account the possibility that the call has 
> side-effects; with the 
> > document() function this is unlikely
> >
> > (b) the semantics of the document() function essentially 
> say that if 
> > you make the same call twice with the same URI, it only 
> gets executed once.
> >
> > In short - don't do it.
> >
> > Michael Kay
> > http://www.saxonica.com/