Re: help:missing value test (node??) is not working

Thursday, June 9, 2011, 1:04:42 PM, krist0ph3r wrote:

> I've found that checking for a node's existence doesn't work unless you check
> for the 1st element in that node. In your example:
>
> <#if mouse[0]??>
> Mouse found
> <#else>
> No mouse found
> &lt;/#if&gt;
>
> however, if mouse is a variable and not a node (and you're checking if it is
> defined, not if it exists), you can just use mouse?? as you have in your
> question.

Yeah, that's something almost everyone runs into. This is documented
at: http://freemarker.org/docs/xgui_imperative_learn.html

The cause is that when you issue someElement.mouse, you get the *list*
of matching nodes (just like with XPath). It can be many nodes, only
one node, or 0 nodes. In all of these case the result list itself
exists, only it's possibly empty. If someElement.mouse would return
nothing instead of a 0-length list, someElement.mouse?? would work,
but then

  <#list someElement.mouse as m>
    ... do something with each m
  </#list>

would fail, which would be again rather unpractical. That said, I
don't know how to solve this properly (I mean, like in a newer XML
wrapper or with language changes). For example, ?? and ! could treat
empty collections as non-existant... but that would be a mess if we
aren't talking about XML queries like this. Or there should be
NonExistantTemplateModel interface with which a value can suggest that
if ?? or ! asks, it doesn't exist... rather dirty hack and maybe it
would have unforeseen consequences.

-- 
Best regards,
 Daniel Dekany


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