Re: [Freemarker-devel] Fixing the XML wrapper regarding the existence ops - Was: Handling missing node values with NodeModel

On 21 Jun 2011, at 18:28, Daniel Dekany <[email protected]> wrote:

> Tuesday, June 21, 2011, 4:36:26 PM, Tom Fennelly wrote:
> 
>> Awesome..... that worked perfect... thanks Daniel !!!!
> 
> No, it's in fact awful, but this is how it is for now. I see two ways
> out of this situation:
> 
> a) Introduce a marker (empty) interface, freemarker.template.DefaultValue.
>   If a TemplatModel implements this interface, that tells FreeMarker
>   that this value is to be used only as the last chance, and the
>   locally provided default (like "anonymous" in name!"anonymous")
>   should take precedence. This feature would be, coincidentally,
>   useful for XML, because if empty XPath result sets are marked with
>   DefaultValue, then something like myXmlNode.foo!"default" would
>   magically work, just like myXmlNode.foo[0]!"default" does. However,
>   the empty results set is not "last chance" value (not a default
>   value), so it's kind of a dirty trick.
> 

I vote for keeping the current notation. I would prefer not deviating from Xpath not to cause surprise (maybe Xpath needs fixing, that's another story)

I am not sure I understand all the consequences of (a) though. Will any empty sequence be replaced with the new default value?

-- Denis.

> b) When people write myXmlNode.foo, they either expect 1 or 0 matches
>   (a single node which is optional), or multiple matches (a list of
>   nodes, even if possibly 0 or 1 long). XPath however doesn't have a
>   notation to express these two intents, so the current XML wrapper
>   doesn't have either. This is mathematically nice and everything,
>   but in practice it leads to all those voodoo tricks (like if and
>   only if myXmlNode.foo returns 1 matching node, the result is both a
>   scalar and a sequence - explain that to Average Joe Jr.) and
>   confusion. So, how about making these two cases different like:
> 
>   - myXmlNode.foo means "I want that *single* node". If there are no
>     matches, that's a undefined variable error, thus `!`, `??` etc.
>     works as expected. If there are multiple marching nodes, that's a
>     TemplateModelException, that can't be suppressed (except with
>     #attempt, of course).
> 
>   - myXmlNode.foo$ means "I want the list of the matching nodes".
>     This would always return a sequence, even if 0 long. Why "$"?
>     It could be read as the "S" of the plural form, "foos". (I think
>     this was originally the idea of Jonathan.) The drawback is of
>     course that this way we deviate from XPath, so XPath expression
>     couldn't be written as myXmlNode["foo//bar"] and like. Instead,
>     they had to be written as myXmlNode("foo//bar") maybe.
> 
> Neither of these are strictly backward-compatible, but since we can
> have multiple wrapper instances and instances can be configured or of
> different classes, that's manageable.
> 
> Another thing... I find it scary that we depend on internal API-s to
> do XPath with Xerces. If they still haven't added the required API-s
> as public, well... in the new wrapper we should just require Jaxen.
> Unless, there are performance problems with that or like... anybody is
> aware of such issues?
> 
> -- 
> Best regards,
> Daniel Dekany
> 
> 
>> On 21/06/2011 15:29, Daniel Dekany wrote:
>>> I recommend reading this:
>>> 
>>>   http://freemarker.org/docs/xgui_imperative_learn.html
>>> 
>>> It answers this question among others. Anyway, you could do it like
>>> this:
>>> 
>>>   ${.vars["com.acme.Travel"].checkIn[0]!}
>>> 
>>> The relevant change I did here is the adding of [0]; the others are
>>> just aesthetical. You see, the result of the XPath query exists even
>>> if it's an empty node list, so what you want to know if it has a 1st
>>> element.
>>> 
>>> 
>>> Tuesday, June 21, 2011, 3:26:15 PM, Tom Fennelly wrote:
>>> 
>>>> Hi.
>>>> 
>>>> I'm using a NodeModel and was wondering how missing nodes can be
>>>> handled?  I'm looking for the equivalent of the
>>>> unsafe_expr!default_expr notation
>>>> (http://freemarker.sourceforge.net/docs/dgui_template_exp.html#dgui_template_exp_missing),
>>>> but something that works for NodeModels (the default did not work for me).
>>>> 
>>>> BTW.. the template's in question use the .vars syntax in the
>>>> expressions.  So, when I do the following I get an exception -
>>>> ${.vars["com.acme.Travel"]["checkIn"]!""}.  The exception is...
>>>>     Expecting a string, date or number here, Expression
>>>> .vars["com.acme.Travel"]["checkIn"]!"" is instead a freemarker.ext.dom.NodeListModel
>>>>     The problematic instruction:
>>>>     ----------
>>>>     ==>  ${.vars["com.acme.Travel"]["checkIn"]!""} [on line 3,
>>>> column 19 in free-marker-template]
>>>>     ----------
>>>> 
>>>>     Java backtrace for programmers:
>>>>     ----------
>>>>     freemarker.core.NonStringException: Error on line 3, column 21 in free-marker-template
>>>> 
>>>> Thanks in advance.
>>>> 
>>>> T.
>> 
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