[Xpath-ng] List constructor syntax

[Uche Ogbuji]

>   Also, if we adopt the XPath 2.0 convention of using an empty list to
>   mean an unknown or invalid value,
> 
> I'm fairly sure that's a bad convention,

I think it's OK in some cases, i.e. as a return from location path selection.  
But I agree that it's a bad general convention.

> one that the xpath2 drafts seem
> to have been moving away from as far as possible.
> 
> Initially they tried to use () as an "unknown" value and give logical
> operators three valued logic sematics using true() false() and ()
> but using () as an exceptional value doesn't fit well with the fact that
> it's a perfectly normal and expected return value for any path
> expression, what used to be called an empty node set...
> So later xpath2 drafts seem to be backing off the 3 valued logic idea.

Very hesitant suggestion: maybe we should introduce a Null value into the core 
as a general-purpose error flag.


-- 
Uche Ogbuji                                    Fourthought, Inc.
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