[Xpath-ng] List constructor syntax

[Uche Ogbuji]

> Hi David,
> 
> > Yes, I know. I'm sure it can be made to work, I'm just not sure what
> > the advantage is, except for the previously mentioned feature that
> > you can get away without having an explicit list constructor.
> 
> Well, I think that's a fairly large advantage :) I also think it would
> simplify things for users if they didn't have to consider the fact
> that some functions might return a singleton list while others return
> an actual item, especially if that has an effect on whether you can
> pass the value on to another function.

I'm not a fan of statically typed function signatures, so this argument 
doesn't really move me.  I would certainly prefer to keep XPath's untyped 
functions, which would move the issue you bring up purely to function 
semantics, which is where I believe it belongs.


> Also, if we adopt the XPath 2.0 convention of using an empty list to
> mean an unknown or invalid value, then we really want every function
> to return a list (so that all functions work in the same way and some
> functions can return this "invalid" value), which means that functions
> will have to be able to take lists as arguments.

The empty-list-as error convention in XPath is, IMHO not a good idea to 
genralize if we move to generic lists.  Again, I think that the error flag is 
a matter of function semantics, not core behavioral model.  For example, I 
would expect a numeric function to return NaN in case of error rather than 
empty list.

I do think we'll have to consider a module for error signalling.

> I don't know -- it just seems cleaner to say that every value is a
> list, if we can get away with it...

Maybe, but I also feel it's cleaner to have a clear separation between lists 
and atoms.

I think we probably need better developed use cases to hash this out.


-- 
Uche Ogbuji                                    Fourthought, Inc.
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