Re: [xsl] things about grouping

[David Carlisle <davidc@xxxxxxxxx>]

On 28/11/2012 07:04, Ihe Onwuka wrote:

> Well XPath has direct support for union and intersect so for
> completeness we would expect it to support difference. Thats
> reasonable isn't it. So lets go to the part of the specification that
> deals with set theoretic operators and see what it says.
>
> The first hint of trouble is in the URL.
>
> http://www.w3.org/TR/xpath-functions/#union-intersection-except
>
> Lets read on and see why there is support for union and intersection
> and not difference.
>
> op:union 	Returns the union of the two sequence arguments, eliminating
> duplicates.
> op:intersect 	Returns the intersection of the two sequence arguments,
> eliminating duplicates.
> op:except 	Returns the difference of the two sequence arguments,
> eliminating duplicates.
>
> So difference is supported except (pun intended) it is called except.
>
> But (sic) the semantic of except  that has been ingrained in us since
> childhood is that when you say  A except B, B takes it context from A.



You are not really suggesting that set difference operation (except) 
should behave differently with respect to the current node than the 
union and intersect operations (or list concatenation are you?

in all these cases A and B are evaluated the same way

A|B
A intersect B
A except B
A,B

If I understand your message correctly you are arguing that it would 
somehow have been simpler if in the third case only the current node 
when evaluating B was somehow different, I'm not sure what exactly,
would you want to evaluate B at every item in A?

In general XPath operators take _values_ (Xpath sequences) not 
expressions so they are always (conceptually) evaluated before the 
operator.  A except B evaluates A and B and then returns all teh items 
in A that are not in B.

The path separator / and filter [] are different in that they are 
specific syntax that changes the current item.

David



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