Subject: Re: [xsl] Re: Re: order of UNIONs From: Jörg Heinicke <joerg.heinicke@xxxxxx> Date: Thu, 15 Nov 2001 08:39:58 +0100 |
Thanks, Dimitre, for the explanations. I must confess, that I don't know much about node-sets in theory, only things I came across when using them in XSL. Are there any nice websites or books (Michael Kay's?) about node-set theory? Joerg > > But does this really make sense? select="group[21]|group[1]" means for me > > 'select the 21st group and add the 1st group'. > > It makes ***perfect sense***. Node-sets are sets. Sets do not have order. Regardless > of the order, in which you add elements, the result is the same set: > > a | b = b | a > > The above is an axiom in set theory. > > What you actually need is kinda bag, or list -- these are very different structures > >from sets. Both allow duplicates, and a list has order. > > I think one of the major problem of the XPath 2.0 Data Model is that they do not > distinguish between lists and sets, trying somehow to say that a node-set is a kind > of list (the actual term used there was "sequence", if I remember well). The > consequences are bad problems, because some operations on lists cannot be performed > 1:1 on sets, and vice versa -- not every list is a set, an operation performed on a > sequence, that is a nodeset, may not yield a nodeset, a nodeset operation performed > on a sequence that is a nodeset, may behave quite differently from the same > operation, performed on a sequence (e.g. eliminating/preserving duplicates on a > union/append operation). > > Cheers, > Dimitre Novatchev. XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list
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