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RE: XPath 2.0 - how much of XQuery should it include?

• To: "Jeni Tennison" <jeni@j...>,"Jonathan Robie" <jonathan.robie@d...>
• Subject: RE: XPath 2.0 - how much of XQuery should it include?
• From: "Dare Obasanjo" <dareo@m...>
• Date: Thu, 9 May 2002 17:26:23 -0700
• Cc: <xml-dev@l...>,<public-qt-comments@w...>
• Thread-index: AcH3s5aQmir/CdffRUSxqhak0XFjAQABEurA
• Thread-topic: XPath 2.0 - how much of XQuery should it include?

> -----Original Message-----
> From: Jeni Tennison [mailto:jeni@j...] 
> Sent: Thursday, May 09, 2002 4:44 PM
> To: Jonathan Robie
> Cc: xml-dev@l...
> Subject: Re:  XPath 2.0 - how much of XQuery should 
> it include?
> 
> 
> Hi Jonathan,
> 
> > And what other features of XPath 2.0 do you think are overkill for 
> > XPath?
> 
> Do you really want to hear? OK...
> 
> The fundamental thing is that XPath is used by XSLT as an 
> expression language. It's used in XSLT to provide values for 
> instructions, not as a programming language in its own right. 
> This is reflected in the fact that it is embedded in 
> attribute values, not used in content.
> 
> If you have an XPath that's going over several lines, then 
> something's going wrong (not that the XPaths I write don't 
> sometimes go over several lines, just that when they do so it 
> usually indicates either a deficiency in the language, or 
> that I'm trying to show off). At a practical level, line 
> breaks in attribute values aren't maintained by XML parsers, 
> so you're likely to lose all that nice formatting if your 
> XSLT stylesheet is ever treated like the XML that it is.
> 
> XPath 2.0 incorporates a number of *statements* that are 
> already provided by XSLT 2.0. The for "expression" and the if 
> "expression" would be classed as statements in any other 
> language. XPath 2.0 also replicates other functionality 
> that's already supported in XSLT: sorting, for example, and 
> variable assignment (though obviously XPath 2.0 can only 
> carry out variable assignment in limited contexts, something 
> that in my opinion undermines the idea that for expressions 
> can solve all the problems of sequence construction in XSLT 
> 2.0). Obviously XQuery needs this -- it has to have 
> statements because it isn't embedded in something that does 
> -- but XPath 2.0 doesn't (or wouldn't if XSLT 2.0 was 
> adjusted to enable it to create sequences of atomic values, 
> something that I believe is entirely feasible).
> 
> Most of what I believe is overkill for XPath 2.0 is stuff 
> that's simply too complicated for an expression language:
> 
>   - for expressions, because XSLT has xsl:for-each, although I do
>     think that a simple mapping operator would be essential if there
>     weren't for expressions
> 
>   - conditional expressions, as they currently are, because XSLT has
>     xsl:if and xsl:choose, although I do think that a simple
>     conditional expression (i.e. test ? true : false) would be vital
>     if there weren't if expressions
> 
>   - quantified expressions, because you could replicate them with a
>     simple mapping operator, and in my experience you don't often
>     need to test that every $x satisfies something when doing a
>     transformation (and the existential qualification of the existing
>     comparisons covers the 'some' expression)
> 
> Other things I feel less strongly about; I wouldn't abandon 
> XPath 2.0 if they remained, but I don't particularly see the 
> point of them (or the requirement, if you want to go by use cases):
> 
>   - comments in XPaths -- if an XPath gets long enough that you need
>     to embed comments in it, you should break it up and use XML
>     comments instead
> 
>   - dereferences -- id(), which does basically the same thing, is
>     very rarely used because people tend to want to make their
>     stylesheets robust in case DTDs aren't available for some reason,
>     so use key() instead. I don't see that dereferencing will be any
>     different, although if it used XML Schema identity constraints it
>     would be a lot more compelling
> 
>   - the "union" operator -- when is it ever a good idea to have more
>     than one symbol for the same operator?
> 
>   - eq/ne/lt/gt/ge/le -- these do exactly the same as =/!=/</>/>=/<=.
>     The only difference for XPath (as far as I can see) is that if the
>     arguments are sequences then they (due to fallback processing)
>     compare the first of the items in those sequences rather than
>     every combination of values of those sequences. I can't think of
>     any occasion in which that's useful.
> 
>   - one of the pair of <</>> or precedes/follows -- but you knew that;
>     I have no idea which one, because most of the time you need to do
>     that kind of test, you know that neither of the nodes is an
>     ancestor of the other so the distinction doesn't apply.
> 
> You didn't want me to go into the functions, did you?...
> 
> Don't get me wrong -- there are some things that I think are 
> good in XPath 2.0. Sequence construction has the promise of 
> being really handy if it's easy -- it's one of those things 
> where we'll appreciate a lot more when we're able to use it. 
> Being able to compare the identity of two nodes is fabulous 
> (although to be honest, it's mainly used for grouping, and 
> that's built-in to XSLT 1.0). General steps... [sound that 
> Homer (Simpson, not the poet) makes when he thinks of beer] ;)
> 
> > Is it a mistake for XPath to try to incorporate so much of 
> XQuery, or 
> > is this what users really want?
> 
> To a certain extent, it's hard to answer this question. We 
> won't know what we want until we start trying to work with 
> XPath 2.0. In particular, sequences of atomic values are such 
> a radical change that anything we say now about what we need 
> in order to manipulate them are guesses. If I were to guess, 
> I'd say that users want XPath 2.0 to be much like XPath 1.0, 
> with a few small changes to address the requirements that 
> we've discovered, through usage, over the past couple of 
> years. They want regular expressions and date formatting. 
> (Honestly, they want dynamic evaluation, but we've already 
> been told in no uncertain terms that that's not going to 
> happen, so really I should stop carping on about it.)
> 
> I do think that it's a lot easier to add features to a 
> language than it is to take them away; XPath 2.0 should be 
> *core* functionality, not XQuery minus node-constructors. And 
> the other users of XPath -- XPointers, XForms, XML Schema, 
> should be considered as well.
> 
> Ahh... it's nice to feel someone might be listening... Let me 
> know if you want me to expand with some use cases.
> 

XPath as XQuery - node constructors + a few more axes nodes might as
well be called XQuery-reduced instead of XPath 2.0 since instead of
being an incremental version of XPath it is primarily XQuery on training
wheels.  


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