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RE: XSLT and XQuery

• To: "Jonathan Robie" <jonathan.robie@s...>,"Joe English" <jenglish@f...>,<www-xml-query-comments@w...>,<xml-dev@l...>
• Subject: RE: XSLT and XQuery
• From: "Michael Rys" <mrys@m...>
• Date: Fri, 4 Jan 2002 17:19:48 -0800
• Thread-index: AcGVedyTHQpvX3enTQaWPVSLFiiZqQADL6CQ
• Thread-topic: XSLT and XQuery

Jonathan, I agree with most of your analysis. The only problem that I
have is whether we should consider the XML element for embedding XQuery
to be specified inside the XQuery WG or not. The reason why I ask that
is that XQuery so far has (IMHO) wisely avoided to deal with
standardizing language embeddings since there would be too many cases to
consider.

So if somebody wants to embed XQuery in Python or System.XML or AxKit or
SQLXML templates, should XQuery prescribe that embedding or should the
designers, users, standardization committees of these environments not
decide on how to embed XQuery?

Best regards
Michael

> -----Original Message-----
> From: Jonathan Robie [mailto:jonathan.robie@s...]
> Sent: Friday, January 04, 2002 15:43 PM
> To: Joe English; www-xml-query-comments@w...; xml-dev@l...
> Subject: Re:  XSLT and XQuery
> 
> I believe it is technically feasible to integrate XSLT and XQuery
along
> the
> lines suggested by James in:
> 
> http://www.jclark.com/xml/construct.html
> 
> However, I don't think it would be a good idea to try to do so before
> XQuery 1.0 is released. This is a cost-benefit decision for me. In the
> rest
> of this message, it may sound like I don't like the idea James
proposes,
> but that's not really true. I think it is an interesting proposal that
> should be considered in due time, but not placed on the critical path
for
> XQuery 1.0. The proposal raises a large number of procedural and
technical
> questions, and I think this should be freely explored over a period of
> time, not made into a new set of requirements for an existing
standards
> effort.
> 
> Here is the problem that James says he is trying to solve:
> 
> >Currently XSLT and XQuery use very similar syntaxes for element and
> >attribute construction. For example,
> >
> ><p>This is a <a href="{@ref}">link</a>.</p> works the same in both
> >XSLT and XQuery.
> 
> There are actually many constructs which are similar, but different,
in
> XSLT and XQuery, just as many of the features of Java and C++ are
similar,
> but different. That does not necessarily mean that Java and C++ should
be
> unified. There are compelling reasons to unify the path expression
> language. I am not yet convinced that there is an equally urgent need
to
> unify element constructors, and if we did so, I would think that would
> best
> be done as part of a deeper integration of the two languages
altogether.
> This is the sort of thing that requires careful exploratory work by a
> small
> group over time, and I don't think it should be placed on the critical
> path
> for XQuery 1.0.
> 
> >However, there are numerous minor differences:
> >
> >o  In XSLT, curly braces interpolate expressions inside attributes,
but
> >are just ordinary characters inside elements. Inside elements,
> >expressions are interpolated by using elements of a distinguished
> >namespace are used instead. In XQuery, curly braces are used
> >uniformly inside attributes and elements.
> 
> XSLT could decide to allow curly braces in elements, with the same
meaning
> as in XQuery. This would not require deep cooperation between the two
> working groups.
> 
> >o Elements and attributes whose names are specified by an expression
> >use a syntax in XQuery that is not well-formed XML. In XSLT, this is
> >done by xsl:element.
> 
> XQuery's element constructors are not well-formed XML only with
attribute
> constructors that do not use the quoted syntax:
> 
>          <foo bar={ //baz } />
> 
> The old computed element constructors have been dropped. Note that you
can
> express the above using quoted element values, with the same meaning:
> 
>          <foo bar="{ //baz }" />
> 
> We have an open issue asking whether we should drop one of the above
> syntaxes.
> 
> >o In XSLT, documents are parsed first by an XML parser. The XSLT
> >processor operates on the infoset produced by the XML parser. This
> >means that use of < and & are subject to the usual XML rules. In
> >particular, in XSLT when < is used in a comparison operator it needs
> >to be escaped as &lt;. In XQuery, parts of the query are processed
> >using an XML-like parse, and parts are processed using an XPath-like
> >parse; the XPath-like parsing does not operate on the results of the
> >XML-like parsing. So you can use "<" as a comparison operator without
> >escaping it. In XSLT, character references are recognized uniformly
as
> >in XSLT; in XQuery, they are recognized only in a few places.
> 
> In that last sentence, I assume that "as in XSLT" was intended to read
"as
> in XML", right?
> 
> >I believe that many users will need to work with both XSLT and
XQuery,
> >and these numerous subtle differences will be extremely confusing to
> >such users.
> 
> I do not think that users are confused to find that "foo[bar < 12]" is
> legal in XQuery. I think they would be more confused to find that they
> have
> to type "foo[bar &lt; 12]". Of course, if you put characters in an XML
> document, they behave as characters do in XML documents, so I am not
being
> critical of XSLT here. I think it would also be quite reasonable to
come
> up
> with a standard dummy element to wrap around an XQuery so that you
*can*
> embed an XQuery in a document in a standard way that will be
recognized by
> all XQuery processors.
> 
>   But requiring all queries to be in an XML document seems like
overkill
> to me.
> 
> >I believe we should take the best aspects of the XSLT and
> >XQuery element construction syntax and unify them into a single
syntax
> >to be used by both XSLT 2.0 and XQuery 1.0.
> 
> Why unify just one syntactic construct? Or is this part of completely
> unifying the two languages?
> 
> Here are some costs I am concerned about:
> 
> First, we have just spent an entire year working together to unify
XPath
> and XQuery, and we are not yet done with that work. Unifying the
remaining
> portion of XQuery and all of XSLT would certainly take a significant
> amount
> of time -- I would estimate it as at least an additional year. James'
> proposal is largely about unifying syntax, but in order to unify the
> syntax
> we would also have to unify the semantics at a very deep level, and we
> would have to take into account the reasons that the two languages
evolved
> differently in the first place. We would also have to answer difficult
> questions about the extent to which we could sacrifice compatibility
with
> existing XSLT standards.
> 
> Second, the two languages often have different syntax for similar but
> slightly different functionality. In fact, in the proposal, James
gives a
> good example of this:
> 
>    if ($x = 1)
>    then
>       <foo/>
>    else
>       <bar/>
> 
>    <xsl:choose>
>      <xsl:when test="$x = 1">
>        <foo/>
>      </xsl:when>
>      <xsl:otherwise>
>        <bar/>
>      </xsl:otherwise>
>    </xsl:choose>
> 
> If we unified the two languages, I assume we would not stop at element
> constructors, so we would want to add an element syntax for
if-then-else,
> and a string syntax for choose-when-otherwise. Then again, we might
decide
> to abandon one or the other of these syntaxes, but the debate about
which
> to abandon would take some time - and we would have this debate for
every
> single feature in which the two languages differ, or else wind up with
a
> language that drops in a bunch of confusingly similar features from
both
> languages.
> 
> Third, the basic approach to programming in XSLT is recursive descent,
> which is a fairly rare pattern of usage in XQuery. If XSLT continues
to be
> programmed as it currently is, and XQuery continues to be programmed
as it
> currently is, then we will have two distinct user communities using
> different subsets of the common language. Are we suggesting that one
> community or the other adopt the standard approaches of the other
> community? If so, do we have any indication that the other community
is
> interested in doing so? If not, will we have a PL-1 situation, where
> people
> can both be programming in the same language, but can't read each
other's
> code?
> 
> Here are the other benefits cited by James:
> 
> >Full XML is supported. It is not necessary to subset XML by removing
> >entity references, comments and processing instructions. Furthermore
> >parsing is consistent with XML. For example, XML has complex rules
about
> >how whitespace in attribute values are normalized; at the moment
XQuery
> >must choose between interpreting attribute values inconsistently with
XML
> >and requiring all XQuery parsers to replicate XML's complex attribute
> >value normalization algorithm.
> 
> Comments and processing instructions are supported in XQuery. Entity
> references are not, except for the built-in character references of
XML.
> If
> people need them, they can embed their queries in an XML document.
That
> does not mean that everybody should be required to do so. People who
do
> not
> need character references should not have to type "foo[bar &lt; 7]"
when
> they mean "foo[bar < 7]".
> 
> The attribute normalization algorithm is easily written as a function.
An
> XQuery implementation can call that function.
> 
> >Parsing XML is a non-trivial exercise. XML parsers are widely
available
> >for all major platforms. Implementation is made easier if
implementors
> can
> >use standard parsers rather than having to write their own nearly-XML
> parsers.
> >
> >XQuery/XSLT may need to embed fragments in other XML vocabularies.
Other
> >XML vocabularies may need to embed XQuery. For example, XQuery might
need
> >to embed fragments of XML Schema. XML Schema has annotations which
might
> >use XQuery to express constraints. If XQuery is well-formed XML, this
is
> >easy and causes no problems. If not, then every level of embedding
adds
> an
> >extra layer of escaping; this quickly becomes totally unworkable.
> 
> I certainly agree that it must be possible to write XQuery, including
> element constructors, in a way that is well-formed XML. This is
possible
> today. It does not require deeper integration with XSLT.
> 
> >Often users want to construct documents that have a lot of
boilerplate
> XML
> >(typically XHTML) and often only a small amount of variable content
that
> >depends on the query. For such documents, it is useful to be able to
use
> >XML tools to edit and otherwise manipulate the boilerplate.
> 
> This feels like the most common of your scenarios. Again, I think that
the
> real requirement is to be able to embed XQuery in well-formed XML.
> 
> >XML takes care of the specifying and detecting character encodings.
If
> you
> >don't use XML, then you have to invent your own mechanisms for
dealing
> >with this. This is a crucial I18N issue.
> >
> >XML provides a mechanism for referring to arbitrary Unicode
characters.
> >XPath was designed to be embedded in XML documents, and so does not
> >provide its own syntax for character references, but instead expects
to
> >rely on XML's mechanism. This all works fine for XSLT. However, with
> >XQuery's parsing model, character references are only recognized in
> >attribute values and literal element content; there's no way to put a
> >character reference in a literal string.
> 
> Again, I think all of the above criteria are met by any approach that
> allows XQuery to be embedded in an XML document. We need not require
that
> all XQueries be embedded in XML documents.
> 
> So here's my take:
> 
> 1. It really is important to be able to embed XQuery in an XML
document.
> We
> should also come up with a trivial, but standard, way of doing this.
It
> basically amounts to agreeing on the name of the wrapper element:
> 
>          <xq:yourNameHere xmlns:xq="http://www.w3.org/2002/XQuery">
>                  //foo
>          </xq:yourNameHere>
> 
> 2. It may be very interesting to think about the potential benefits of
> integrating XSLT and XQuery. If there is a compelling benefit to
> integrating the languages, we should consider doing so after the
release
> of
> XQuery 1.0.
> 
> Jonathan
> 
> 
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