Templates in XQuery

[William Candillon]

Hi,

We tried to capture the solution proposed by John as an xquery
design pattern and came up with the translator pattern (see:
http://www.balisage.net/Proceedings/vol5/html/Candillon01/BalisageVol5-Candillon01.html#pm).

The code sample of this pattern is available at http://patterns.28msec.com.
Examples can be executed with Zorba or online at http://try.zorba-xquery.com.

Best regards,

William

On Mon, Sep 6, 2010 at 10:58 AM, John Snelson
<http://x-query.com/mailman/listinfo/talk> wrote:
> On 05/09/10 12:26, Brett Zamir wrote:
>>
>>   On 9/5/2010 5:16 PM, Martin Probst wrote:
>>>>>>
>>>>>> I wouldn't agree. To me, XQuery is an XML aggregation language. I
>>>>>> never do any significant XML transformation work using XQuery.
>>>>>
>>>>> Outside of the important omission of templates or the handful of
>>>>> features
>>>>> from XSL which have no equivalents in XQuery, what transformation
>>>>> feature of
>>>>> XSL cannot be expressed in XQuery and more succinctly so?
>>>>
>>>> That's enough isn't it?  :)  The template processing model is the
>>>> killer feature, the thing that makes XSLT the best at transforming
>>>> XML.
>>>
>>> That's my perspective as well. It might not seem much to you, but
>>> templates make all the difference in the programming model. As I said,
>>> to me XSLT is a set of rules that are applied as
>>> filters/transformation rules on a tree structure. That's a major
>>> difference from XQuery where I go out and point the interpreter at
>>> certain XML parts I'd like to find, both in mental model and in
>>> implementation, I think (not ever having implemented an XSLT processor
>>> though).
>>
>> The use cases are simply:
>> 1) Make it possible for the many people who already prefer the shorter
>> and visually contrastive XQuery syntax to be able to do so for their
>> whole document templating needs (i.e., for cases when they formerly
>> would have needed to use the XSL syntax, let them use the same XSL
>> engine they used before but which now might support XQuery+templates as
>> an alternative).
>> 2) Provide another option to XSL users not yet familiar with XQuery
>> syntax but who might like to learn something which might make them more
>> productive in their work by taking advantage of the shorter and visually
>> contrastive XQuery syntax.
>
> I've given plenty of thought in the past to adding XSLT-like templating
> facilities to XQuery - even going so far as to implement some experimental
> syntax for it in XQilla. The trouble is, XSLT's XML syntax has allowed every
> instruction to be given a large number of options as attributes or
> sub-elements, and that syntax is very hard to then translate into a grammar
> like XQuery's. So I no longer believe that adding grammar for XSLT
> compatible templates definitions to XQuery is something worth doing.
>
> Of course, just adding stylesheet import and apply-templates to XQuery is
> probably feasible, and hopefully that will be done at some point in the
> future.
>
> However, recently I've been considering implementing a templating framework
> in XQuery. With XQuery 1.1's higher order functions facility, it would be
> simple enough to build - for instance, what would people think about
> something like this:
>
> let $stylesheet := stylesheet((
>  template(function($n) { $n instance of element(foo) },
>    function($n) { <new_foo>{ apply-templates($n/*) }</new_foo> }),
>
>  template(function($n) { exists($n/self::bar/parent::foo) },
>    function($n) { () })
> ))
> return apply-templates(doc("mydoc.xml"), $stylesheet)
>
> I've got plenty of details to think through, but a simple structure of
> predicate functions and action functions, along with an apply-templates()
> function that handles it could meet the needs of anyone transforming XML in
> XQuery.
>
> John
>
> --
> John Snelson, Senior Engineer                  http://twitter.com/jpcs
> MarkLogic Corporation                         http://www.marklogic.com
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