Hi Sebastian,
Hi Jean-Marc,

On this problem, a quick comment coming into my mind, since I faced a comparable problem recently. Without changing XQUERY or BaseX, it is possible to emulate overloading using the introspection module of BaseX. But you need a map which keys and values are functions (not possible with BaseX map module, but possible with XQUERY).


Thank you for you input. Using the inspection module helped.

It’s not perfect, because the dispatcher still has to import custom code modules, but at least I don’t have to change my code everywhere I want to use the custom functions:

—— — dispatch module

module namespace dispatch="http://extensions/dispatch";
import module namespace custom="http://extensions/custom“; (: is it possible to read this via query:invoke? or something :)

declare function dispatch:fun($id as xs:string,$arg1){
    let $default := for $f in inspect:functions() 
                      where exists(inspect:function($f)[annotation[@name='dispatch:default'][literal[1]=$id]]) 
                      return $f
    let $custom :=  for $f in inspect:functions()
                      where exists(inspect:function($f)[annotation[@name='dispatch:custom'][literal[1]=$id][literal[2]=$arg1]])
                      return $f
    return if (count($custom)=1) then $custom($arg1)
           else $default($arg1)
};

——— custom module
module namespace custom="http://extensions/custom";
import module namespace dispatch="http://extensions/dispatch";

declare %dispatch:custom('someid','13')
        function custom:canbenamedwhatever($t as item()) {'custom'};

— — — test run
import module namespace dispatch="http://extensions/dispatch";

declare %dispatch:default('someid') 
        function local:NameAsYouLike($t as item()) { 'default' };

dispatch:fun('someid',12) (: yields „default“ :)
,dispatch:fun('someid',13) (: yields „custom“ :)



This is not a generalized solution, but it meets my requirements (i.e. only one argument). 
Pro/cons
+ I wrote my first annotation that actually does something ;-)
+ the code is reasonably well decoupled
- I have to call the dispatcher directly - no automagic no function overload
- the dispatcher has to import the custom modules, it’t just one line, but still this is a dependency.

With such a map, keys are computed using the BaseX introspection module for your prod functions, since values are computed using the BaseX introspection module for your custom functions.

By „map of functions“ did you mean what "inspection:functions()“ is doing?


All calls are then concentrated through a dispatcher : something like

call($function,$argument) { if contains($MapOfFunctions, $function) then map:get($MapOfFunctions,$function)(argument) else $function($argument) }.

I tested this trick very recently actually, to overload the xquery:eval BaseX function. I named the resulting function evil_eval (a very dangerous function) !


eval ist evil enough… ;-)
I don’t get it: how would you be able to overload the query eval function?

Hope this help

Yes it did.


Cheers

Jean-Marc


Have fun, 
 Sebastian







2013/11/30 Wiemer, Sebastian <Sebastian.Wiemer@adesso.de>
Hi Christian, 

Hi Sebastian,

thanks for your e-mail; I’m pleased to see you are still working with BaseX.
And we still love BaseX :-) 
Thanks for keeping up the good work!

BTW: I unfortunately missed your and Alexanders presentation at the MarkUp Forum: are there any slides available?



So just by dropping a file like „custom.xqm" in the restxq path, we can
customize the system.

…sounds like a nice idea.

The drawback is that we can only use this for HTTPS requests from the
client, but not from the server, because we use HTTPS. So we cannot use
RESTXQ requests server side with http:send-request. (Or do we?)

I guess I have no answer to that, as I haven’t actually come across
this challenge yet. This might be completely off the track, but maybe
it’s also a question of how Jetty is configured? Did you already check
out the jetty.xml file and possible settings?

Yes, I checked jetty configuration (but I gave up), and it seems to me, that jetty.xml or web.xml might not be the right place for a solution after all.

What Florian and I probably are trying to accomplish here is a way to introduce aspect oriented or rule based
programming to XQuery to decouple custom extensions from product code.

This is similar to imported modules in XSLT: If you import template rules, which are more specific,
they will be applied first. Could this be applied to XQuery as well? (But in XSLT you would have to 
import the custom code and hence change your code. We can do that in XQuery as well, but it
is not completely decoupled, is it?)

Florian and I came around with an idea, that does not use HTTP requests and maybe clarify
our goal: how about introducing a „dispatch" annotation to XQuery?

Let’s assume we have a function „age“ that takes an age as integer and a name as string and prints a message.
A customization would be to be more specific for teenagers. But we don’t want to change the code.
the annotation „dispatch:declare“ declares a function to be potentially dispatched: if there is not a more specific
function with a „dispatch:replace“ annotation, than that function is called. 
Maybe "dispatch:prepend" and "dispatch:append“ would be nice as well.
Also it would come in very handy if the replacing function is allowed be updating or not updating.

product.xqm
%dispatch:declare(age($age,$name))
declare function prod:age($age as xs:integer,$name as xs:string){
  $name||" is "||$age||" years old."
};

prod:age(14,'Emily’) = „Emily is 14 years old."

custom.xqm
%dispatch:replace(age(19>$age>13,$name))
declare function custom:age($age as xs:integer,$name as xs:string){
  $name||" is a "||$age||“ years old teenager."
};


So a use of „prod:age“ would yield the result of „custom:age“:
prod:age(14,'Emily’) = „Emily is a 14 years old teenager.“ 


Comments on this idea are very appreciated.

Sebastian


Christian


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