[rules-users] Drools and Clojure

[Mark Proctor]

André Thieme wrote:
> Mark Proctor schrieb:
>   
>> Map( this['c'] == 206 )
>>
>> That should work, we do support MVEL syntax for maps and arrays - we 
>> just don't suppor method calls, yet.
>>     
>
> Hello Mark. I just tested it and it indeed works for me.
> Although as I understand it, this will be compiled into an expression
> using eval (or it will even be only interpreted).
> So, if that is true it can't bring any performance advantage.
>   
yes, no performance benefit. As this form of use is a "return value" 
really, it should be possible to index (if we assume that nest objects 
do not change). Although it's a fair bit of work to do this, but 
interesting work - if anyone wants to help out :)
> The syntax is nothing I worry about. In Clojure, which is a Lisp, I have
> macros and can remove any obstacles in the syntax I like. It is trivial
> to develop new domain specific languages for rules. So, my rule syntax
> for Clojure will look very lispy, and each user is free to change and
> extend it. I will also allow the RHS to be written in Clojure code, no
> Java needed.
>   
Btw if you are doing "lispy" stuff:
http://blog.athico.com/2008/02/drools-clips.html
http://blog.athico.com/2008/06/drools-clips-progress.html
>
> But back to your example: I noticed something very interesting:
> when I use the MVEL dialect for the Map lookup, then I get no NPE
> anymore when I check in a Map which does *not* have the key I test
> for.
> So, now I have two versions of my rule:
>
> package droolsandclojure;
> import java.util.Map;
>
> rule "Clojure test 1"
>    when
>     m:Map()
>     eval((Integer)m.get("c") == 206)
>    then
>     System.out.println("Match: " + m);
> end
>
> and
>
> package droolsandclojure;
> import java.util.Map;
>
> rule "Clojure MVEL test 2"
>   dialect "mvel"
>    when
>     m:Map( this["c"] == 206 )
>    then
>     System.out.println("Match: " + m);
> end
>
>
> Only one of these two rules is used, not both at the same time.
> When I use rule 1 then I can not insert Maps into my session which do
> not have a key "c". If I try it and run my code I get a NPE.
>
> When I use rule 2, the MVEL version, this is different. Now I can
> insert any Maps and will not get an exception. The rule will simply
> just not execute the RHS.
>
> How can rule 1 be changed so that it will not put a constraint on the
> objects which are allowed to go into the session without throwing a NPE?
>
>   
see "Null-Safe Bean Navigation"
http://mvel.codehaus.org/MVEL+2.0+Property+Navigation
> And another interesting thing I noticed:
> to both rules I added the line
> global String s;
> and in the LHS's of both rules I removed the "c" and put s at its place.
> Then the first thing I did after creating a session was to
> (.setGlobal session "s" "c")
>
> The rule 1 accepted this change. I can use s instead of "c". But there
> is still the limitation that I can only insert Maps into the session
> which do have the String "c" as key. Otherwise: NPE.
> The rule 2 (mvel) however does not accept s as a placeholder. I get:
> Exception executing predicate this[s] == 206
> [Thrown class org.drools.RuntimeDroolsException]
>
> Any ideas how to get rid of the NPE?
>   
We'll have to look into this, probably a bug.

Mark

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