Hi Richard, The "when" section matches fact (object) patterns, so your requirement could be written as:- when there is a Test Fact 'A' and another Test Fact 'B' where "the 'x' property of 'A'" equals "the 'y' property of fact 'B' plus 1" then..... This would become:- rule "Rule 1" when Test( $y : y ) Test( x == ($y + 1) ) then System.out.println("Rule 1 matches"); end By default both patterns can match the same object, so test data:- A : x = 1, y = 1 B : x = 2, y = 1 Will cause the rule to activate twice:- A and B B and B With kind regards, Mike _____ From: rules-users-bounces(a)lists.jboss.org [mailto:rules-users-bounces@lists.jboss.org] On Behalf Of Richard Bremner Sent: 02 May 2008 08:53 To: rules-users(a)lists.jboss.org Subject: [rules-users] newbie question on "when" condition syntax Hi, say I have a class with two fields, x and y: public class Test { int x = 1; int y = 2; getters/setters... } I would like a rule which fires when y = x + 1 my initial thought would be something like: rule "Rule 1" when test : Test (y == x + 1) then System.out.println("Rule 1 matches."); end but this is invalid syntax and I can not find any examples of such a rule. I'm doing my best but reading the manual etc. I'm struggling with the syntax here and can't figure it out. any help really appreciated!

Hi Richard, Glad I can be of help (this forum as a whole is actually very helpful). Reading your email you understand me correctly (you even picked up on the implicit AND between pattern matches). With a bit of imagination complex problems can be solved with Drools; just like programming you need to adjust your mindset sometimes and try a different tact. Personally, if your rules are not user-facing, I wouldn't worry about DSL - even if they do don't worry for the time being! Make sure to read the excellent manual, many people have put a lot of work into this and it is invaluable. Read about "dialect" to answer your question about rule language, to save me copy and pasting. Good luck, Mike _____ From: rules-users-bounces(a)lists.jboss.org [mailto:rules-users-bounces@lists.jboss.org] On Behalf Of Richard Bremner Sent: 02 May 2008 11:38 To: Rules Users List Subject: Re: [rules-users] newbie question on "when" condition syntax Hi Mike, Thanks very much, after a few minutes of staring I think I have understood your very well put answer! If I understand you right, what is happening is: Test( $y : y ) This means, WHEN there is an instance of Test, assign the value of its "y" property to $y AND WHEN there is an instance of Test where it's "x" property is equal to $y + 1 THEN fire. I think I understand that. I am going to need to do this with some pretty complicated formulae, containing up to 8 different variables. I have a fixed data set and need to find certain patterns in them based on the formulae. I have most of the program figured out, my only trouble is expressing my formulae in the correct syntax. I will try to extend this principle you have explained to me and hope it can cope with the extra complication... unless there would be a better way to achieve this? I am really sorry but I am totally new to drools - but am committed to using it on a project so I guess I'm going to get to know it pretty well :-) I read briefly that one can define a DSL, I might look at this eventually to build an easier mechanism for expressing my formulae - if that can be achieved using a DSL. Can the whole rule be defined in Groovy/Java - if so I might be easier doing that. I have so much work to do! many thanks Richard 2008/5/2 Anstis, Michael (M.) <manstis1(a)ford.com&gt;: Hi Richard, The "when" section matches fact (object) patterns, so your requirement could be written as:- when there is a Test Fact 'A' and another Test Fact 'B' where "the 'x' property of 'A'" equals "the 'y' property of fact 'B' plus 1" then..... This would become:- rule "Rule 1" when Test( $y : y ) Test( x == ($y + 1) ) then System.out.println("Rule 1 matches"); end By default both patterns can match the same object, so test data:- A : x = 1, y = 1 B : x = 2, y = 1 Will cause the rule to activate twice:- A and B B and B With kind regards, Mike _____ From: rules-users-bounces(a)lists.jboss.org [mailto:rules-users-bounces@lists.jboss.org] On Behalf Of Richard Bremner Sent: 02 May 2008 08:53 To: rules-users(a)lists.jboss.org Subject: [rules-users] newbie question on "when" condition syntax Hi, say I have a class with two fields, x and y: public class Test { int x = 1; int y = 2; getters/setters... } I would like a rule which fires when y = x + 1 my initial thought would be something like: rule "Rule 1" when test : Test (y == x + 1) then System.out.println("Rule 1 matches."); end but this is invalid syntax and I can not find any examples of such a rule. I'm doing my best but reading the manual etc. I'm struggling with the syntax here and can't figure it out. any help really appreciated! _______________________________________________ rules-users mailing list rules-users(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/rules-users

On Fri 2.May'08 at 10:05:20 +0100, Anstis, Michael (M.) wrote: Actually, please correct me if I am mistaken, but is this condition correct? I believe that the original question is that for a single instance of class Test t, fire the rule when t.x == t.y + 1 (where 't' refers to the *same* instance of class Test). I believe the rule above would match any instance of class Test whose x value is equal to the value of the instance field y from *any* instance of class Test plus 1. Does that make sense?

Yes, newest mvel version works fine, but that problem only happened when using mvel dialect. For java, just do as Chris mentioned: when Test( y == (x+1) ) then And it will work. The () is needed to tell the engine that what is inside is a semantic code block, i.e., an expression that will return a value to be compared to y. []s Edson 2008/5/2 Chris Woodrow <woodrow.chris(a)gmail.com&gt;: -- Edson Tirelli JBoss Drools Core Development Office: +55 11 3529-6000 Mobile: +55 11 9287-5646 JBoss, a division of Red Hat @ www.jboss.com

Thanks Chris, that does work. oh man, I tried something so similar to that with the parenethisis last night using my "programming by trial and error" methodology. many thanks to everyone who replied, it's all been a great help! Richard Chris Woodrow wrote: -- View this message in context: http://www.nabble.com/newbie-question-on-%22when%22-condition-syntax-tp17... Sent from the drools - user mailing list archive at Nabble.com.

Ahhh I wondered about that, thanks. Again, now I see it, it makes complete sense. This drools stuff is actually pretty damn good! Richard Chris Woodrow wrote: -- View this message in context: http://www.nabble.com/newbie-question-on-%22when%22-condition-syntax-tp17... Sent from the drools - user mailing list archive at Nabble.com.