[rules-users] Golfer example finds two identical solutions - the explanation

Hi there, some people complained that the Golfer-example from the JBoss-Drools distribution finds two identical solutions. I investigated a little and found the "problem": The rule is underspecified: It binds five Golfer objects, but only four of those are completely specified by their respective condition elements. The fifth, the "Freds right neighbour", lacks that completeness... What happens is the following: The "unknown golfer" can be bound to two different facts: I had the "unknown" golfer printed out - and see what happened: Fred 1 orange, Joe 2 blue, Bob 4 plaid, Tom 3 red unknown Tom 2 blue Fred 1 orange, Joe 2 blue, Bob 4 plaid, Tom 3 red unknown Joe 2 blue As the unknown golfer is NOT compared to the actual neighbor, Joe, there are two possible instantiations for our "unknown" golfer... The solution would be to constraint the unknown-golfer (= Fred's right neighbour) to the set of (Bob, Joe, Tom)... an eval sounds great - too bad the "in" constraint doesn't work... // freds right neighbour is either Tom, Bob or Joe! eval (($fn == $joe) || ($fn == $bob) || ($fn == $tom)) The simple (but un-elegant) solution is setting an agenda-group rule- attribute, prohibing the rule from firing twice. In his book on JESS, E. Friedemann-Hill shows JESS to deliver only a single solution ... I translated the Drools-version literally to his JESS version (I had to create GolferColor and GolferPosition classes, but needed only 32 facts in working memory, instead of 64 with the combined Golfer-drools version).... regards, Gernot Dr. Gernot Starke Doing IT Right --- Willi-Lauf Allee 43, D-50858 Köln gs(a)gernotstarke.de, +49 (0) 177 - 728 2570 http://www.gernotstarke.de Blog: http://it-and-more.blogspot.com **************************************** Das freie Portal für Software-Architekten: http://www.arc42.de