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
[EMAIL PROTECTED],
+49 (0) 177 - 728 2570
http://www.gernotstarke.de
Blog: http://it-and-more.blogspot.com
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