Re: [rules-users] Easy mistake... - drools operator precedence

On 02/18/2013 02:35 PM, Geoffrey De Smet wrote: > > Op 18-02-13 14:00, Willem van Asperen schreef: >> On 02/18/2013 11:11 AM, Willem van Asperen wrote: >>> Hi All, >>> >>> Just wanted to share with you an easy mistake. This is done running >>> Drools 5.5.0.Final. >>> >>> Imagine a fact >>> >>> |Cheese (|| >>> || creationTick : double|| >>> || preservationTicks : double|| >>> ||)| >>> >>> Then the following does not do what you expect: >>> >>> |rule "dump old cheese"|| >>> || when|| >>> || $c : Cheese( $sbd : creationTick + preservationTicks )|| >>> || CurrentTime( tick >= $sbd )|| >>> || then|| >>> || System.out.println("||we have to dump "+$c);|| >>> ||end| >>> >>> You would expect that $c is dumped when current time has passed >>> creationTick + preservationTicks. But no. The variable $sbd is >>> bound to creationTick /before/ the preservationTicks are added!! I >>> must say that I do not quite understand how ($sbd : creationTick) + >>> preservationTicks resolves to "true" to make the premise succeed... >>> Maybe because it is != 0? >>> >>> I found that it should be: >>> >>> |rule "dump old cheese"|| >>> || when|| >>> || $c : Cheese( $sbd : (creationTick + preservationTicks) )|| >>> || CurrentTime( tick >= $sbd )|| >>> || then|| >>> || System.out.println("we have to dump "+$c);|| >>> ||end| >>> >>> This makes sense. Now $sbd is bound to the result of the addition. >>> But it is an easy trap!! >>> >>> Regards, >>> Willem >> Hi All, >> >> Coming back to this... What will be evaluated in the following snippet: >> >> rule "dump old cheese" >> when >> CurrentTime( $tick : tick ) >> $c : Cheese( $tick >= creationTick + preservationTicks ) >> then >> System.out.println("we need to dump "+$c); >> end >> >> I assume this evaluates if $tick is larger or equal to (creationTick >> + preservationTicks), right? > I think so too. > It looks like the : operator has a higher precedence then + (and > therefor >= too), which is a bit surprising indeed. > Personally, I never mix the : operator with other operators, to avoid > having to worry about the precendence. So you would write: |rule "dump old cheese"|| || when|| || $c : Cheese( $ct : creationTick, $p : preservationTicks )|| || CurrentTime( tick >= $ct + $p )|| || then|| || System.out.println("we have to dump "+$c);|| ||end |

Assuming + takes precedence over >=. The reason I am binding the sum to creationTick and preservationTicks to a variable is that I am using that variable in multiple places and do not want the engine to have to calculate the sum for every occurrence. Also: it makes the rule more readable in my view. >> Otherwise it would say >> >> step 1: $tick > creationTick + preservationTicks >> step 2: true + preservationTicks >> >> which would fail, obviously because you cannot add a double to a >> boolean. >> >> Regards, >> Willem >> >> >> _______________________________________________ >> rules-users mailing list >> rules-users(a)lists.jboss.org >> https://lists.jboss.org/mailman/listinfo/rules-users > _______________________________________________ rules-users mailing list rules-users(a)lists.jboss.org https://lists.jboss.org/mailman/listinfo/rules-users