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

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.

> 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