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

[Geoffrey De Smet]

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
>
>
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