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

[Geoffrey De Smet]

Op 18-02-13 16:34, Willem van Asperen schreef:
> 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
> |
Yes, although I ' d probably create a method 
Cheese.getCreationTickPlusPreseverationTicks() (with a better name)
and use that.
> 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
>>>
>>>
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>>
>
>
>
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