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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