[rules-users] Constants?

[Wolfgang Laun]

Here's another one for your collection of constant workarounds:
   function int slackTicks(){ returns 42; }
-W

On 18/02/2013, Willem van Asperen <willem at van.asperen.org> wrote:
> Hi All,
>
> I have a bunch of rules that evaluate using some constant variable (say
> "slackTicks"). Currently, in my rules, I write down the value of this
> "slack" each and every time I use it, for example:
>
> |TransferSchedule( tick > currentTick - 10d )|
>
> where 10 is the slack.
>
> A solution would be to insert constant facts:
>
> |Constant( key = "slack", $slackTicks : value )||
> ||TransferSchedule( tick > currentTick - $slackTicks )|
>
> Or inject a global properties object:
>
> |global Properties constants||
> ||
> ||$slackTicks : Double.valueOf(constants.getPropertyValue("model.slack",
> "0"))|
>
> Or, the last one I can think of: a public final static on the fact class:
>
> |TransferSchedule( tick > currentTick - TransferSchedule.SLACK_TICKS )||
> |
> These all cross the boundary between Java and Drools. In my Java code I
> have to insert the Constant facts or inject the global. When my drools
> developers think of some new ways to create the rules, I run the risk of
> having to go in and modify the .properties file or, worse, change the
> Java code.
>
> To be able to define a "constant" would be a clean solution:
>
> |declare||constant double SLACK_TICKS = 10||;
> ||||
> ...
> ||TransferSchedule( tick > currentTick - SLACK_TICKS )||
> ...
> |
> This separates the rules from the Java code and give the rules developer
> all freedom to do whatever they want to make constant.
>
> Ah, just thought of another way to do it. You can insert the facts from
> within Drools itself:
>
> |declare Constant||
> ||    key : String||
> ||    value : Object||
> ||end||
> ||
> ||rule initialize||
> ||    salience 100000||
> ||    when||
> ||        // always||
> ||    then||
> ||        insert( new Constant( "slack", 10d ) );||
> ||        //... other constants go here||
> ||end||
> |
> I will still have to "capture" the constants that I need in every rule
> with a Constants( key == "...", $c : value ) premise, though... And you
> would need to add some more Constant classes to cater to contain type
> safety...
>
> What's your thought? What strategy do you use?
>
> Regards,
> Willem
>