[rules-users] Constants?

[Wolfgang Laun]

 function int slackTicks(){ return 42; }  // oops

On 19/02/2013, Wolfgang Laun <wolfgang.laun at gmail.com> wrote:
> 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
>>
>