rule

The rule keyword declares a rule.

Purpose

This keyword is used to declare a rule.

Context

At the top level of rulesets

Syntax

rule ruleName {[priority = value;] 
               [property propertyName = value;]
when {condition1 ... conditionn [evaluate (expression)]}
then {[action1 ... actionm]}
[else {[action1 ... actionp]}]
};

Description

A rule in IRL is composed of three parts:

A header part, which defines the name of the rule, its priority, and properties.
The condition part, which begins with the keyword when, also referred to by its side of the rule.
The action part, which begins with the keyword then, also referred by its side of the rule.

When the condition part ends with an evaluate statement, the action part can have an else part, executed if the evaluate statement returns false. If it returns true, the then part is executed.

If a formal comment (/**...*/) precedes the rule definition, the comment is saved so that it can be retrieved later using the API.

Example

rule VideoCallBilling {
   when {
      ?c:Customer(?p:phoneNo);
      ?v:collect (new videoCallCollection())
         Usage(phoneNo == ?p; type == videoCall)
         where (size() > 0);
   }
   then {
      float ?t = 0.;
      Enumeration ?enum = ?v.elements();
      while (?enum.hasMoreElements()) {
         Usage ?x = (Usage)enum.nextElement();
         ?t += ?x.charge(); 
      }
      System.out.println("Dear "+ ?c.name + "Your bill for
                         Video conference calls is : "+ ?t);
   }
};

The VideoCallBilling rule collects appropriate Usage objects, sums the charge, and prints the result with the client’s name. The first condition returns the Customer object in variable ?c, with the field phoneNo stored in variable ?p. The second condition is a collect statement. This statement returns a videoCallCollection object in variable ?v. This collection object contains Usage objects with the field phoneNo equal to the variable ?p, phoneNo from the previous condition, and field type equal to videoCall. The where part of the collect statement verifies that one or more Usage objects have been collected. The then part of the rule declares two variables ?t, initialized to 0, and variable ?enum, set to the elements of the collection using the default collection method elements(). Using the while statement, the elements of the collection are enumerated and the Usage objects are accessed and referenced using the variable ?x. Variable ?t and the method charge()are used to sum the charge for each Usage object. The println statement prints the client’s name and the total charge.

rule CheckTemperature {
    priority = 1,000,000 + 1;
    when {
       Sensor(type==Temperature; value>150);
    }
    then {
       insert Alarm();
    }
  };

The CheckTemperature rule has a priority of 1,000,001. If an object Sensor with a field type equal to Temperature has a value greater than 150, the condition part is true and the action part is executed. An object Alarm is inserted into the working memory using the command insert.