exists

The exists keyword tests whether a class condition is true.

Purpose

This keyword is used in the condition part of a rule to test whether the condition is true.

Context

Rule conditions

Syntax

exists condition;

Description

Use the exists statement in the condition part of a rule, on a simple condition, an in statement, or a from statement. The exists statement tests whether the condition is true. The contrary of exists is not (avoid not exists).

The exists statement returns true when any object in the working memory matches the condition.

It returns true for a simple condition if the working memory contains at least one object that can match the condition.
It also returns true with an in or from statement when the in or from statement returns true.

Because the condition does not discriminate which object was matched, you cannot bind an exists statement to an external variable. Any variable bound within the exists statement is local to the statement. No statement of the rule can contain any reference to the bound variable.

You can use the special variable ?this within the exists statement to designate the current working memory object being tested.

Example

The GeneralCustomerRequest rule uses the exists statement to verify that an item requested by a customer exists. The exists statement returns true when a single instance of the object is found, which is faster than providing all the items that match the customer’s request.

rule GeneralCustomerRequest {
   when {
      CustomerRequest(?item:request);
      exists Item(?this == ?item);
   }
   then {
      System.out.println(?item + " are in stock.");
   }
};

In the CustomerRequest condition, the variable ?item is matched with the field request. In the exists statement, the Item object is represented by the variable ?this. The statement tests whether the Item object is equal to the variable ?item. If an Item object is matched, the println command in the action part prints that the items are in stock.