The from Conditional Element allows users to specify a source for patterns to reason over. This allows the engine to reason over data not in the Working Memory. This could be a sub-field on a bound variable or the results of a method call. It is a powerful construction that allows out of the box integration with other application components and frameworks. One common example is the integration with data retrieved on-demand from databases using hibernate named queries.

The expression used to define the object source is any expression that follows regular MVEL syntax. I.e., it allows you to easily use object property navigation, execute method calls and access maps and collections elements.

Here is a simple example of reasoning and binding on another pattern sub-field:

rule "validate zipcode"
when
    Person( $personAddress : address ) 
    Address( zipcode == "23920W") from $personAddress 
then
    # zip code is ok
end

With all the flexibility from the new expressiveness in the Drools engine you can slice and dice this problem many ways. This is the same but shows how you can use a graph notation with the 'from':

rule "validate zipcode"
when
    $p : Person( ) 
    $a : Address( zipcode == "23920W") from $p.address 
then
    # zip code is ok
end

Previous examples were reasoning over a single pattern. The from CE also support object sources that return a collection of objects. In that case, from will iterate over all objects in the collection and try to match each of them individually. For instance, if we want a rule that applies 10% discount to each item in an order, we could do:

rule "apply 10% discount to all items over US$ 100,00 in an order"
when
    $order : Order()
    $item  : OrderItem( value > 100 ) from $order.items
then
    # apply discount to $item
end

The above example will cause the rule to fire once for each item whose value is greater than 100 for each given order.

The next example shows how we can reason over the results of a hibernate query. The Restaurant pattern will reason over and bind with each result in turn:

The collect Conditional Element allows rules to reason over collection of objects collected from the given source or from the working memory. A simple example:

import java.util.ArrayList

rule "Raise priority if system has more than 3 pending alarms"
when
    $system : System()
    $alarms : ArrayList( size >= 3 )
              from collect( Alarm( system == $system, status == 'pending' ) )
then
    # Raise priority, because system $system has
    # 3 or more alarms pending. The pending alarms
    # are $alarms.
end

In the above example, the rule will look for all pending alarms in the working memory for each given system and group them in ArrayLists. If 3 or more alarms are found for a given system, the rule will fire.

The collect CE result pattern can be any concrete class that implements tha java.util.Collection interface and provides a default no-arg public constructor. I.e., you can use default java collections like ArrayList, LinkedList, HashSet, etc, or your own class, as long as it implements the java.util.Collection interface and provide a default no-arg public constructor.

Both source and result patterns can be constrained as any other pattern.

Variables bound before the collect CE are in the scope of both source and result patterns and as so, you can use them to constrain both your source and result patterns. Although, the collect( ... ) is a scope delimiter for bindings, meaning that any binding made inside of it, is not available for use outside of it.

Collect accepts nested from elements, so the following example is a valid use of collect:

import java.util.LinkedList;

rule "Send a message to all mothers"
when
    $town : Town( name == 'Paris' )
    $mothers : LinkedList() 
               from collect( Person( gender == 'F', children > 0 ) 
                             from $town.getPeople() 
                           )
then
    # send a message to all mothers
end

The accumulate Conditional Element is a more flexible and powerful form of collect Conditional Element, in the sense that it can be used to do what collect CE does and also do things that collect CE is not capable to do. Basically what it does is it allows a rule to iterate over a collection of objects, executing custom actions for each of the elements, and at the end return a result object.

The general syntax of the accumulate CE is:

<result pattern> from accumulate( <source pattern>,
                                  init( <init code> ),
                                  action( <action code> ),
                                  reverse( <reverse code> ),
                                  result( <result expression> ) )

The meaning of each of the elements is the following:

It is easier to understand if we look at an example:

rule "Apply 10% discount to orders over US$ 100,00"
when
    $order : Order()
    $total : Number( doubleValue > 100 ) 
             from accumulate( OrderItem( order == $order, $value : value ),
                              init( double total = 0; ),
                              action( total += $value; ),
                              reverse( total -= $value; ),
                              result( total ) )
then
    # apply discount to $order
end

In the above example, for each Order() in the working memory, the engine will execute the init code initializing the total variable to zero. Then it will iterate over all OrderItem() objects for that order, executing the action for each one (in the example, it will sum the value of all items into the total variable). After iterating over all OrderItem, it will return the value corresponding to the result expression (in the above example, the value of the total variable). Finally, the engine will try to match the result with the Number() pattern and if the double value is greater than 100, the rule will fire.

The example used java as the semantic dialect, and as such, note that the usage of ';' is mandatory in the init, action and reverse code blocks. The result is an expression and as such, it does not admit ';'. If the user uses any other dialect, he must comply to that dialect specific syntax.

As mentioned before, the reverse code is optional, but it is strongly recommended that the user writes it in order to benefit from the improved performance on update and retracts.

The accumulate CE can be used to execute any action on source objects. The following example instantiates and populates a custom object:

rule "Accumulate using custom objects"
when
    $person   : Person( $likes : likes )
    $cheesery : Cheesery( totalAmount > 100 )
                from accumulate( $cheese : Cheese( type == $likes ),
                                 init( Cheesery cheesery = new Cheesery(); ),
                                 action( cheesery.addCheese( $cheese ); ),
                                 reverse( cheesery.removeCheese( $cheese ); ),
                                 result( cheesery ) );
then
    // do something
end

rule "Apply 10% discount to orders over US$ 100,00"
when
    $order : Order()
    $total : Number( doubleValue > 100 ) 
             from accumulate( OrderItem( order == $order, $value : value ),
                              sum( $value ) )
then
    # apply discount to $order
end

rule "Average profit"
when
    $order : Order()
    $profit : Number() 
              from accumulate( OrderItem( order == $order, $cost : cost, $price : price )
                               average( 1 - $cost / $price ) )
then
    # average profit for $order is $profit
end

/*
 * Copyright 2007 JBoss Inc
 * 
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * 
 *      http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * Created on Jun 21, 2007
 */
package org.drools.base.accumulators;


/**
 * An implementation of an accumulator capable of calculating average values
 * 
 * @author etirelli
 *
 */
public class AverageAccumulateFunction implements AccumulateFunction {

    protected static class AverageData {
        public int    count = 0;
        public double total = 0;
    }

    /* (non-Javadoc)
     * @see org.drools.base.accumulators.AccumulateFunction#createContext()
     */
    public Object createContext() {
        return new AverageData();
    }

    /* (non-Javadoc)
     * @see org.drools.base.accumulators.AccumulateFunction#init(java.lang.Object)
     */
    public void init(Object context) throws Exception {
        AverageData data = (AverageData) context;
        data.count = 0;
        data.total = 0;
    }

    /* (non-Javadoc)
     * @see org.drools.base.accumulators.AccumulateFunction#accumulate(java.lang.Object, java.lang.Object)
     */
    public void accumulate(Object context,
                           Object value) {
        AverageData data = (AverageData) context;
        data.count++;
        data.total += ((Number) value).doubleValue();
    }

    /* (non-Javadoc)
     * @see org.drools.base.accumulators.AccumulateFunction#reverse(java.lang.Object, java.lang.Object)
     */
    public void reverse(Object context,
                        Object value) throws Exception {
        AverageData data = (AverageData) context;
        data.count--;
        data.total -= ((Number) value).doubleValue();
    }

    /* (non-Javadoc)
     * @see org.drools.base.accumulators.AccumulateFunction#getResult(java.lang.Object)
     */
    public Object getResult(Object context) throws Exception {
        AverageData data = (AverageData) context;
        return new Double( data.count == 0 ? 0 : data.total / data.count );
    }

    /* (non-Javadoc)
     * @see org.drools.base.accumulators.AccumulateFunction#supportsReverse()
     */
    public boolean supportsReverse() {
        return true;
    }

}

drools.accumulate.function.average = org.drools.base.accumulators.AverageAccumulateFunction

Forall is the Conditional Element that completes the First Order Logic support in Drools. The syntax is very simple:

forall( <select pattern> <constraint patterns> )

The forall Conditional Element will evaluate to true when all facts that match the <select pattern> match all the <constraint patterns>. Example:

rule "All english buses are red"
when
    forall( $bus : Bus( type == 'english') 
                   Bus( this == $bus, color = 'red' ) )
then
    # all english buses are red
end

In the above rule, we "select" all Bus object whose type is "english". Then, for each fact that matchs this pattern we evaluate the following patterns and if they match, the forall CE will evaluate to true. Another example:

rule "all employees have health and dental care programs"
when
    forall( $emp : Employee()
            HealthCare( employee == $emp )
            DentalCare( employee == $emp )
          )
then
    # all employees have health and dental care
end

Forall can be nested inside other CEs for complete expressiveness. For instance, forall can be used inside a not CE:

rule "not all employees have health and dental care"
when 
    not forall( $emp : Employee()
                HealthCare( employee == $emp )
                DentalCare( employee == $emp )
              )
then
    # not all employees have health and dental care
end

As a side note, forall Conditional Element is equivalent to writing:

not( <select pattern> and not ( and <constraint patterns> ) )

Also, it is important to note that forall is a scope delimiter, so it can use any previously bound variable, but no variable bound inside it will be available to use outside of it.