(Conditional) Named consequences in Drools 5.5

by Mario Fusco - September 7, 2012
Rules Article

Until now Drools rules have been always expressed in the form:

rule "name"
when
    LHS (conditional element)
then
    RHS (consequence)
end

Sometimes this could be somewhat limiting and leads to verbose and difficult to be maintained repetitions like in the following example:

rule "Give 10% discount to customers older than 60"
when
    $customer : Customer( age > 60 )
then
    modify($customer) { setDiscount( 0.1 ) };
end

rule "Give free parking to customers older than 60"
when
    $customer : Customer( age > 60 )
    $car : Car ( owner == $customer )
then
    modify($car) { setFreeParking( true ) };
end

It is already possible to partially overcome this problem by making the second rule extending the first one like in:

rule "Give 10% discount to customers older than 60"
when
    $customer : Customer( age > 60 )
then
    modify($customer) { setDiscount( 0.1 ) };
end

rule "Give free parking to customers older than 60"
    extends "Give 10% discount to customers older than 60"
when
    $car : Car ( owner == $customer )
then
    modify($car) { setFreeParking( true ) };
end

Anyway, starting from Drools 5.5, it is possible to define more labelled consequences other than the default one in a single rule, so, for example, the 2 former rules can be compacted in only one like it follows:

rule "Give 10% discount and free parking to customers older than 60"
when
    $customer : Customer( age > 60 )
    do[giveDiscount]
    $car : Car ( owner == $customer )
then
    modify($car) { setFreeParking( true ) };
then[giveDiscount]
    modify($customer) { setDiscount( 0.1 ) };
end

This last rule has 2 consequences, the usual default one, plus another one named “giveDiscount” that is activated, using the keyword do, as soon as a customer older than 60 is found in the knowledge base, regardless of the fact that he owns a car or not. The activation of a named consequence can be also guarded by an additional condition like in this further example:

rule "Give free parking to customers older than 60 and 10% discount to golden ones among them"
when
    $customer : Customer( age > 60 )
    if ( type == "Golden" ) do[giveDiscount]
    $car : Car ( owner == $customer )
then
    modify($car) { setFreeParking( true ) };
then[giveDiscount]
    modify($customer) { setDiscount( 0.1 ) };
end

The condition in the if statement is always evaluated on the pattern immediately preceding it. In the end this last, a bit more complicated, example shows how it is possible to switch over different conditions using a nested if/else statement:

rule "Give free parking and 10% discount to over 60 Golden customer and 5% to Silver ones"
when
    $customer : Customer( age > 60 )
    if ( type == "Golden" ) do[giveDiscount10]
    else if ( type == "Silver" ) break[giveDiscount5]
    $car : Car ( owner == $customer )
then
    modify($car) { setFreeParking( true ) };
then[giveDiscount10]
    modify($customer) { setDiscount( 0.1 ) };
then[giveDiscount5]
    modify($customer) { setDiscount( 0.05 ) };
end

Here I wanted to give a 10% discount AND a free parking to Golden customers over 60, but only a 5% discount (without free parking) to the Silver ones. I achieved this result by activating the consequence named “giveDiscount5” using the keyword break instead of do. In fact do just schedules a consequence in the agenda, allowing the remaining part of the LHS to continue of being evaluated as per normal, while break also blocks any further pattern matching evaluation. Note, of course, that the activation of a named consequence not guarded by any condition with break doesn’t make sense (and generates a compile time error) since otherwise the LHS part following it would be never reachable.

— updated —
To clarify some of comments. Here is an if..do rule, from above

rule "Give free parking to customers older than 60 and 10% discount to golden ones among them"
when
    $customer : Customer( age > 60 )
    if ( type == "Golden" ) do[giveDiscount]
    $car : Car ( owner == $customer )
then
    modify($car) { setFreeParking( true ) };
then[giveDiscount]
    modify($customer) { setDiscount( 0.1 ) };
end

That rule is the equivalent of the two single rules:

rule "Give free parking to customers older than 60 and 10% discount to golden ones among them"
when
    $customer : Customer( age > 60 )
    $car : Car ( owner == $customer )
then
    modify($car) { setFreeParking( true ) };
end

rule "Give free parking to customers older than 60 and 10% discount to golden ones among them - 1"
when
    $customer : Customer( age > 60 )
    eval ( $customer.type == "Golden" )
then
    modify($customer) { setDiscount( 0.1 ) };
end

The same rule but a ‘if..break’ instead:

rule "Give free parking to customers older than 60 and 10% discount to golden ones among them - 2"
when
    $customer : Customer( age > 60 )
    if ( type == "Golden" ) break[giveDiscount]
    $car : Car ( owner == $customer )
then
    modify($car) { setFreeParking( true ) };
then[giveDiscount]
    modify($customer) { setDiscount( 0.1 ) };
end

Notice the negation of the eval:

rule "Give free parking to customers older than 60 and 10% discount to golden ones among them - 1"
when
    $customer : Customer( age > 60 )
    eval ( ! ($customer.type == "Golden") )
    $car : Car ( owner == $customer )
then
    modify($car) { setFreeParking( true ) };
end

rule "Give free parking to customers older than 60 and 10% discount to golden ones among them - 2"
when
    $customer : Customer( age > 60 )
    eval ( $customer.type == "Golden" )
then
    modify($customer) { setDiscount( 0.1 ) };
end

If we take the last, more complex rule:

rule "Give free parking and 10% discount to over 60 Golden customer and 5% to Silver ones"
when
    $customer : Customer( age > 60 )
    if ( type == "Golden" ) do[giveDiscount10]
    else if ( type == "Silver" ) break[giveDiscount5]
    $car : Car ( owner == $customer )
then
    modify($car) { setFreeParking( true ) };
then[giveDiscount10]
    modify($customer) { setDiscount( 0.1 ) };
then[giveDiscount5]
    modify($customer) { setDiscount( 0.05 ) };
end

That translates into the following three rules. Notice for the free parking I explicitly add in all the negations that are implicit to the above rule.

rule "Give free parking and 10% discount to over 60 Golden customer and 5% to Silver ones - 1"
when
    $customer : Customer( age > 60 )
    eval( $customer.type == "Golden" )
then
    modify($customer) { setDiscount( 0.1 ) };
end

rule "Give free parking and 10% discount to over 60 Golden customer and 5% to Silver ones - 2"
when
    $customer : Customer( age > 60 )
    eval( !( $customer.type == "Golden" ) && $customer.type == "Silver" )
then
    modify($customer) { setDiscount( 0.05 ) };
end

rule "Give free parking and 10% discount to over 60 Golden customer and 5% to Silver ones - 3"
when
    $customer : Customer( age > 60 )
    eval( ( $customer.type == "Golden" ) || 
             ( !( $customer.type == "Golden" ) && !( $customer.type == "Silver" ) ) ) ) 
    $car : Car ( owner == $customer )
then
    modify($car) { setFreeParking( true ) };
end

We have further work coming down the line, that will allow nested patterns inside of the branch’s. You can see an early draft proposal of this here: https://community.jboss.org/wiki/BranchCEs

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