http://blog.athico.com/2012/09/conditional-named-consequences-in.html
(Conditional) Named consequences in Drools 5.5
Posted by Mario Fusco
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/elsestatement:
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._______________________________________________
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