[rules-users] Best practice for 0..1 relations

[Christopher Dolan]

What's the best way to encode a fact that's needed in the RHS but is not important in the LHS?

Consider a contrived example of computing total household income for single or married persons. I can think of two ways to encode this rule, but I don't like either of them:

Style 1: one rule for each scenario
    rule "household income, single"
        when
            $p1 : Person()
            not Relation(person1 == $p1, type == "spouse")
        then
            insertLogical(new Income($p1.getIncome()));
    end
    rule "household income, married"
        when
            $p1 : Person()
            Relation(person1 == $p1, type == "spouse", $p2: person2)
        then
            insertLogical(new Income($p1.getIncome() + $p2.getIncome()));
    end

Style 2: a single rule with a collection
    rule "household income "
        when
            $p1 : Person()
            $rels : List() from collect(Relation($p1 == person1, type == "spouse"))
        then
            insertLogical(new Income($p1.getIncome() + ($rels.size() == 0 ? 0 : $rels.get(0).getPerson2().getIncome()));
    end


(please ignore the bug that the income may get inserted twice because people are spouses of each other)


Style 1 is more verbose, but more straightforward: it's how I think of the problem intuitively. Style 2 is much more compact, and is more maintainable if I need to add more predicates or a more complicated RHS. But the idea of needing a List when I know there will be exactly 0 or 1 related facts just seems wrong.

I've searched for some LHS syntax that assigns a variable without participating in boolean evaluation, but I've failed to find anything.

Chris