Drools 5.6 mvel null safe property navigation

Question

I am trying to upgrade drools from version 5.1 to 5.6, in 5.1 we used mvel null safe property access as below, but after upgrading mvel property access operator (.?) no more recognised by drools. Is there any other way to achieve this in 5.6? Thanks.

rule "Hello world"
    salience -1
    dialect "mvel"
when
    $c : Hello ( eval( a.?b.?c.?d == null ) )
then
    invalidate($c, "d is missing");
end

Answer 1

Use !. , available in 5.5.0, possibly earlier.