The LHS of your rule is wrong. I think you need to use an eval() if you want to invoke methods that contains parameters.
I also remembered another way to get process variables in the RHS, but I don't remember if it has some caevats:
when
$processInstance: WorkflowProcessInstance()
then
String monkey = (String)$processInstance.getVariable("monkey"); //This will return "chimp"
$processInstance.setVariable("character","nice");
end