The idea was the Parameter was in the routing rule itself, this way the function getFunctionValue would be run at compile time (as it's value won't change) rather than running it each time within the rule definition.

e.g.

Class MyRoutingRule Extends RuleDefinition {

Parameter myClassParameter = {#class(FunctionSet).GetFunctionValue()}

XData RuleDefinition {
  //use myClassParameter in here
}

}

I thought I'd also tried using the $parameter function as well, but perhaps I have to specify the class name as the first argument, even if the class is itself?

Thanks.