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This seems to me to be a classic example of poorly implementing first-class functions in an OO language to solve a simple problem. Or rather, this solution bestows a level of needless complexity and jargon that would make a LISPer cry.

As soon as you want to start encoding "rules" for "encoding the allowed transformations of your data," then congratulations, you've invented something called a "function." Functions are rules for expressing what you are allowed to do.

    fn wield(Wizard, Wand) // Wizards can wield wands.
    fn wield(Warrior, Sword) // Warriors can wield swords.
    fn capability(Player, wield_fn=wield(Player, Weapon)) // Players can wield weapons.

Now there's no way to violate these rules unless your users can compile functions and pass them into your code.

>There is no need to figure out “which class does the rule about paladins vs werewolves go in?” The rule goes in a rulebook, end of story. Like I said, Inform7 is amazing.

I've never seen someone so impressed by global functions before.

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(derive ::wizard ::person) (derive ::warior ::person) (derive ::weapon ::thing) (derive ::dagger ::weapon) (derive ::sword ::weapon) (derive ::staff ::weapon) ;; defines dispatch function that returns (inheritable) categories to dispath ;; it's very simple in this case (defmulti wield! (fn [p1 p2 thing] [(type p2) (type thing)])) (defmethod wield! [::person ::thing] [p1 p2 thing] (say! p1 p2 " refuse")) (defmethod wield! [::person ::dagger] [p1 p2 thing] (take! p2 p1 thing) (say! p1 p2 "accepts.")) (defmethod wield! [::wizard ::staff] [p1 p2 thing] (take! p2 p1 thing) (say! p1 p2 " gladly accepts.")) (defmethod wield! [::warrior ::sword] [p1 p2 thing] (take! p2 p1 thing) (say! p1 p2 " gladly accepts."))

define class <weapon> (<object>) end; define class <sword> (<weapon>) end; define class <wand> (<weapon>) end; define class <person> (<object>) end; define class <wizard> (<person>) end; define class <warrior> (<person>) end; define method wield (who :: <wizard>, what :: <wand>) => () ... end; define method wield (who :: <warrior>, what :: <sword>) => () ... end;