"By opaque, do you mean encapsulated?" By opaque, I mean the only thing you can do is execute them. While this is encapsulated, it isn't at the right level of

"By opaque, do you mean encapsulated?" By opaque, I mean the only thing you can do is execute them. While this is encapsulated, it isn't at the right level of control. " but the GUI layer is (imo) definitely the wrong place to do it. When's the last time you wanted to filter, prioritize, secure, or distribute UI events?" Ironically, the GUI is already that way. Under the hood, enumerated values were passed to your program by the windowing system. It has to be, because there's a serialization layer there. GUI programs already have no choice but to be structured this way, and indeed, also certainly have some sort of filtering mechanism in them to do things like ignore events it doesn't want to handle. You're probably programming at an abstraction layer that is already sitting above that. You think this isn't useful in a GUI precisely because the GUI framework you're using implemented this pattern for you already! "Your hybrid idea is interesting! Is that doable with Rust in practice?" Really it's just creating objects that have various inspectable attributes as needed, that also have some trait or something that allows them to be called in a uniform manner. It's not too hard, it's just a bit more structure around what is already there; you basically copy & paste the closure into the method of a new object, and make the capture explicit in the object.