I think the reasonable alternative here is Clojure's metadata tagging, which would allow for more "principled" execution. Saying "it all has to be in the types

I think the reasonable alternative here is Clojure's metadata tagging, which would allow for more "principled" execution. Saying "it all has to be in the typesystem" can _also_ become a cludge (see languages that have built-in null handling vs languages that just require you to use Option and the like)