A clojure.spec.gen.alpha generator that picks a multimethod implementation from the known set.

multimethod_selector.clj

(defn multimethod-selector
  "Returns a generator that picks one dispatch value from the known
   dispatch values of a multimethod. Defers the lookup of dispatch
   values until sampling time, so any defmethods evaluated after the
   generator is created may still be selected."
  [s]
  #(sgen/bind
     (sgen/int)
     (fn [i]
       (let [ms (keys (methods s))]
         (sgen/return (nth ms (mod i (count ms))))))))