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

multimethod_selector.clj

(require '[clojure.spec.gen.alpha :as sgen])

;; original
(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))))))))

;; (marginally) simpler?
(defn multimethod-selector2
  "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/return ::unused)
    (fn [_]
      (sgen/elements (keys (methods s))))))

(defmulti sound first)
(defmethod sound :dog [_] "woof")
(defmethod sound :cat [_] "meow")

(def gen ((multimethod-selector sound)))
(def gen2 ((multimethod-selector2 sound)))

(gen/sample gen)  ;; (:cat :dog ...)
(gen/sample gen2) ;; (:cat :dog ...)

(defmethod sound :duck [_] "quack")

(gen/sample gen)  ;; (:cat :duck :dog ...)
(gen/sample gen2) ;; (:cat :duck :dog ...)