example.clj

(effect Rand
  (rand-int :: (-> int int))
  (rand     :: (-> float)))

(define with-rng :: (forall [A e] (-> int (-> A ! <Rand ...e>) A ! <...e>))
  (fn [seed thunk]
    (let [_ (_rng-seed seed)]
      ((handler
        (return v -> v)
        ((rand-int n) k -> (resume k (_rand-int n)))
        ((rand)       k -> (resume k (_rand))))
       thunk))))

;; toy impl, have no arrays or transients
(define shuffle :: (forall [F A] :where [(Seq F)] (-> (F A) (Vector A) ! <Rand>))
  (fn [xs]
    (loop [remaining (seq->vec xs) acc []]
      (if (empty? remaining)
        acc
        (let [i (rand-int (count remaining))]
          (recur (seq-append (take i remaining)
                             (drop (+ i 1) remaining))
                 (conj acc (nth remaining i))))))))

(with-rng 27 (fn [] (shuffle '(1 2 3 4))))) ; gradually typed, so no effect annotation necessary