A Thorny Issue In Clojure Java Interop

thorny.clj

(ns a-thorny-issue.core
  (require [clojure [reflect :as r] [pprint :as pp]]
           [clojure.java [javadoc :as jd]])
  (import [java.util.concurrent Executors Future ExecutorService]))

(comment (pp/print-table [:name :parameter-types :return-type] 
                         (sort-by :name (:members 
                                         (r/reflect java.util.concurrent.ExecutorService))))

         (jd/javadoc java.util.concurrent.ExecutorService))

(def an-executor (Executors/newSingleThreadExecutor))

(defn submit-callable [c ex]
  (.submit ex c))

(defn some-hard-work []
  34)

(instance? Callable some-hard-work)
(instance? Runnable some-hard-work)

(instance? Future (submit-callable some-hard-work an-executor))

(.get (submit-callable some-hard-work an-executor))

(defn submit-callable-reify [c ex]
  (.submit ex (reify Callable (call [_] (c)))))

(instance? Future (submit-callable-reify some-hard-work an-executor))

(.get (submit-callable-reify some-hard-work an-executor))

(defn submit-callable-hint [^Callable c ex]
  (.submit ex c))

(defn submit-callable-hint2 [c ex]
  (.submit ex ^Callable c))

(defn submit-callable-cast [c ex]
  (.submit ex (cast Callable c)))

(defn submit-callable-hint3 [c ex]
  (let [^Callable the-fn c]
    (.submit ex the-fn)))

(defn submit-callable-hint4 [c]
  (let [ex (Executors/newSingleThreadExecutor)
        ^Callable the-fn c]
    (.submit ex the-fn)))

(defn submit-callable-hint5 [c ex]
  (let [the-ex ex
        ^Callable the-fn c]
    (.submit ex the-fn)))

;; Success!!
(defn submit-callable-hint6 [^Callable c ^ExecutorService ex]
  (.submit ex c))