;; An Fn is {code, env}: it may have captured, and the caller neither knows
;; nor cares. A CFn is the bare code address, one word, and cannot capture.
(defn apply-fn  [f (Fn  [i32] i32)  x i32] i32 (f x))
(defn apply-cfn [f (CFn [i32] i32)  x i32] i32 (f x))

(defn bump [x i32] i32 (+ x 1))

(defn main [] ()
  (let [n 10]
    ;; An fn takes its types from the position it is written in.
    (println (apply-fn (fn [x] (+ x n)) 5))   ; captures n
    (println (apply-fn bump 5))               ; a defn captures nothing
    (println (apply-cfn bump 5))              ; so it fits either position
    ;; Widening is one way: a CFn goes where an Fn is wanted.
    (println (apply-fn (fn [x] (* x 2)) 21))))