return example

example.clj

(ns example
  (:require
   [cloroutine.core :refer [cr]]))

;; ref: https://github.com/leonoel/cloroutine

(def ^:dynamic *tail*)

(defn gen-seq [gen]
  (binding [*tail* (lazy-seq (gen-seq gen))]
    (gen)))

(defn return [x]
  (cons x *tail*))

(defn no-op [] nil)

(defmacro generator [& body]
  (let [cnt (count body)
        [front back] (split-at (dec cnt) body)]
    `(gen-seq (cr {return no-op} (do ~@front (return ~@back)) nil))))

(defmacro defn-x [name args & body]
  `(defn ~name ~args
     (-> (generator
           ~@body)
         (first))))

(defn-x helloworld [a]
  (when (even? a)
    (return a))
  :b)

(helloworld 10)
;;=> 10
(helloworld 9)
;;=> :b