lazy-macro.clj

(defmacro lazy
  "Returns a lazy sequence of calls to (f). Arguments are evaluated once as needed.
   (lazy f a b c d) --> (a, (f a b), (f (f a b) c), (f (f (f a b) c) d))"
  ([f x]
    `(lazy-seq
      (list ~x)) )
  ([f x form & more]
    `(lazy-seq
      (let [x# ~x]
        (cons x#
          (lazy ~f (~f x# ~form) ~@more)))) ) )

(defmacro lazy->  [& args] `(lazy ->  ~@args))
(defmacro lazy->> [& args] `(lazy ->> ~@args))

(def ex1 (lazy + 1 2 3 4 5 6))
(def ex2 (lazy * 1 2 3 4 5 6))
(def ex3 (lazy-> 1 (+ 1) (+ 4) (- 2)))
(def ex4
  (lazy->>
    (#(do (println "(Evaluating first)") 1))
    (#(do (println "(Evaluating second)") (+ 1 %)))
    (#(do (println "(Evaluating third)") (- 2 %)))))