check-let.clj

(defmacro check-let [b-vec & body]
  (if-not (empty? b-vec)
    (let [v (take 3 b-vec)
          r (apply vector (drop 3 b-vec))]
      `(if-let [~@(take 2 v)]
         (check-let ~r ~@body)
         ~(nth v 2)))
    `(do ~@body)))

;; turns this:

(if-let [a 1]
  (if-let [b 2]
    (if-let [c 3]
      (if-let [d 4]
        {:success true :result (+ a b c d)}
        {:success false :message "Couldn't find D"})
      {:success false :message "Couldn't find C"})
    {:success false :message "Couldn't find B"})
  {:success false :message "Couldn't find A"})

;; into this:

(check-let [a 1    {:success false :message "Couldn't find A"}
             b 2    {:success false :message "Couldn't find B"}
             c 3    {:success false :message "Couldn't find C"}
             d 4    {:success false :message "Couldn't find D"}]
  {:success true :result (+ a b c d)})



;;
;; real world example: web request handler that inserts a new record
;;


;; this:

(defn new-addition [thought-id]
  (fn [req]
    (if-let [addition-text (:comment (:params req))]
      (if-let [thought (find-thought! thought-id)]
        (if-let [user (current-user! req)]
          (let [addition (make-addition addition-text thought user)]
            (insert! :additions addition)
            (json-success (merge addition {:html (render-addition addition)})))
          (json-fail "No valid user."))
        (json-fail "No thought by that id."))
      (json-fail "Text can't be blank."))))

;; into this:

(defn new-addition [thought-id]
  (fn [req]
    (check-let [addition-text (:comment (:params req))   (json-fail "Text can't be blank.")
                thought       (find-thought! thought-id) (json-fail "No thought by that id.")
                user          (current-user! req)        (json-fail "No valid user.")]
      (let [addition (make-addition addition-text thought user)]
        (insert! :additions addition)
        (json-success addition)))))