hacking maybe-monad in protocol-monads

gistfile1.clj

#_(defn plus [[mv & mvs]]
  (plus-step mv mvs))

(defmacro plus [mvs]
  `(let [mvs# (quote ~mvs)
         mv# (eval (first mvs#))]
     (monads.core/plus-step mv# (rest mvs#))))

(deftype maybe-monad [v]
  clojure.lang.IDeref
  (deref [_]
    v)

  Monad
  (do-result [_ v]
    (maybe-monad. v))
  (bind [mv f]
    (if (= mv maybe-zero-val)
      maybe-zero-val
      (f @mv)))

  MonadZero
  (zero [_]
    maybe-zero-val)
  (plus-step [mv mvs]
    (let [mv (loop [mv mv
                    rvs mvs]
               (let [mv (if (= (class mv) maybe-monad)
                          mv
                          (eval mv))]
                 (if (not= maybe-zero-val mv)
                   mv
                   (if (empty? rvs)
                     nil
                     (recur (first rvs)
                            (rest rvs))))))]
      (if (nil? mv)
        maybe-zero-val
        mv))))

;; the test will now pass, i.e. it won't throw an exception

(deftest maybe-plus
  (is (= :bogus
         @(m/plus [(m/maybe :bogus)
                   (m/do m/maybe
                         [_ (m/maybe 1)]
                         (throw (Exception. "Should not be thrown")))]))))