(ns sandbox.monad)

(defn merge-meta
  [x m]
  (vary-meta x merge m))

(defn copy-meta
  [x m]
  (merge-meta x (meta m)))

(defprotocol Monad
  :extend-via-metadata true
  (bind [this f]))

(defn maybe-nothing
  [x]
  (merge-meta x {:nothing? true}))

(defn maybe-just
  [x]
  (merge-meta x {:nothing? false}))

(defn maybe-wrap-nil
  [f]
  (fn [m]
    (if-let [n (f m)]
      (maybe-just n)
      (maybe-nothing m))))

(defn maybe
  [x]
  (letfn [(bind' [this f]
            (if (:nothing? (meta this))
              this
              (merge-meta (f this)
                          (dissoc (meta this) :nothing?))))]
    (merge-meta x {`bind bind'
                   :nothing? (not x)})))

(defn or-else
  [m else]
  (if-not (:nothing? (meta m))
    m
    else))

(defn inc-if-not-3
  [[x]]
  (when-not (= x 3)
    [(inc x)]))

(def monadic-inc-if-not-3 (maybe-wrap-nil inc-if-not-3))

(or-else (bind
          (bind
           (bind
            (maybe [3])
            monadic-inc-if-not-3)
           monadic-inc-if-not-3)
          monadic-inc-if-not-3)
         "foo")