gistfile1.clj

(ns cara.type.core)

;(declare Nothing Just)

(defprotocol PDataType
  (get-constructors [self]))

(defprotocol PConstructor
  (get-datatype [self]))

(declare Maybe)

(deftype Nothing []
  PConstructor
  (get-datatype [this]
    Maybe))

(defprotocol PJust
  (from-just [this]))

(deftype Just [value]
  PConstructor
  (get-datatype [this]
    Maybe)
  PJust
  (from-just [this]
    value))

(def Maybe (reify
             PDataType
             (get-constructors [this]
               [Nothing Just])))

(def nothing (Nothing.))
(defn just [value]
  (Just. value))

(defn maybe-case* [value nothing-func just-func]
  (cond (instance? Just value) (just-func (from-just value))
        (instance? Nothing value) (nothing-func)
        :else (throw (Exception. (str value " is not an instance of the Maybe data type")))))

(defn t []
  (dotimes [_ 100000]
    (maybe-case* (just :a) #(str "nothing") #(str "just " %))))

(defprotocol PMonad
  (return [self value]))

(defprotocol PMonadValue
  (bind [self func]))

(def MaybeM
  (reify PMonad
    (return [self value]
      (just value))))

(extend-type Just
  PMonadValue
  (bind [self func] (func (from-just self))))

(extend-type Nothing
  PMonadValue
  (bind [self func] nothing))