Monads

core.clj

(ns monads.core
  (:use [clojure.algo.monads
         :only (domonad with-monad m-lift m-seq m-reduce m-when
                identity-m
                sequence-m
                maybe-m
                state-m fetch-state set-state
                writer-m write
                cont-m run-cont call-cc
                maybe-t)]))

;; identity monad

(with-monad identity-m (m-bind 10 inc))

;; sequence monad

(with-monad sequence-m (m-bind [1 2 3] (fn [x] [(inc x)])))

(with-monad sequence-m
  (m-bind [1 2 3] (fn [x]
                    (m-bind [4 5 6] (fn [y] [x y])))))

(domonad sequence-m
  [x [1 2 3]
   y [4 5 6]]
  [x y])

(for [x [1 2 3]
      y [4 5 6]]
  [x y])

;; maybe monad

(reduce + [1 nil 3])       ;; => Null Pointer Exception

(with-monad maybe-m
  (m-reduce + [1 nil 3]))  ;; => nil

(domonad maybe-m
         [x 1
          y 2
          z 3]
         (+ x y z))