1. Meikel Brandmeyer
  2. monad

Source

monad / src / de / kotka / monad / maybe.clj

;-
; Copyright 2008 (c) Meikel Brandmeyer.
; All rights reserved.
;
; Permission is hereby granted, free of charge, to any person obtaining a copy
; of this software and associated documentation files (the "Software"), to deal
; in the Software without restriction, including without limitation the rights
; to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
; copies of the Software, and to permit persons to whom the Software is
; furnished to do so, subject to the following conditions:
;
; The above copyright notice and this permission notice shall be included in
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;
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; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
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; THE SOFTWARE.

(clojure.core/ns de.kotka.monad.maybe
  (:use
     de.kotka.monad
     [clojure.contrib.def :only (defvar)]))

(declare Nothing Just)

(derive `Nothing `MZero)
(derive `Just    `Monad)

(defvar
  nothing
  (make-monad `Nothing nil)
  "nothing represents the empty value of the maybe monad.")

(defn is-nothing?
  "Predicate for testing, whether a given monad represents nothing."
  [m]
  (= (monad-type m) `Nothing))

(defn is-just?
  "Predicate for testing, whether a given monad represents some value."
  [m]
  (= (monad-type m) `Just))

(defmulti
  #^{:arglists '[(monad) (monad default)]
     :doc
  "Retrieve the value from a maybe monad. Returns the default in case the monad
  represents nothing. Or throws an exception in case no default was given."}
  from-maybe
  #(monad-type %))

(defmethod from-maybe `Just
  [m & _]
  (monad m))

(defmethod from-maybe `Nothing
  ([_]
   (throw (Exception. "Tried to retrieve a value from nothing")))
  ([_ default]
   default))