+; Copyright 2008 (c) Meikel Brandmeyer.
+; 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
+; all copies or substantial portions of the Software.
+; THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+; IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+; FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+; AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+; LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+; OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ [clojure.contrib.def :only (defvar)]))
+ "nothing represents the empty value of the maybe monad."}
+ (return ::type `nothing))
+ "just returns a monad representing the passed-in something."
+ (return ::type [something]))
+ "Predicate for testing, whether a given monad represents nothing."
+ "Predicate for testing, whether a given monad represents some value."}
+ "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."
+ (throw (Exception. "Tried to retrieve a value from nothing"))))
+ (if (is-just? m) (first m) default)))
+ (if (= `nothing m) m (f (first m))))