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Meikel Brandmeyer committed 6bb3f72

Added the error monad

* monad/error.clj: new file

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 			<arg value="de.kotka.monad"/>
 			<arg value="de.kotka.monad.maybe"/>
 			<arg value="de.kotka.monad.state"/>
+			<arg value="de.kotka.monad.error"/>
 		</java>
 	</target>
 

src/de/kotka/monad/error.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
+; 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
+; THE SOFTWARE.
+
+(clojure.core/ns de.kotka.monad.error
+  (:use
+     de.kotka.monad
+     de.kotka.monad.maybe
+     [clojure.contrib.def :only (defvar)]))
+
+(derive ::success :de.kotka.monad.maybe/just)
+(derive ::failure :de.kotka.monad.maybe/nothing)
+
+(defn return-result
+  [x]
+  (return ::success x))
+
+(defn throw-error
+  [e]
+  (return ::failure e))
+
+(defmulti
+  #^{:arglists '[(result-or-error handler)]
+     :doc
+  "Returns the result in case it was one. In case of an error, call the
+  handler and return whatever it returns."}
+  catch-error
+  (fn [m _] (monad-type m)))
+
+(defmethod catch-error ::success
+  [m _]
+  m)
+
+(defmethod catch-error ::failure
+  [m h]
+  (h (monad m)))