singleton.clj: new lib for initializing global singleton instances

1 files changed, 27 insertions, 0 deletions

diff --git a/src/clojure/contrib/singleton.clj b/src/clojure/contrib/singleton.clj
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+;;; singleton.clj: singleton functions
+
+;; by Stuart Sierra, http://stuartsierra.com/
+;; April 9, 2009
+
+;; Copyright (c) Stuart Sierra, 2009. All rights reserved.  The use
+;; and distribution terms for this software are covered by the Eclipse
+;; Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
+;; which can be found in the file epl-v10.html at the root of this
+;; distribution.  By using this software in any fashion, you are
+;; agreeing to be bound by the terms of this license.  You must not
+;; remove this notice, or any other, from this software.
+
+
+(ns singleton)
+
+(defn singleton
+  "Returns a memoized version of a function with no arguments.  The
+  memoized version caches the function's return value.
+
+  This is useful for lazily creating global objects that are expensive
+  to initialize.  Warning: Make sure you really want a single global
+  instance, and not one instance per thread."
+  [f]
+  (let [instance (atom nil)
+        make-instance (fn [_] (f))]
+    (fn [] (or @instance (swap! instance make-instance)))))