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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)))))
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