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1 ;;; singleton.clj: singleton functions
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2
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3 ;; by Stuart Sierra, http://stuartsierra.com/
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4 ;; April 14, 2009
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5
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6 ;; Copyright (c) Stuart Sierra, 2009. All rights reserved. The use
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7 ;; and distribution terms for this software are covered by the Eclipse
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8 ;; Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php)
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9 ;; which can be found in the file epl-v10.html at the root of this
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10 ;; distribution. By using this software in any fashion, you are
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11 ;; agreeing to be bound by the terms of this license. You must not
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12 ;; remove this notice, or any other, from this software.
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13
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14
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15 ;; Change Log:
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16 ;;
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17 ;; April 14, 2009: added per-thread-singleton, renamed singleton to
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18 ;; global-singleton
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19 ;;
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20 ;; April 9, 2009: initial version
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21
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22
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23 (ns
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24 ^{:author "Stuart Sierra",
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25 :doc "Singleton functions"}
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26 clojure.contrib.singleton)
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27
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28 (defn global-singleton
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29 "Returns a global singleton function. f is a function of no
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30 arguments that creates and returns some object. The singleton
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31 function will call f just once, the first time it is needed, and
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32 cache the value for all subsequent calls.
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33
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34 Warning: global singletons are often unsafe in multi-threaded code.
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35 Consider per-thread-singleton instead."
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36 [f]
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37 (let [instance (atom nil)
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38 make-instance (fn [_] (f))]
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39 (fn [] (or @instance (swap! instance make-instance)))))
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40
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41 (defn per-thread-singleton
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42 "Returns a per-thread singleton function. f is a function of no
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43 arguments that creates and returns some object. The singleton
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44 function will call f only once for each thread, and cache its value
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45 for subsequent calls from the same thread. This allows you to
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46 safely and lazily initialize shared objects on a per-thread basis.
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47
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48 Warning: due to a bug in JDK 5, it may not be safe to use a
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49 per-thread-singleton in the initialization function for another
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50 per-thread-singleton. See
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51 http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=5025230"
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52 [f]
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53 (let [thread-local (proxy [ThreadLocal] [] (initialValue [] (f)))]
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54 (fn [] (.get thread-local))))
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