Mercurial > lasercutter
comparison src/clojure/contrib/singleton.clj @ 10:ef7dbbd6452c
added clojure source goodness
author | Robert McIntyre <rlm@mit.edu> |
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date | Sat, 21 Aug 2010 06:25:44 -0400 |
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9:35cf337adfcf | 10:ef7dbbd6452c |
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1 ;;; singleton.clj: singleton functions | |
2 | |
3 ;; by Stuart Sierra, http://stuartsierra.com/ | |
4 ;; April 14, 2009 | |
5 | |
6 ;; Copyright (c) Stuart Sierra, 2009. All rights reserved. The use | |
7 ;; and distribution terms for this software are covered by the Eclipse | |
8 ;; Public License 1.0 (http://opensource.org/licenses/eclipse-1.0.php) | |
9 ;; which can be found in the file epl-v10.html at the root of this | |
10 ;; distribution. By using this software in any fashion, you are | |
11 ;; agreeing to be bound by the terms of this license. You must not | |
12 ;; remove this notice, or any other, from this software. | |
13 | |
14 | |
15 ;; Change Log: | |
16 ;; | |
17 ;; April 14, 2009: added per-thread-singleton, renamed singleton to | |
18 ;; global-singleton | |
19 ;; | |
20 ;; April 9, 2009: initial version | |
21 | |
22 | |
23 (ns | |
24 ^{:author "Stuart Sierra", | |
25 :doc "Singleton functions"} | |
26 clojure.contrib.singleton) | |
27 | |
28 (defn global-singleton | |
29 "Returns a global singleton function. f is a function of no | |
30 arguments that creates and returns some object. The singleton | |
31 function will call f just once, the first time it is needed, and | |
32 cache the value for all subsequent calls. | |
33 | |
34 Warning: global singletons are often unsafe in multi-threaded code. | |
35 Consider per-thread-singleton instead." | |
36 [f] | |
37 (let [instance (atom nil) | |
38 make-instance (fn [_] (f))] | |
39 (fn [] (or @instance (swap! instance make-instance))))) | |
40 | |
41 (defn per-thread-singleton | |
42 "Returns a per-thread singleton function. f is a function of no | |
43 arguments that creates and returns some object. The singleton | |
44 function will call f only once for each thread, and cache its value | |
45 for subsequent calls from the same thread. This allows you to | |
46 safely and lazily initialize shared objects on a per-thread basis. | |
47 | |
48 Warning: due to a bug in JDK 5, it may not be safe to use a | |
49 per-thread-singleton in the initialization function for another | |
50 per-thread-singleton. See | |
51 http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=5025230" | |
52 [f] | |
53 (let [thread-local (proxy [ThreadLocal] [] (initialValue [] (f)))] | |
54 (fn [] (.get thread-local)))) |