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1 ;;; fcase.clj -- simple variants of "case" for Clojure
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2
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3 ;; by Stuart Sierra, http://stuartsierra.com/
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4 ;; April 7, 2008
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5
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6 ;; Copyright (c) Stuart Sierra, 2008. 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 ;; This file defines a generic "case" macro called "fcase" which takes
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16 ;; the equality-testing function as an argument. It also defines a
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17 ;; traditional "case" macro that tests using "=" and variants that
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18 ;; test for regular expressions and class membership.
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19
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20
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21 ;; Note (December 23, 2008): This library has been supplanted by the
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22 ;; inclusion of "condp" in clojure.core as of Clojure SVN rev. 1180.
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23
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24
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25 (ns
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26 ^{:author "Stuart Sierra",
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27 :doc "This file defines a generic \"case\" macro called \"fcase\" which takes
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28 the equality-testing function as an argument. It also defines a
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29 traditional \"case\" macro that tests using \"=\" and variants that
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30 test for regular expressions and class membership.
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31
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32
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33 Note (December 23, 2008): This library has been supplanted by the
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34 inclusion of \"condp\" in clojure.core as of Clojure SVN rev. 1180."}
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35
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36 clojure.contrib.fcase
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37 (:refer-clojure :exclude (case)))
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38
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39
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40 (defmacro fcase
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41 "Generic switch/case macro. 'fcase' is short for 'function case'.
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42
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43 The 'compare-fn' is a fn of two arguments.
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44
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45 The 'test-expr-clauses' are value-expression pairs without
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46 surrounding parentheses, like in Clojure's 'cond'.
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47
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48 The 'case-value' is evaluated once and cached. Then, 'compare-fn'
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49 is called once for each clause, with the clause's test value as its
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50 first argument and 'case-value' as its second argument. If
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51 'compare-fn' returns logical true, the clause's expression is
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52 evaluated and returned. If 'compare-fn' returns false/nil, we go to
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53 the next test value.
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54
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55 If 'test-expr-clauses' contains an odd number of items, the last
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56 item is the default expression evaluated if no case-value matches.
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57 If there is no default expression and no case-value matches, fcase
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58 returns nil.
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59
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60 See specific forms of this macro in 'case' and 're-case'.
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61
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62 The test expressions in 'fcase' are always evaluated linearly, in
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63 order. For a large number of case expressions it may be more
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64 efficient to use a hash lookup."
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65 [compare-fn case-value &
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66 test-expr-clauses]
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67 (let [test-val-sym (gensym "test_val")
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68 test-fn-sym (gensym "test_fn")
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69 cond-loop (fn this [clauses]
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70 (cond
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71 (>= (count clauses) 2)
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72 (list 'if (list test-fn-sym (first clauses) test-val-sym)
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73 (second clauses)
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74 (this (rest (rest clauses))))
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75 (= (count clauses) 1) (first clauses)))]
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76 (list 'let [test-val-sym case-value, test-fn-sym compare-fn]
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77 (cond-loop test-expr-clauses))))
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78
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79 (defmacro case
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80 "Like cond, but test-value is compared against the value of each
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81 test expression with =. If they are equal, executes the \"body\"
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82 expression. Optional last expression is executed if none of the
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83 test expressions match."
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84 [test-value & clauses]
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85 `(fcase = ~test-value ~@clauses))
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86
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87 (defmacro re-case
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88 "Like case, but the test expressions are regular expressions, tested
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89 with re-find."
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90 [test-value & clauses]
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91 `(fcase re-find ~test-value ~@clauses))
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92
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93 (defmacro instance-case
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94 "Like case, but the test expressions are Java class names, tested with
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95 'instance?'."
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96 [test-value & clauses]
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97 `(fcase instance? ~test-value ~@clauses))
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98
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99 (defn in-case-test [test-seq case-value]
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100 (some (fn [item] (= item case-value))
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101 test-seq))
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102
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103 (defmacro in-case
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104 "Like case, but test expressions are sequences. The test expression
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105 is true if any item in the sequence is equal (tested with '=') to
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106 the test value."
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107 [test-value & clauses]
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108 `(fcase in-case-test ~test-value ~@clauses))
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