闭包范围-大小写宏

Question

在R.KentDybvig的著作““计划编程语言第4版””中，作者在第86页为接受条件范围的case语句编写了一个define-syntax (Scheme宏)。我想我应该在Clojure试试这个。

这是结果。

我怎样才能改善这一点？我对范围运算符使用:ii、:ie、:ei和:ee，分别表示包容性、包容性、排他性、排他性和排他性。还有更好的选择吗？

我选择扩展到cond，而不是离散的if语句，因为我觉得将来对cond宏的任何改进都会给我带来好处。

(defmacro range-case [target & cases]
  "Compare the target against a set of ranges or constant values and return
   the first one that matches. If none match, and there exists a case with the
   value :else, return that target. Each range consists of a vector containing
   3 terms: a lower bound, an operator, and an upper bound. The operator must
   be one of :ii, :ie, :ei, or :ee, which indicate that the range comparison
   should be inclusive-inclusive, inclusive-exclusive, exclusive-inclusive,
   or exclusive-exclusive, respectively.
   Example:
     (range-case target
                 [0.0 :ie 1.0] :greatly-disagree
                 [1.0 :ie 2.0] :disagree
                 [2.0 :ie 3.0] :neutral
                 [3.0 :ie 4.0] :agree
                 [4.0 :ii 5.0] :strongly-agree
                 42 :the-answer
                 :else :do-not-care)
   expands to
     (cond
       (and (<= 0.0 target) (< target 1.0)) :greatly-disagree
       (and (<= 1.0 target) (< target 2.0)) :disagree
       (and (<= 2.0 target) (< target 3.0)) :neutral
       (and (<= 3.0 target) (< target 4.0)) :agree
       (<= 4.0 target 5.0) :strongly-agree
       (= target 42) :the-answer
       :else :do-not-care)
    Test cases:
      (use '[clojure.test :only (deftest is run-tests)])
      (deftest unit-tests
        (letfn [(test-range-case [target]
                                 (range-case target
                                             [0.0 :ie 1.0] :greatly-disagree
                                             [1.0 :ie 2.0] :disagree
                                             [2.0 :ie 3.0] :neutral
                                             [3.0 :ie 4.0] :agree
                                             [4.0 :ii 5.0] :strongly-agree
                                             42 :the-answer
                                             :else :do-not-care))]
      (is (= (test-range-case 0.0) :greatly-disagree))
      (is (test-range-case 0.5) :greatly-disagree)
      (is (test-range-case 1.0) :disagree)
      (is (test-range-case 1.5) :disagree)
      (is (test-range-case 2.0) :neutral)
      (is (test-range-case 2.5) :neutral)
      (is (test-range-case 3.0) :agree)
      (is (test-range-case 3.5) :agree)
      (is (test-range-case 4.0) :strongly-agree)
      (is (test-range-case 4.5) :strongly-agree)
      (is (test-range-case 5.0) :strongly-agree)
      (is (test-range-case 42) :the-answer)
      (is (test-range-case -1) :do-not-care)))
    (run-tests)"
  `(cond
    ~@(loop [cases cases ret []]
        (cond
         (empty? cases)
         ret

         (odd? (count cases))
         (throw (IllegalArgumentException.
                 (str "no matching clause: " (first cases))))

         (= :else (first cases))
         (recur (drop 2 cases) (conj ret :else (second cases)))

         (vector? (first cases))
         (let [[lower-bound operator upper-bound] (first cases)
               clause (second cases)

               [condition clause]
               (case operator
                     :ii `((<= ~lower-bound ~target ~upper-bound) ~clause)
                     :ie `((and (<= ~lower-bound ~target)
                                (< ~target ~upper-bound)) ~clause)
                     :ei `((and (< ~lower-bound ~target)
                                (<= ~target ~upper-bound)) ~clause)
                     :ee `((< ~lower-bound ~target ~upper-bound) ~clause)
                     (throw (IllegalArgumentException.
                             (str "unknown operator: " operator))))]
           (recur (drop 2 cases) (conj ret condition clause)))

         :else
         (let [[condition clause]
               `[(= ~target ~(first cases)) ~(second cases)]]
           (recur (drop 2 cases) (conj ret condition clause)))))))

更新：下面是包含米凯拉和柯塔拉克建议的更改的修订版

(defmacro range-case [target & cases]
  "Compare the target against a set of ranges or constant values and return
   the first one that matches. If none match, and there exists a case with the
   value :else, return that target. Each range consists of a vector containing
   one of the following patterns:
     [upper-bound]                 if this is the first pattern, match any
                                   target <= upper-bound
                                   otherwise, match any target <= previous
                                   upper-bound and <= upper-bound
     [< upper-bound]               if this is the first pattern, match any
                                   target < upper-bound
                                   otherwise, match any target <= previous
                                   upper-bound and < upper-bound
     [lower-bound upper-bound]     match any target where lower-bound <= target
                                   and target <= upper-bound
     [< lower-bound upper-bound]   match any target where lower-bound < target
                                   and target <= upper-bound
     [lower-bound < upper-bound]   match any target where lower-bound <= target
                                   and target < upper-bound
     [< lower-bound < upper-bound] match any target where lower-bound < target
                                   and target < upper-bound
   Example:
     (range-case target
                 [0 < 1] :strongly-disagree
                 [< 2]     :disagree
                 [< 3]     :neutral
                 [< 4]     :agree
                 [5]       :strongly-agree
                 42          :the-answer
                 :else       :do-not-care)
   expands to
     (cond
       (and (<= 0 target) (< target 1)) :strongly-disagree
       (and (<= 1 target) (< target 2)) :disagree
       (and (<= 2 target) (< target 3)) :neutral
       (and (<= 3 target) (< target 4)) :agree
       (<= 4 target 5) :strongly-agree
       (= target 42) :the-answer
       :else :do-not-care)
    Test cases:
      (use '[clojure.test :only (deftest is run-tests)])
      (deftest unit-tests
        (letfn [(test-range-case [target]
                                 (range-case target
                                             [0 < 1] :strongly-disagree
                                             [< 2]   :disagree
                                             [< 3]   :neutral
                                             [< 4]   :agree
                                             [5]     :strongly-agree
                                             42      :the-answer
                                             :else   :do-not-care))]
      (is (= (test-range-case 0) :strongly-disagree))
      (is (= (test-range-case 0.5) :strongly-disagree))
      (is (= (test-range-case 1) :disagree))
      (is (= (test-range-case 1.5) :disagree))
      (is (= (test-range-case 2) :neutral))
      (is (= (test-range-case 2.5) :neutral))
      (is (= (test-range-case 3) :agree))
      (is (= (test-range-case 3.5) :agree))
      (is (= (test-range-case 4) :strongly-agree))
      (is (= (test-range-case 4.5) :strongly-agree))
      (is (= (test-range-case 5) :strongly-agree))
      (is (= (test-range-case 42) :the-answer))
      (is (= (test-range-case -1) :do-not-care))))
    (run-tests)"
  (if (odd? (count cases))
    (throw (IllegalArgumentException. (str "no matching clause: "
                                           (first cases))))
    `(cond
      ~@(loop [cases cases ret [] previous-upper-bound nil]
          (cond
           (empty? cases)
           ret

           (= :else (first cases))
           (recur (drop 2 cases) (conj ret :else (second cases)) nil)

           (vector? (first cases))
           (let [condition (first cases)
                 clause (second cases)

                 [case-expr prev-upper-bound]
                 (let [length (count condition)]
                   (cond
                    (= length 1)
                    (let [upper-bound (first condition)]
                      [(if previous-upper-bound
                         `(and (<= ~previous-upper-bound ~target)
                               (<= ~target ~upper-bound))
                         `(<= ~target ~upper-bound))
                       upper-bound])

                    (= length 2)
                    (if (= '< (first condition))
                      (let [[_ upper-bound] condition]
                        [(if previous-upper-bound
                           `(and (<= ~previous-upper-bound ~target)
                                 (< ~target ~upper-bound))
                           `(< ~target ~upper-bound))
                         upper-bound])
                      (let [[lower-bound upper-bound] condition]
                        [`(and (<= ~lower-bound ~target)
                               (<= ~target ~upper-bound))
                         upper-bound]))

                    (= length 3)
                    (cond
                     (= '< (first condition))
                     (let [[_ lower-bound upper-bound] condition]
                       [`(and (< ~lower-bound ~target)
                              (<= ~target ~upper-bound))
                        upper-bound])

                     (= '< (second condition))
                     (let [[lower-bound _ upper-bound] condition]
                       [`(and (<= ~lower-bound ~target)
                              (< ~target ~upper-bound))
                        upper-bound])

                     :else
                     (throw (IllegalArgumentException. (str "unknown pattern: "
                                                            condition))))

                    (and (= length 4)
                         (= '< (first condition))
                         (= '< (nth condition 3)))
                    (let [[_ lower-bound _ upper-bound] condition]
                      [`(and (< ~lower-bound ~target) (< ~target ~upper-bound))
                       upper-bound])

                    :else
                    (throw (IllegalArgumentException. (str "unknown pattern: "
                                                           condition)))))]
             (recur (drop 2 cases)
                    (conj ret case-expr clause)
                    prev-upper-bound))

           :else
           (let [[condition clause]
                 `[(= ~target ~(first cases)) ~(second cases)]]
             (recur (drop 2 cases) (conj ret condition clause) nil)))))))

Answer 1

我也会投票赞成一些稍微冗长但不那么丑陋的读物。

 (range-case target
   [(<= 0.0) (< 1.0)] :greatly-disagree
   [(<= 1.0) (< 2.0)] :disagree
   [(<= 2.0) (< 3.0)] :neutral
   [(<= 3.0) (< 4.0)] :agree
   (<= 4.0 5.0)       :strongly-agree
   42 :the-answer
   :else :do-not-care)

这可能是一个可行的替代办法。

Answer 2

一些想法：

• 为操作符设置一个默认值(例如，在典型问题中，":ie“可能是最自然的)
• 将其中一个边界默认为前一个或下面的上限/下限，这样您就不需要重复相同的边界值。
• 考虑如果是假设，而不是cond，这样您就可以进行间隔二分法(如果您期望得到大量的情况，这将是一次性能上的胜利)。

另一种选择可能是使宏在案例级别上工作，如下所示：

(cond
  (in-range target [0.0 1.0]) :greatly-disagree)
  (in-range target [1.0 2.0]) :disagree)
  ...)

我个人喜欢这一点，因为如果需要，您可以将您的范围测试与其他谓词混合使用。