SICP、Continuation Passing Style 和 Clojure 的蹦床答案

【问题标题】：SICP, Continuation Passing Style and Clojure's trampolineSICP、Continuation Passing Style 和 Clojure 的蹦床
【发布时间】：2014-12-14 13:40:16
【问题描述】：

我正在与 SICP 合作，练习 2.29-b 让我有机会在穿越移动设备和分支时享受延续传球风格的乐趣。

简而言之，每个手机都有左右分支，由长度和数字重量或另一个手机组成。该问题要求找出给定手机的总重量。

经过第一个相互递归的解决方案，很简单，我尝试并成功实现了一个cps的解决方案：

(defn total-weight-cps [mobile]
  (letfn 
    [(branch-weight-cps
      [branch kont]
      (let [structure (branch-structure branch)]
        (if (mobile? (branch-structure branch))
          (do (println "then " structure) (kont (traverse-mobile-cps structure identity)))
          (do (println "else " structure) (kont structure)))))

     (traverse-mobile-cps
      [mobile kont]
      (branch-weight-cps (left-branch mobile)
                         (fn [left-weight]
                           (branch-weight-cps (right-branch mobile)
                                              (fn [right-weight] (kont (+ left-weight right-weight)))))))]

    (traverse-mobile-cps mobile identity)))

此时，我已尝试使用蹦床来保留我的堆栈。但它有以下例外：

java.lang.ClassCastException: sicp_clojure.2_1_exercises_2_24_2_32$total_weight_STAR_$traverse_mobile_cps__6694$fn__6695$fn__6696$fn__6697 cannot be cast to java.lang.Number
Numbers.java:126 clojure.lang.Numbers.add
.../git/sicp-clojure/src/sicp_clojure/2_1_exercises_2_24_2_32.clj:185 sicp-clojure.2-1-exercises-2-24-2-32/total-weight*[fn]
core.clj:5801 clojure.core/trampoline
core.clj:5806 clojure.core/trampoline
RestFn.java:439 clojure.lang.RestFn.invoke
.../git/sicp-clojure/src/sicp_clojure/2_1_exercises_2_24_2_32.clj:186 sicp-clojure.2-1-exercises-2-24-2-32/total-weight*

使用trampoline的代码，继优秀link之后，为：

(defn total-weight* [mobile]
  (letfn 
    [(branch-weight-cps
      [branch kont]
      (let [structure (branch-structure branch)]
        (if (mobile? (branch-structure branch))
          (do (println "then " structure) (kont (traverse-mobile-cps structure identity)))
          (do (println "else " structure) (kont structure)))))

     (traverse-mobile-cps
      [mobile kont]
      (branch-weight-cps (left-branch mobile)
                         (fn [left-weight]
                           (branch-weight-cps (right-branch mobile)
                                              (fn [right-weight] #(kont (+ left-weight right-weight)))))))]
    (trampoline traverse-mobile-cps mobile identity)))

最后是一些示例数据：

(def branch11 (make-branch 1 1))
(def branch22 (make-branch 2 2))
(def branch36 (make-branch 3 6))
(def branch43 (make-branch 4 3))

(def mobile11-43 (make-mobile branch11 branch43))
(def mobile36-22 (make-mobile branch36 branch22))

(def branch5m1143 (make-branch 5 mobile11-43))
(def branch7m3622 (make-branch 7 mobile36-22))
(def mobile5m1143-7m3622 (make-mobile branch5m1143 branch7m3622))

(total-weight* mobile5m1143-7m3622)

为什么会爆炸？

【问题讨论】：

你问它为什么会爆炸？你从来没有真正留下你的问题。

标签： clojure continuations sicp continuation-passing

【解决方案1】：

按照我帖子中的相同链接，我已经解决了我的实现问题：

(defn total-weight* [mobile]
  (letfn
    [(branch-weight-cps
      [branch kont]
      (let [structure (branch-structure branch)]
        (if (mobile? (branch-structure branch))
          (fn [] (traverse-mobile-cps structure kont))
          (fn [] (kont structure)))))

     (traverse-mobile-cps
      [mobile kont]
      (branch-weight-cps (left-branch mobile)
                         (fn [left-weight]
                           (branch-weight-cps (right-branch mobile)
                                              (fn [right-weight] #(kont (+ left-weight right-weight)))))))]
    (trampoline traverse-mobile-cps mobile identity)))

【讨论】：