在这个简单的例子中展示一流的功能 [英] Demonstrate first-class functions in this simple example

问题描述

请使用这两个重叠的 Clojure 函数为初学者演示代码重用的一流函数(或其他一些函数式编程概念).基本上，使用函数式编程方法简化下面的代码块.

Please demonstrate first-class functions (or some other functional programming concept) for code reuse for a beginner using these two overlapping Clojure functions. Basically, simplify the code block below using functional programming methods.

作为比较，你会如何用你选择的其他函数式语言来做到这一点?

As a comparison, how would you do it in your other functional language of choice?

insertR 和 insertL 是简单的首次出现插入函数.如您所见，它们仅相差一行.

insertR and insertL are simple first-occurrence insertion functions. As you can see, they only differ by one line.

;test case
(def newkey :cake)
(def oldkey :and)
(def lat '(:bacon :lettuce :and :tomato :and :jelly)) ; list of keywords

(defn insertR [newkey oldkey lat]
  (if (empty? lat)  
    '()
    (if (= (first lat) oldkey)
        (cons oldkey (cons newkey (rest lat))) ;;; diff line
        (cons (first lat) (insertR newkey oldkey (rest lat))))))

(defn insertL [newkey oldkey lat]
  (if (empty? lat)
    '()
    (if (= (first lat) oldkey)
        (cons newkey lat)  ;;; diff line
        (cons (first lat) (insertL newkey oldkey (rest lat))))))

这是输出.

=> (insertR newkey oldkey lat)
(:bacon :lettuce :and :cake :tomato :and :jelly)
=> (insertL newkey oldkey lat)
(:bacon :lettuce :cake :and :tomato :and :jelly)

推荐答案

以下是使用一流函数重构此问题的示例:

Here's an example of refactoring this using first-class functions:

(defn insert [newkey oldkey lat f]
  (if (empty? lat)
    '()
    (if (= (first lat) oldkey)
      (f oldkey newkey lat)
      (cons (first lat) (insert newkey oldkey (rest lat) f)))))

(defn insertL [newkey oldkey lat]
  (insert newkey oldkey lat 
    (fn [oldkey newkey lat] (cons newkey lat))))

(defn insertR [newkey oldkey lat]
  (insert newkey oldkey lat 
    (fn [oldkey newkey lat] (cons oldkey (cons newkey (rest lat))))))

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