Clojure 根据条件更改绑定本地的值

Question

根据以下条件更改本地的惯用方法是什么？在这里，我根据某些条件更改 x 的值。

(defn person-story
 [person]
  (let [x (str "My name is " (:firstname person) " " (:lastname person) "." )
    x (if (:showaddress person) (str x " I live in " (:address person) ".") x)
    x (if (:showage person) (str x " I am " (:age person) " yrs old. ") x)
    x (if (seq (:hobby person)) (str x " I like " (clojure.string/join ", " (:hobby person)) ".") x)]
x))

(person-story {:firstname "John" :lastname "Doe" :age 45 :showage false :address "67 Circe Ave" :showaddress true :hobby ["movie" "music" "money"]})

那会输出：

"My name is John Doe. I live in 67 Circe Ave. I like movie, music, money."

如果我用 java 来做，我会做这样的事情：

    StringBuilder sb = new StringBuilder("My name is ");
    sb.append(person.get("firstname")).append(" ");
    sb.append(person.get("lastname")).append(" ");
    if (showaddress) sb.append("I live in ").append(person.get("address")).append(" ");
    if (showage) sb.append("I am ").append(person.get("age")).append(" yrs old. ");
    List<String> hobbies = person.get("hobby");
    if ( hobbies != null && !hobbies.isEmpty()) sb.append("I like "). append(StringUtils.join(hobbies, ", "));
    return sb.toString()

实现我在 clojure 中实现的相同目标的最佳方法是什么？对不起标题，我想不出更好的标题。

---

解决方案

谢谢 xsc 和 amalloy，两个答案都很棒。我接受了 amalloy 的回答，因为它展示了一种全新的解决问题的方法，但两者都赞成。 以下是两种建议的解决方案的 sn-ps：

合金的方法：

(defn person-story2 [person]
  (let [tests [[:showaddress #(format " I live in %s." (:address %))]
               [:showage #(format " I am %s yrs old." (:age %))]
               [(comp seq :hobbies) #(format " I like %s." (clojure.string/join ", " (:hobbies %)))]]]
    (apply str (format "My name is %s %s." (:firstname person) (:lastname person))
           (for [[test f] tests
                 :when (test person)]
             (f person)))))


(person-story2 {:firstname "John" :lastname "Doe" :showage true :age 50  :showaddress true :address "Universal Studios" :hobbies ["movies" "music" "money"]})

输出：

"My name is John Doe. I live in Universal Studios. I am 50 yrs old. I like movies, music, money."

xsc的方法：

(defn person-story 
  [{:keys [firstname lastname address showaddress age showage hobbies] :as person}]
  (cond-> 
   (str "My name is " firstname " " lastname ". ")
   showaddress (str "I live in " address ". ")
   showage (str "I am " age " yrs old. ")
   (seq hobbies) (str "I like " (clojure.string/join ", " hobbies))))

(person-story {:firstname "John" :lastname "Doe" :showage false :age 50 :address "Universal Studios" :showaddress true :hobbies ["movies" "music" "money"]})

输出：

"My name is John Doe. I live in Universal Studios. I like movies, music, money"

Answer 1

从 Clojure 1.5 开始，cond->/cond->> 的工作方式与 ->/->> 类似，但条件决定是否执行单个步骤：

(cond->
  (str "My name is " (:firstname p) " " (:lastname p) ".")
  (:showaddress p) (str "I live in " (:address p) ".")
  (:showage p)     (str "I am " (:age p) " yrs old.")
  ...)

这将是条件字符串构建的惯用解决方案。或者，您可以使用以下内容：

(clojure.string/join
  [(str "My name is " (:firstname p) " " (:lastname p) ".")
   (if (:showaddress p)
     (str "I live in " (:address p) "."))
   ...])

这利用了nil 在连接字符串时将被忽略的事实。

Answer 2

为避免重复任何无用的内容（例如，if 条件或您正在遍历所有内容的 x），请定义您要运行的测试的列表，以及在测试成功时应用的函数。然后你可以减少那个列表。在通过重复调用 str 构建字符串的简单情况下，您可以剪掉中间人一点，只需通过 apply 自己调用 str：

(defn person-story [person]
  (let [tests [[:showaddress #(format " I live in %s." (:address %))]
               [:showage #(format " I am %s yrs old." (:age %))]
               [(comp seq :hobby) #(format " I like %s." (clojure.string/join ", " (:hobby %)))]]]
    (apply str (format "My name is %s %s." (:firstname person) (:lastname person))
           (for [[test f] tests
                 :when (test person)]
             (f person)))))