Question

我正在尝试使用for-macro显示menu-items变量中的项目：

(defrecord MenuItem
  [select-char description])

(def menu-items [(MenuItem. "1" "add an expense")
         (MenuItem. "2" "add an income")
         (MenuItem. "0" "exit")])

(defn display-menu [items]
  (for [item items]
    (println (:select-char item))))

(defn menu-prompt [items]
  (display-menu items)
  (read-val ">>>"))

(println menu-items)
(menu-prompt menu-items)

然而，只有＆gt;＆gt;＆gt;显示提示。有人可以解释为什么会这样，以及如何显示这些项目吗？

Answer 1

这是“懒虫”的案例

用于生成一个惰性序列，该序列仅在读取时进行评估对display-menu的调用只返回对列表的引用然后继续它的结婚方式没有做任何事情。

将其包裹在对doall

的调用中

user> (def a (for [x (range 10)] (println "doing work " x)))
#'user/a
user> a
(doing work  0
doing work  1
doing work  2
doing work  3
doing work  4
doing work  5
doing work  6
doing work  7
doing work  8
doing work  9
nil nil nil nil nil nil nil nil nil nil)

如果您使用doall或dorun，那么它将立即完成工作。

user> (dorun (for [x (range 10)] (println "doing work " x)))
doing work  0
doing work  1
doing work  2
doing work  3
doing work  4
doing work  5
doing work  6
doing work  7
doing work  8
doing work  9
nil
user> (doall (for [x (range 10)] (println "doing work " x)))
doing work  0
doing work  1
doing work  2
doing work  3
doing work  4
doing work  5
doing work  6
doing work  7
doing work  8
doing work  9
(nil nil nil nil nil nil nil nil nil nil)

迭代一系列记录

1 个答案: